Slope Stability Vol2
Slope Stability Vol2
Slope Stability Vol2
Final Report
December 2005
Sponsored by
the Iowa Highway Research Board
(IHRB Project TR-489)
and
the Iowa Department of Transportation
(CTRE Project 03-127)
Iowa State University’s Center for Transportation Research and Education is the umbrella organization for the following centers and programs: Bridge Engineering Center • Center for Weather Impacts on Mobility
and Safety • Construction Management & Technology • Iowa Local Technical Assistance Program • Iowa Traffic Safety Data Service • Midwest Transportation Consortium • National Concrete Pavement
Technology Center • Partnership for Geotechnical Advancement • Roadway Infrastructure Management and Operations Systems • Statewide Urban Design and Specifications • Traffic Safety and Operations
About the PGA
Disclaimer Notice
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and the accuracy of the information presented herein. The opinions, findings, and conclusions
expressed in this publication are those of the authors and not necessarily those of the sponsors.
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Technical Report Documentation Page
CHARACTERIZATION: VOL. II
Final Report
December 2005
Principal Investigator
David J. White
Co-Principal Investigator
Vernon R. Schaefer
Research Assistant
Hong Yang
Sponsored by
A report from
Ames, IA 50010-8632
Phone: 515-294-8103
Fax: 515-294-0467
www.ctre.iastate.edu
TABLE OF CONTENTS
ACKNOWLEDGMENTS ..........................................................................................................XIII
INTRODUCTION ...........................................................................................................................1
Objectives ............................................................................................................................2
General Information...........................................................................................................20
Additional Discussion......................................................................................................136
Summary ..........................................................................................................................139
Conclusions......................................................................................................................139
v
RECOMMENDATIONS.............................................................................................................141
REFERENCES ............................................................................................................................143
APPENDIX..................................................................................................................................147
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LIST OF FIGURES
Figure 1. Existing slope failure at Highway 330 in Jasper County, Iowa (White 2003) .................1
Figure 4. In situ borehole shear test results showing cohesion intercept and friction angle............7
Figure 6. Schematic diagram for the Bromhead ring shear apparatus (Kakou et al. 2001).............9
Figure 11. Landform regions of Iowa (Prior 1991) and locations of the slopes investigated........18
Figure 16. Ring shear test results for the glacial till in slope 1......................................................26
Figure 24. Shear strength parameter values for the weak shale layer (layer 2) in slope 2 ............33
Figure 32. Slope 4, looking east (photo taken by Yang, 04/01/05) ...............................................45
Figure 43. Location of slope 6 (Hwy 34, MP175.3S, Wapello Co.) .............................................56
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Figure 45. Cross-section for slope 6 ..............................................................................................58
Figure 49. Location for slope 7 (Hwy 169, Winterset, Madison Co.) ...........................................63
Figure 53. Ring shear test results for shales in slope 7..................................................................65
Figure 54. Slope profile for stability analysis for slope 7..............................................................66
Figure 55. Analysis 13 assuming homogeneous soils and circular slip surface for slope 7 ..........66
Figure 56. Different shear strength for the weak shale layer 2 in the slope 7 ...............................67
Figure 57. Location of slope 8 (Hwy 169, 2 miles south of Afton, Union Co.) ............................71
Figure 63. Location of slope 9 (Hwy 169, 2 miles south of Afton, Union Co.) ............................77
Figure 69. Location of slope 10 (Hwy 169, 4 miles south of Afton, Union Co.) ..........................83
Figure 75. Location of slopes 11-14 (Hwy E57, 4.5 miles west of Luther, Boone Co.) ...............89
Figure 86. Slope 13, looking southwest (photo taken by Yang, 10/10/04) .................................103
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Figure 91. Slope 14, looking southeast (photo taken by Yang, 10/10/04)...................................108
Figure 96. Location of slope 15 (Sugar Creek Project, Ottumwa, Wapello Co.) ........................119
Figure 102. Plastic limit versus liquid limit for the shales at slope 15 ........................................123
Figure 103. Atterberg limits versus clay fraction for the shales at slope 15................................123
Figure 104. Direct shear test results for samples of boring CH1009 at slope 15 ........................124
Figure 105. Consolidated drained triaxial test for highly weathered shale at 0.6-1.2m in
Figure 106. Results of ring shear test for the highly weathered shale at 0.6-1.2m in boring
Figure 107. Idealized section for slope stability analysis for slope 15 ........................................125
Figure 108. Sensitivity of FS on effective friction angle of the highly weathered shale for the
Figure 109. Sensitivity of FS on effective cohesion of the highly weathered shale for the
Figure 110. Sensitivity of FS on unit weight of the highly weathered shale for the Idealized
Figure 111. Sensitivity of FS on water table level in the river for the Idealized section for
slope 15 ............................................................................................................................127
Figure 112. Sensitivity of FS on the water table level within the slope for the Idealized
Figure 113. Sensitivity of FS on the elevation of the highly weathered shale for the
Figure 114. Slope analysis assuming circular slip surface for the Real section for slope 15 ......128
Figure 115. Slope analysis assuming block slip surface for the Real section for slope 15 .........129
Figure 116. Slope analysis assuming circular slip surface for Idealized section for slope 15.....129
Figure 117. Slope analysis assuming block slip surface for the Idealized section for slope 15 ..130
Figure 120. Shear strength parameter values required for giving FS of unity for slope 15.........131
Figure 121. Atterberg limits for all the soils in slopes 1 to 14.....................................................136
Figure 122. Atterberg limits versus clay fraction for all the soils in slopes 1 to 14 ....................137
Figure 123. Residual friction angel versus plasticity for soils in slopes 1 to 14..........................137
Figure 124. Residual friction angel versus plasticity for soils in slope 15 ..................................138
Figure A2. X-Ray Diffraction results for the shale at depth of 0.3 m for slope 1 .......................147
ix
Figure A5. Borehole log for BH3 at slope 2................................................................................149
Figure A7. X-Ray Diffraction results for the shale at depth of 0.6m in BH4 for slope 2............150
Figure A9. X-Ray Diffraction results for the till at depth of 0.6m in BH2 for slope 3 ...............151
Figure A11. X-Ray Diffraction results for the till at depth of 0.6m in BH2 for slope 4 .............152
Figure A13. X-Ray diffraction results for the till at depth of 0.6m in BH2 for slope 5 ..............153
Figure A15. X-Ray diffraction results for the shale at depth of 0.3m for slope 6 .......................154
Figure A23. X-Ray Diffraction results for the shale at depth of 0.6m for slope 9 ......................159
Figure A37. Direct shear test results for slope 15 (part 1 of 4) ...................................................166
Figure A38. Direct shear test results for slope 15 (part 2 of 4) ...................................................166
Figure A39. Direct shear test results for slope 15 (part 3 of 4) ...................................................167
Figure A40. Direct shear test results for slope 15 (part 4 of 4) ...................................................167
Figure A41. Ring shear test results for the shales at slope 15 (part 1 of 2) .................................168
Figure A42. Ring shear test results for the shales at slope 15 (part 2 of 2) .................................168
Figure A43. XRD result (1 of 10) (CH1003, 5.6-5.9m, highly weathered shale) .......................169
Figure A44. XRD result (2 of 10) (CH1003, 12.75m, slightly weathered shale)........................169
Figure A45. XRD result (3 of 10) (CH1004, 8.2-8.65m, moderately weathered shale)..............170
Figure A46. XRD result (4 of 10) (CH1004, 11.22m, slightly weathered shale)........................170
Figure A47. XRD result (5 of 10) (CH1005, 5.5-6.1m, highly weathered shale) .......................171
Figure A48. XRD result (6 of 10) (CH1005, 7.15-7.3m, highly weathered shale) .....................171
Figure A49. XRD result (7 of 10) (CH1005, 9.2-10.7m, slightly weathered shale)....................172
Figure A50. XRD result (8 of 10) (CH1007, 2.0-2.6m, highly weathered shale) .......................172
x
Figure A51. XRD result (9 of 10) (CH1009, 2.4-2.7m, highly weathered shale) .......................173
Figure A52. XRD result (10 of 10) (CH1010, 0.6-1.2m, highly weathered shale) .....................173
LIST OF TABLES
Table 6. Soil Properties used for the slope analysis for slope 2 ....................................................34
Table 10. Soil properties used for the slope analysis for slope 3...................................................41
Table 20. Summary of basic property test results for slope 7........................................................67
Table 21. Summary of ring shear test results for slope 7...............................................................67
Table 22. Soil properties used for the slope analysis for slope 7...................................................68
Table 25. Summary of basic properties for the soils in slope 8.....................................................74
xi
Table 41. Summary of BST results for slope 15..........................................................................132
Table 44. Summary of direct shear test results for slope 15........................................................134
Table 45. Summary of triaxial and unconfined compression test results for slope 15 ................134
Table 46. Summary of ring shear test results for slope 15...........................................................135
Table 47. Parameters used for the slope analysis for slope 15 ....................................................135
Table 48. Summary of the results of probabilistic analysis for the Real section and the
Table 49. Statistics of the shear strength parameter values from the BST for slopes 1 to 14 .....138
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ACKNOWLEDGMENTS
The Iowa Department of Transportation and the Iowa Highway Research Board sponsored this
study under contract TR-489. The authors are grateful for the sponsorship.
The authors would like to thank the following persons: Gary Kretlow Jr. (Iowa DOT) and Robert
Stanley (Iowa DOT), for their help in collecting desk information on the slope failures; Dr.
Muhannad Suleiman and Bhooshan Karnik (CH2M Hill), for their help during part of the field
work; Donald Davidson Jr., Matt Birchmier, and Sherry Voros, for their help in the laboratory
work; and Scott Schlorholtz, for his advice in the XRD tests.
The authors also would like to acknowledge the following undergraduate students: Ashley
Schwall, who performed the XRD tests; Martinique Martineau, Matz D. Jungmann, Andy S.
Floy, Jesse J. Clark, Christopher E. Milner, and Michael K. Richardson who helped in field tests.
The findings, opinions, recommendations, and conclusions expressed in this report are those of
the authors and do not necessarily reflect the views of the sponsor and administrations.
xiii
EXECUTIVE SUMMARY
Soil slope instability concerning highway infrastructure is an ongoing problem in Iowa, as slope
failures endanger public safety and continue to result in costly repair work. This research
consists of field investigations addressing both the characterization and reinforcement of such
slope failures. The research methods and findings of these investigations are summarized in
Volume 1 of this report. Research details of the independent characterization and reinforcement
investigations are provided in Volumes 2 and 3, respectively. Combined, the field investigations
offer guidance on identifying the factors that affect slope stability at a particular location and
also on designing slope reinforcement using pile elements for cases where remedial measures are
necessary.
Research Summary
Characterization of slope failures is complicated, because the factors affecting slope stability can
be difficult to discern and measure, particularly soil shear strength parameters. Extensive
research has been conducted on slope stability investigations and analysis. The current research,
however, focused on applying an infrequently-used testing technique comprised of the Borehole
Shear Test (BST). This in-situ test rapidly provides effective (i.e., drained) shear strength
parameter values of soil. Using the BST device, fifteen Iowa slopes (fourteen failures and one
proposed slope) were investigated and documented. Particular attention was paid to highly
weathered shale and glacial till soil deposits, which have both been associated with slope failures
in the southern Iowa drift region. Conventional laboratory tests, including direct shear tests,
triaxial compression tests, and ring shear tests were also performed on undisturbed and
reconstituted soil samples to supplement BST results. The shear strength measurements were
incorporated into complete evaluations of slope stability using both limit equilibrium and
probabilistic analyses.
Remediation of slope failures requires stabilization alternatives that address causes of slope
instability. Slope reinforcement using pile elements can be an effective method of remediation in
preventing slope movements in weak soils where enhanced drainage does not provide adequate
stability. Soil load transfer to pile elements from the downslope soil movement as occurs in slope
failures is a complex soil–structure interaction problem. Soil–structure interactions for small-
diameter, grouted pile elements subject to lateral soil movement were investigated by conducting
full-scale pile load tests, in which piles installed through a shear box into stable soil were loaded
by uniform lateral translation of soil. Instrumentation of the shear boxes and pile reinforcement
indicated the load distributions that developed along the piles. The load test analyses which
followed the pile load tests support the claim that the distributed loads which are mobilized
during pile loading depend on the relative displacement between the soil and pile elements. The
reliable estimation of these load distributions is important, because the influence of piles on the
global stability of the slope depends directly on the pile loading condition.
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Research Conclusions
The following conclusions were drawn from slope stability case histories:
• The Borehole Shear Test often measures peak shear strength parameters, which are
generally not operative for a slope failure, and sometimes measures the soften shear
strength when the measurements are taken near the slip surface. Factors of safety for case
histories of slope failures calculated using BSTs were generally greater than unity.
• The ring shear test using reconstituted samples gives residual shear strength parameter
values corresponding to relatively large shear displacements. Factors of safety for case
histories of slope failures calculated using ring shear test results were generally less than
unity.
• Back calculated shear strengths for slope failures that provided factors of safety equal to
unity were generally between shear strengths from ring shear tests and Borehole Shear
Tests. Slope failures can be attributed to soil softening or progressive failure and may
have been caused by high water tables.
• For some slope failures, the use of the BST are useful in better estimating the operative
(or the mobilized) shear strength in conjunction with the residual shear strength and back
calculated shear strength.
• For the slope failures, the glacial tills generally have lower clay fraction and lower
plasticity index than the clay shales. All the tills are classified as low plasticity clay (CL)
according to Unified Soil Classification System, while most of the shales are classified as
high plasticity clay (CH).
• The peak BST results for the slope failures show that, the glacial tills and the clay shales
have similar average values of effective friction angle, which are 22.5o and 22.1o,
respectively; but the glacial tills have considerably lower average value of effective
cohesion (11.6 kPa) than the clay shales (17.7 kPa). However, the glacial tills have higher
residual shear strength (residual friction angle of 8.4o to 26.9o) than the clay shales
(residual friction angle of 6.2o to 15.1o).
• Sensitivity analyses showed that soil shear strength is the most sensitive parameter
affecting factors of safety. Water table location additionally has a significant influence on
slope stability.
• Probabilistic slope stability analyses are useful when a relatively large amount of input
parameters are available, such as shear strengths obtained from BSTs. The probability of
slope failure is evaluated based on statistical distribution of soil shear strengths.
• The installation of slender piles in weak soils offers considerable resistance to lateral soil
movement, with improvement factors from the load tests ranging from 1.2 to 6.6.
Improvement factors are defined as a ratio of peak loads for reinforced tests and
unreinforced tests.
• Pile section moment capacities were mobilized, indicating that a “flexible” pile failure
mode was achieved. The depth of maximum moment and pile failure ranged from 1.8 to
5.4 pile diameters below the shear plane.
xvi
• The relative soil-pile displacement at the soil surface indicates the behavioral stages of
small-diameter piles as (1) mobilization of soil shear stresses and elastic bending of pile,
(2) mobilization of pile concrete compressive strength, and (3) incipient pile failure due
to pile moment capacity mobilization. The behavioral characteristics of slender piles are
controlled by structural pile behavior through moment-curvature relationships as much as
they are by soil behavior.
• Displacement-based lateral response analysis methods which use soil p-y curves
accurately predict the deflection and bending moment of piles subject to lateral soil
movement. From these pile behavior characteristics, pile shear may be calculated and
applied to the limit equilibrium equation for evaluating global stability of reinforced
slopes.
The research findings are expected to benefit civil and geotechnical engineers of government
transportation agencies, consultants, and contractors dealing with slope stability, slope
remediation, and geotechnical testing in Iowa. In-situ BST measurements provide reliable, site-
specific soil parameters for design applications which can lead to substantial cost savings over
using empirical estimations for critical soil properties. As the BST is an alternative to expensive
and time-consuming laboratory testing, the device is particularly useful in obtaining relatively
large amounts of data necessary for probabilistic analyses. Procedures for incorporating Borehole
Shear tests into practice are documented in Volume 2 of this report. Nevertheless, some training
may be required for effective and appropriate use. The BST is primarily intended to test cohesive
soils. The device can produce erroneous results in gravelly soils. Additionally, the quality of
boreholes affects test results, and disturbance to borehole walls should be minimized before test
performance. A final limitation of widespread Borehole Shear testing may be its limited
availability, as only about 4 to 6 test devices are currently being used in Iowa.
The research presented in Volume 3 demonstrates with experimental testing how lateral forces
develop along stabilizing piles to resist slope movements. This report then documents a step-by
step procedure that can be used by both state and county transportation agencies to design slope
reinforcement using slender piles. A state department of transportation may develop training
seminars for all local transportation agencies to provide further guidance in using the proposed
design method. This effort may be coordinated with the authors and might be extended so far as
to conduct a pilot study to demonstrate the intended process of designing and evaluating the
reinforcement solution. While slope reinforcement with slender piles by county transportation
agencies is encouraged, such action is recommended to be coordinated with the state department
of transportation. This organization can document all such remediation projects to better guide
counties using successful and unsuccessful experiences, as the DOT will have working
knowledge of other unstable slope characteristics and corresponding reinforcement designs. The
proposed slope reinforcement solution has not yet been demonstrated at an Iowa slope failure
site. As a result, difficulty in scheduling and bidding a pile reinforcement project and evaluating
the effectiveness of the measure may impede successful implementation. Obtaining experience
and feedback through data collection or visual inspection, however, will promote incorporation
of the research findings into standard slope remediation practice.
xvii
Successful implementation of innovative slope stability reinforcement and characterization
solutions can be evaluated by documenting the number of slopes reinforced with pile elements
and those investigated using BST measurements, respectively. Cost savings of incorporating
Borehole Shear testing into site investigation practice will be made evident by comparing costs
corresponding to designs for geostructures making use of accurate and reliable soil properties
(obtained from BST measurements) to those designs using estimated soil properties and higher
factors of safety. Calculating long-term cost savings of slope reinforcement using piles
considering maintenance costs associated with alternatives and the cost for rebuilding a failed
drainage remediation, for example, can indicate the progress and consequences of
implementation.
xviii
INTRODUCTION
Problem Statement
Figure 1. Existing slope failure at Highway 330 in Jasper County, Iowa (White 2003)
Slope failures are complex events and the factors that affect slope stability are difficult to
measure, particularly shear strength parameter values of the soil and ground water conditions.
Ideally, the stability problems can be discovered and addressed before a slope failure occurs.
However, once a failure occurs or a potential failure is identified, information and knowledge of
the major factors resulting in the failure are required to develop an effective remediation plan.
It is necessary to evaluate the stability of the concerned slopes, or to investigate the causes of the
slope failures, in a rapid and effective way. Although various test methods are available for field
investigation, this study focused on the use of the Borehole Shear Test (BST), which has been
considered as a simple and quick in-situ testing technique (Handy 1986). The investigations were
supplemented by other laboratory tests. Particular emphasis was given to the characterization of
the clay shales which have been associated with many slope failures in Iowa.
Objectives
Chapter 2 provides background information relevant to the study, including (1) Regional geology
of Iowa; (2) General considerations in slope investigation; (3) Borehole Shear Test and Rock
Borehole Shear Test; (4) Residual shear strength and ring shear test; (5) Factor of safety and
limit equilibrium slope analysis; and (6) Probabilistic slope analysis.
Chapter 3 presents the field and lab investigation results, slope analysis results and their
discussions and conclusions for 15 case histories of slopes. The chapter starts with some general
information and overview followed by the details for the 15 case histories.
Chapter 4 summarizes and concludes the results and findings in the study. Finally, Chapter 5
makes some recommendations.
BACKGROUND INFORMATION
In this chapter, some background information that is relevant to the research is provided. This
includes the regional geology of Iowa; general considerations in slope investigation; Borehole
Shear Test (BST) and Rock BST; residual shear strength and ring shear test; liquid equilibrium
slope analysis and probabilistic slope analysis.
Iowa is commonly divided into seven regions based on the various landforms found in each
region (Prior 1976 and 1991). Those regions include the Des Moines Lobe, Southern Iowa Drift
Plain, Loess Hills, Iowan Surface, Northwest Iowa Plains, Paleozoic Plateau, and Alluvial Plains
and are shown in Figure 2. Each region has its own unique landforms and landscape formed by
various processes. Most of the landforms of Iowa were formed by water erosion or glacial
erosion. Various geologic materials also have influenced the formation of the landforms. The
following descriptions of each region are adopted after Prior (1976 and 1991).
Deposits and landforms on the Des Moines Lobe are the best examples of recent glacial erosion
and deposition in the state. The Des Moines Lobe landforms formed during the last glacial
advance into Iowa about 12,000 to 14,000 years ago. The landforms exhbit rough edges or end
moraines, lakes and flat areas, with circular ponds or depressions. Most of the landscape is
covered with glacial drift left behind by the glacier. Glacial drift is a deposit of boulders, gravel,
sand, silt, and clay left behind by a glacier or by the streams and rivers that drained off the
melting ice. In places boulders can be found along fences or in the fields. Present day the rivers
that flow across the Lobe have deposited sand and gravel layers (alluvium).
The Southern Iowa Drift Plain is the largest in Iowa. The landscapes are characterized by gently
rolling hills and valleys. They have been formed by hundreds of thousands of years of erosion
and stream development on what was once a landscape similar to that in the Des Moines Lobe
region. Often trees or even forests grow in the valleys. Rivers, streams, or creeks at the bottoms
of the valleys with their numerous upstream tributaries form a drainage pattern that looks like the
branches of a tree. Underlying much of the region is a thin layer of loess, a thick layer of glacial
drift, and finally bedrock of limestone, shale, and sandstone. Alluvium is common on the flood
plain of the region's drainages. Paleosols also are found in the region.
Loess Hills
The Loess Hills landform region is located along the west edge of Iowa. It formed periodically
during the last 150,000 years. Loess is windblown silt that was picked up by winds off the
Missouri River valley floor during and between glacial advances and retreats. Loess is thickest
along the west edge of Iowa and gradually thins as you go eastward toward central Iowa. Loess
is deposited on top of older glacial drift and bedrock. Streams and rivers have eroded valleys in
the loess and deposited alluvium on their flood plains. There also are deposits of colluvium in the
valleys. The landform region is characterized by steep-sided hills and ridges and tree-covered
ravines or side valleys.
Iowan Surface
The Iowan Surface is one of the most difficult regions to interpret geologically. Recent studies
indicate that the region formed mainly due to intense erosion in a cold, tundra-like climate. The
region is characterized by almost flat land, occasional long hills that early observers called
"dolphin-backed hills," and rivers and streams. In the northern part of the region there are
numerous sinkholes or depressions caused by the collapse of underground caves and caverns.
Glacial drift similar to that found in the Southern Iowa Drift Plain and limestone bedrock
underlie the region, and loess remains on the tops of the elongated hills, which geologists call
Paha after a Native American word that describes a hill. Colluvium and alluvium are found on
some slopes and along flood plains. Erratics (boulders moved by the glaciers from Canada and
Minnesota) are common and sometimes very large.
The Northwest Iowa Plains are the highest, driest, and least tree-covered region in the state. The
region is characterized by a landscape that is similar to the Iowa Surface: flat to very gently
rolling, with long parallel hills and subtle valleys. Trees are typically found only where planted
around farmsteads or in some valley bottoms. Glacial drift underlies a thin layer of loess that
covers most of the region.
Paleozoic Plateau
The contrast between the Paleozoic Plateau region of Iowa and all of the rest of the state is very
obvious. Outcrops of solid bedrock (mostly limestone) are very common. Only a few scattered
patches of glacial deposit exist in the region. Valleys are deep, steep, and make great scenic
vistas as viewed from the uplands. The bedrock that controls the shape of the land in this region
formed in warm tropical sea floors between 300-500 million years ago. The bedrock forms the
famous "bluffs" along the edge of the Mississippi River's flood plain. Caves are common and
sinkholes or depressions often filled with water are found in portions of this landform region.
Alluvial Plains
This landform is located adjacent to the Mississippi and Missouri Rivers and other large rivers in
the state. Characterized by landscapes developed by water erosion and deposition along a river's
flood plain it is wide and flat, with features typical of a flowing river. Alluvium deposited by the
river and glacial drift or bedrock underlie the region.
Many factors are involved in soil slope stability evaluation and analysis. Among those that need
to be considered, the main ones include (1) geologic conditions, including soil properties and
shear strength; (2) site topography; (3) ground water conditions; (4) construction effects; and (5)
seismicity (Abramson et al. 2002; Duncan 1996). Among these factors, shear strength of soil, site
topography and ground water conditions are the most critical for embankment slopes and cut
slopes. Therefore, these factors will be given particular consideration in slope stability
investigation.
Though many apparatus and methods can be used for slope stability investigation, it is neither
possible nor necessary to use all of them. Thus, this research will be limited to the use of a few
apparatus, which includes in-situ Borehole Shear Test (BST) and some conventional laboratory
test devices. The BST is used to rapidly measure in-situ shear strength parameters in a borehole
that is drilled either mechanically or by hand-augering. Conventional laboratory tests such as
direct shear test, triaxial compression test and ring shear test will also be performed on
undisturbed soil samples. As BST is relatively less used, its details and testing procedures are
given in the following section.
The shear strength of soil is perhaps the most critical factor in slope stability analysis. Many
apparatus and methods have been used to obtain the shear strength parameters through both field
measurements (e.g., standard penetration test and cone penetration test, etc.) and laboratory
measurements (e.g., direct shear test and triaxial test, etc.). Among the various test equipment
and apparatus, the Borehole Shear Test (BST) is unique in that it gives a rapid, direct and
accurate in situ measurement of both effective cohesion and friction angle (Handy 1986).
The fundamental consideration involved in the BST is to perform a series of direct shear tests on
the inside of a borehole (Handy and Fox 1967; Wineland 1975). A BST apparatus is shown in
Figure 3. Tests are conducted by expanding diametrically opposed contact shear plates into a
borehole under a constant known normal stress, allowing the soil to consolidate, and then by
pulling vertically the shear plates and measuring the shear stress. Data points are plotted on
Mohr-Coulomb shear envelope (Figure 4) by measuring the maximum shear resistance at
successively higher increments of applied normal stresses. Depending on soil type, the total
testing time for a typical test with 4 to 5 data points is approximately 30 to 60 minutes
(Lutenegger and Hallberg 1981). Because drainage times are cumulative, the BST is normally a
consolidated-drained test (Lutenegger and Tierney 1986).
The BST has been successfully used by a number of researchers in different soil conditions,
including sandy, silty and clayey soils and shales (e.g., Demartincourt and Bauer 1983; Handy
1986; Lutenegger and Tierney 1986; Millian and Escobar 1987); soft marine clays (Lutenegger
and Timian 1987; Demartinecourt and Bauer 1983); hard clays (Handy et al. 1985) and stiff soil
(Lutenegger et al. 1978); and unsaturated soils (Miller et al. 1998). Recently, White and Handy
(2001) also used the BST to study preconsolidation pressures and soil modulii. In addition, the
BST has been used to study a few landslide case histories (e.g., Tice and Sams 1974; Handy
1986). The studies show that the BST is particularly useful for quickly and accurately acquiring
the in-situ shear strength parameters of the soil within the slip zone of an active landslide. After
the slide activates, soil cohesion appears to become essentially zero (Handy 1986).
A Rock Borehole Shear Test (RBST) is also a portable direct shear device used to evaluate rock
shear strength in-situ. The device was developed by Handy and associates at Iowa State
University (Handy et al. 1976). The operation mechanism of the RBST is similar to that of the
BST, except that the RBST is designed to cater for much higher normal and shear stresses. The
maximum rock shear strength that may be measured is 45 MPa, and the range of applied normal
stress is 0-86 MPa (Handy et al. 1976). The RBST device consists of three basic parts, i.e. the
shear head assembly, the pulling jack, and the console (Figure 5). A number of authors (e.g.,
Higgins and Rockaway 1979) have reported successful uses of the RBST in measuring the shear
strength of rock.
80
n = 9
R2 = 0.997
60
φ=22.6
τ (kPa)
40
20
c = 16 kPa
0
0 20 40 60 80 100
σ (kPa)
Figure 4. In situ borehole shear test results showing cohesion intercept and friction angle
Skempton (1964, 1985) described the residual strength as the minimum strength of the soil after
large displacement. Lambe and Whitman (1979) expressed the residual strength as the ultimate
strength of soil in the ultimate conditions during shearing. The shear strength of the soil can drop
from its peak value to the residual value after large displacement, and the drop can be significant
for materials with large amounts of clay minerals, particularly platy minerals. The formation of
the shear surface and achieving the residual strength results in the formation of a new fabric,
particularly in material with high clay content. The drop in strength is attributed to the clay
particle reorientation parallel to the direction of shearing (Lambe and Whitman 1979; Bromhead
1992). While cohesion provides much of the peak strength, the material has little cohesion once a
shear surface is formed (Skempton 1964). Residual strength has been correlated with soil index
properties such clay content and Atterberg limit by many researchers (e.g., Voight 1973; Kanji
1974; Lupini et al. 1981; Mesri and Cepeda-Diaz 1986; Collotta et al. 1989; and Stark and Eid
1994). Residual strength is often related to long-term stability problems and for areas with
landslide history, bedding planes or folded strata (Skempton 1985). The drop in residual strength
from peak strength may cause reactivation of old landslides.
Residual strength parameters are often determined using a rotational ring shear test device. A
few types of ring shear apparatus have been reported by Hvorslev (1939), La Gatta (1970),
Bishop et al. (1971) and Bromhead (1979). The Bromhead ring shear apparatus (Figures 6 and
7) has become widely used due to its simplicity in operation compared to other previous models.
A full description of the apparatus can be found in the technical literature by WF Engineering
(1988). In the apparatus, the ring shaped specimen has an internal diameter of 7 cm and an
external diameter of 10 cm. Drainage is provided by two porous bronze stones fixed to the upper
platen and to the bottom of the container.
Currently, a few testing procedures have been proposed for the use of the Bromhead ring shear
apparatus. Stark and Vetell (1992) have shown that the single stage test procedure provides a
good estimation of the residual strength at effective normal stress less than 200 kPa. When the
effective normal stress is greater than 200 kPa, consolidation of the specimen during the test
causes settlement of the upper platen into the lower platen giving higher residual strength values.
Anayi et al. (1988) have pointed out that in the preshearing test procedure, the preshearing
facilitates the creation of a shear plane and reduces the amount of length of the horizontal
displacement required to reach the residual condition. This procedure causes extrusions of a
substantial amount of soil during the shear process and therefore, as in the case of the single
stage test procedure, gives higher measured residual strength values. Stark and Vetell (1992) also
concluded that in the multistage test procedure an additional strength, probably due to wall
friction as the top platen settles into the specimen container, develops during consolidation and
shear process; hence they proposed the flush test procedure in which, increasing the thickness of
the specimen prior to shear reduces the wall friction and gives more trustworthy measured
values. This procedure takes substantial time to reach the residual condition when it is conducted
at low rate of displacement. In this study, the test procedures (multistage test procedures)
described in ASTM (2002g) (D6467-99) were adopted to determine the residual strength of soils.
The soil specimen is pre-sheared at a relatively large displacement rate and followed by
subsequent shearing under small displacement rate under a few different normal stresses. The
plot of shear stress versus normal stress gives the Mohr-Coulomb failure envelope and the
residual shear strength parameter values.
Figure 6. Schematic diagram for the Bromhead ring shear apparatus (Kakou et al. 2001)
Factor of Safety
Once the slope geometry and subsoil conditions of a slope have been determined, stability of a
slope can be evaluated using either published chart solutions or a computer analysis. The primary
objectives of a slope stability analysis normally include: (1) to evaluate how safe a slope is, or to
calculate the factor of safety for a slope before its failure; and (2) to find out the failure
mechanism if a slope has failed in order to provide necessary information for the remedial
design.
Stability of a slope is usually analyzed by methods of limit equilibrium, and the factor of safety
over the so-called critical slip surface is computed. The factor of safety is defined as the ratio
between the shear strength and the shear stress required for the equilibrium of the slope:
Shear strength
Factor of Safety = (1)
Shear stress required for equilibrium
which can be expressed as
c + σtanφ
F= (2)
τ eq
where F = factor of safety, c = soil cohesion, φ = soil friction angle, σ = normal stress on the slip
surface, and τer = shear stress required for equilibrium.
Deterministic slope stability analysis as obtained through equilibrium analysis computes the
factor of safety based on a fixed set of conditions and material parameters. In practice, however,
there involve many sources of uncertainty in slope stability analysis, e.g., spatial uncertainties
10
(site topography and stratigraphy, etc.) and data input uncertainties (in-situ soil characteristics,
soil properties, etc). Probabilistic slope stability analysis allows for the consideration of such
uncertainty and variability of the input parameters. Since Borehole Shear Test, which can
produce large amount of soil shear strength data in short time, will be the primary in-situ
investigation method in the study, it will be an advantage to perform probabilistic analysis to
account for the shear strength variability. Handy (1986) illustrated the possible application of
probabilistic analysis involving the use of shear strength parameters obtained from BST in a case
study.
The details of the equilibrium analysis and probabilistic analysis for slope stability are discussed
in the following sections.
In equilibrium analysis, the potential sliding mass is subdivided into a series of slices (Figure 8),
and a general limit equilibrium formulation (Fredlund et al. 1981; Chugh 1986) can be used in
the factor of safety computation. The equations of static that can be generated include
1. Summation of forces in a vertical direction for each slice, where the resulted equations
are solved for the normal forces at the bases of the slices;
2. Summation of forces in a horizontal direction for each slice is used to compute the
interslice normal forces, where the resulted equations are applied in an integration
manner across the sliding mass;
3. Summation of moments about a common point for all slices, where the resulted equations
can be rearranged and solved for the moment equilibrium factor of safety, Fm; and
4. Summation of forces in a horizontal direction for all slices, giving rise to a force
(a) Division of sliding mass into slices (b) Forces acting on a typical slice
Figure 8. Method of slices for slope analysis (Chowdhury 1978)
Even with the above static equations, the analysis is still indeterminate, and a further assumption
is made regarding the direction of the resultant interslice forces. The direction is assumed to be
described by an interslice force function. The factors of safety can then be computed based on
11
moment equilibrium (Fm) and force equilibrium (Ff). These factors of safety may vary depending
on the percentage of the interslice force function used in the computation.
Using the same general limit equilibrium formulation, it is also possible to specify a variety of
interslice force conditions and satisfy only the moment or force equilibrium conditions. The
assumptions made to the interslice forces and the selection of overall force (Ff) or moment (Fm)
equilibrium in the factor of safety equation, give rise to the various methods of analysis. A
rigorous method satisfies both moment and force equilibrium (Ff = Fm).
The available computational methods for slope stability include: (1) Ordinary method of slices
(Fellennius 1927); (2) Bishop (1995) simplified method; (3) Janbu (1968) simplified method; (4)
Lowe and Karafiath (1960) method; (5) Modified Swedish method (US Army Corps of
Engineers 1970); (6) Spencer (1967) method; (7) Bishop (1955) rigorous method; (8) Janbu
(1968) generalized method; (9) Sarma (1973) method; and (10) Morgenstern-Price method
(Morgenstern and Price 1965). These available methods are categorized by the assumptions
made for solving the equations generated in the methods of slices. Fredlund and Krahn (1977),
Duncan (1996) and Abramson et al. (2002) made a comprehensive review and summary on these
computational methods.
Among the 10 methods that can be used to determine the factor of safety, the simplified Bishop
(1955) method, Janbu (1968) method and Morgenstern-Price (1965) method are popular because
factor of safety value can be quickly calculated for most slip surfaces (Abramson et al. 2002).
However, factor of safety generally varies depending on the selected slip surface. Therefore it is
essential to perform a complete, iterative search for the critical slip surface to ensure obtaining
the minimum factor of safety, regardless of the computation method of analysis (Duncan 1996).
Probabilistic slope stability analysis quantifies the probability of failure of a slope. In general, the
input parameters in a probabilistic analysis are considered as the mean values of the parameters,
and the variability of the parameters can be specified by entering the standard derivations of the
parameters.
Since soils are naturally formed materials, consequently their physical properties vary from point
to point. The variability of soil properties is a major contributor to the uncertainty in the stability
of a slope. Laboratory results on natural soils indicate that most soil properties can be considered
as random variables conforming to the normal distribution function (Lumb 1966; Tan 1993),
which is often referred to as the Gaussian distribution function that is written as:
1 ⎡
1
⎛ x − μ ⎞ 2 ⎤
f(x) =
exp ⎢−
⎜ ⎟ ⎥ (3)
σ
2π ⎢⎣
2
⎝
σ
⎠
⎥⎦
12
A normal curve is bell shaped, symmetric and with the mean value exactly at middle of the
curve. A normal curve is fully defined when the mean value, μ and the standard deviation, σ are
known. Theoretically, the normal curve will never touch the x axis, since the relative frequency,
f(x), will be nonzero over the entire range. However, for practical purposes, the relative
frequency can be neglected after ±5 times standard deviation, σ, away from the mean value.
Statistical Analysis
In slope stability analysis, trial factors of safety are assumed to be normally distributed. As a
result, statistical analysis can be conducted to determine the mean, standard deviation, the
probability density function and the probability distribution function of the slope stability
problem. The equations used in the statistical analysis are summarized as follows (Lapin 1983):
1 n
Mean factor of safety, μ: μ
=
∑ Fi
n i =0
(4)
1 n
Standard deviation, σ: σ
=
∑ ( Fi − μ ) 2
n i =0
(5)
1 ⎡
1
⎛ F
− μ ⎞ 2 ⎤
σ
2π ⎢⎣
2
⎝
σ
⎠
⎥⎦
−∞
⎪⎩
σ
2π ⎢⎣
2
⎝
σ
⎠
⎥⎦
⎪⎭
where Fi = the trial factors of safety; n = number of trial factors of safety; and F = factor of
safety. An example of probability density function and the corresponding probability distribution
function are presented in Figures 9 and 10, respectively.
A factor of safety is really an index indicating the relative stability of a slope. It does not
represent the actual risk level of the slope due to the variability of input parameters. With
probabilistic analysis, two indices, which are known as probability of failure and reliability
index, are available to quantify the stability or the risk level of a slope.
The probability of failure is the probability of obtaining a factor of safety less than 1.0, as
illustrated in Figure 10. It is computed by integrating the area under the probability density
function for factors of safety less than 1.0. The probability of failure can be interpreted in two
ways: (1) if a slope were to be constructed many times, what percentage of such slopes would
fail; or (2) the level of confidence that can be placed in a design (Mostyn and Li 1993).
Nevertheless, the probability of failure is a good index showing the actual level of stability of a
slope. In addition, there is also no direct relationship between factor of safety and probability of
failure. In other words, a slope with a higher factor of safety may not be more stable than a slope
13
with a lower factor of safety (Harr 1987). For example, a slope with factor of safety of 1.5 and a
standard deviation of 0.5 will have a much higher probability of failure than a slope with factor
of safety of 1.2 and a standard deviation of 0.1.
The reliability index provides a more meaningful measure of stability than the factor of safety.
The reliability index (β) is defined in terms of the mean (μ) and the standard deviation (σ) of the
trial factors of safety as (Christian et al. 1994):
μ −1.0
β= (8)
σ
The reliability index describes the stability of a slope by the number of standard deviations
separating the mean factor of safety from its defined failure value of 1.0. It can also be
considered as a way of normalizing the factor of safety with respect to its uncertainty. When the
shape of the probability distribution is known, the reliability index can be related directly to the
probability of failure.
14
Probabilistic slope stability analyses can be performed using a few methods. One simple but
versatile computational procedure is the Monte Carlo simulation (e.g., Tobutt, 1982; Hammond
et al. 1992; Chandler 1996) which involves (1) the selection of a deterministic solution
procedure; (2) decisions regarding which input parameters are to be modeled probabilistically
and the representation of their variability in terms of a normal distribution model using the mean
value and standard deviation; (3) the estimation of new input parameters and the determination
of new factors of safety many times; (4) the determination of some statistics of the computed
factor of safety, the probability density and the probability distribution of the problem.
The critical slip surface is first determined based on the mean value of the input parameters using
any of the limit equilibrium methods. Probabilistic analysis is then performed on the critical slip
surface, taking into consideration the variability of the input parameters. The variability of the
input parameters is assumed to be normally distributed with specified mean values and standard
deviations.
During each Monte Carlo trial, the input parameters are updated based on a normalized random
number. The factors of safety are then computed based on these updated input parameters. By
assuming that the factors of safety are also normally distributed, the mean and the standard
deviations of the factors of safety are determined. The probability distribution function is then
obtained from the normal curve. The number of Monte Carlo trials in an analysis is dependent on
the number of variable input parameters and the expected probability of failure. In general, the
15
number of required trials increases as the number of variable input increases or the expected
probability of failure becomes smaller. It is not unusual to do thousands of trials in order to
achieve an acceptable level of confidence in a Monte Carlo probabilistic slope stability analysis
(Mostyn and Li 1993).
16
In this chapter the investigation of 15 case histories is described. The chapter leads off with an
overview of the methods used to investigate the slopes, including the filed and laboratory
methodologies used and a description of the analyses undertaken.
This study includes 15 case histories of slope in total, which are located besides Highways 34,
169, E57, and 63, involving counties of Monroe, Wapello, Madison, Union, and Boone in Iowa.
The locations of the slopes are shown in Figure 11, and the overall information for the slopes is
summarized in Table 1. The circles and numbers in the figure indicate the approximate locations
and slope numbers in the study, respectively. The major field investigations were carried out
between August 2003 and November 2004, and the main laboratory tests were conducted
between August 2004 and May 2005.
The slopes are mainly comprised of either clay shale or glacial till, which are commonly
encountered in Iowa. Among the 15 slopes, one is a proposed embankment slope that is currently
under design (Slope 15, Sugar Creek Project); one is a slope that is not failed (Slope 4); the
remainders are all considered failed with apparent failure features. The failed slopes include both
embankment slopes (comprising compacted fill) and back-slopes (formed by cutting). The slopes
in the study are generally gentle and of small scale with slope angle ranging from 11o to 23o and
height ranging from 6 to 23m (Table 1).
Extensive field investigations and laboratory tests were performed for the slopes. Field
investigations include measurement of slope geometry, boring and soil sampling, in-situ
Borehole Shear Test (BST) and groundwater table measurement. Mechanical drilling of
boreholes using rotary drilling rig was mainly concentrated on Slope 15 (Sugar Creek Project). A
total of 10 boreholes were drilled by CH2M Hill. Slope 7 (Winterset) also has two mechanically
drilled boreholes. The remainders of the boreholes for the study were drilled manually using a
hand auger due to the site restraints. The mechanically drilled boreholes were as deep as 12m,
while the manually drilled boreholes could only reach a maximum depth of 4.2m (14 ft) with 3 to
3.6 m for most cases. These depths appear to be sufficient to provide the necessary subsurface
information for the slopes since most of the slides in the study are of relatively small scale (Table
1).
The number of the borehole drilled for each of the slopes ranged from 1 to 4 except for Slope 15,
and the number of BSTs performed in each borehole also varied from 1 to 4, both of which
depended on the complexity of the site conditions. Ground water levels were monitored and
measured, normally within 2 days after boring. The BSTs provided in-situ shear strength
parameter values of the soils, which are necessary for the slope analysis together with the ground
water conditions.
Laboratory investigations mainly comprised ring shear tests, basic property tests (grain size
analysis and Atterberg limits test), mineralogy analyses, soil classifications, natural water
17
contents and density measurements on the representative soil samples from each slope. Extensive
direct shear tests and a few triaxial compression tests, unconfined compression tests and
consolidation tests were also performed on undisturbed soil samples for Slope 15. All these lab
investigations provided further information for the slope study.
Based on the results of field and lab investigation, each slope was analyzed to evaluate the
possible factors causing the slope failure or the potential slope instability using limit equilibrium
method. Slope stability analyses were performed for all the slopes. Probabilistic analysis was
also performed for Slope 15 (Sugar Creek Project) due to the relatively large amount of soil
parameters obtained. The computer program Slope/W (Geo-slope 2004) was used to perform all
the computations.
For the slopes, three types of soil shear strength parameter values were obtained, which were the
in-situ soil strength parameter values from the BST, residual shear strength parameter values
from ring shear test, and the possible mobilized shear strength parameter values at failure from
back-calculation. The possible failure surface was also estimated based on the failure features of
each slope together with back-calculations. This information should be useful when designing a
remediation measure using piles. For example, for a slope failure with relatively large
displacement, the residual shear strength can be considered along the failure surface, and the
piles need to be penetrate through the failure surface. More information can be found in Volume
III of the report.
11,12,13,14
1,2,3,4 5,6 15
8,9,10
Figure 11. Landform regions of Iowa (Prior 1991) and locations of the slopes investigated
18
19
General Information
The information on the area geology for each slope was obtained from the Soil Survey Reports
(USDA 1975, 1978, 1981a, 1981b, 1984). The details of history of the slopes were generally not
well documented. The relevant information was acquired through personal communications with
staff of Soils Design of IaDOT and the residents nearby the slopes.
BSTs and ring shear tests were performed according to the procedures as described in Chapter 2.
Direct shear tests were performed following ASTM (2002d) (D3080-03) (Standard test Method
for Direct Shear Test of Soils under Consolidated Drained Conditions) on undisturbed soil
samples under saturated conditions. A shearing rate of 0.02 mm/min was applied, which was
sufficiently low for the test to produce effective shear strength parameters. The triaxial tests were
performed in accordance with ASTM (2002e) (D4767-95) (Standard Test Method for
Consolidated Undrained Triaxial Compression Test for Cohesive Soils), and the unconfined
compression tests with ASTM (2002f) (D2166-00) (Standard Test Method for Unconfined
Compressive Strength of Cohesive Soil).
The grain size distributions and Atterberg limits of the soil samples were determined following
ASTM (2002a) (D422-63) (Standard Test Method for Particle-Size Analysis of Soils) and ASTM
(2002b) (D4318-00) (Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index
of Soils), respectively. The soils were classified using methods in ASTM (2002c) (D2487-00)
(Standard Classification of Soils for Engineering Purposes (Unified Soil Classification System)).
Soil density was determined using small, relatively “undisturbed” soil samples when obtained
from hand auger. Densities were taken for a number of samples and the average value was taken
as the representative density value.
X-ray diffraction (XRD) analysis is commonly used to determine the composition of a material.
Detailed information about XRD can be found in many literatures such as Cullity (1978) and
Moore and Reynolds (1997). In this study, XRD analyses were conducted to investigate the soil
clay mineralogy. Random oriented bulk sample from air dry soil was used in the test, and the
corresponding x-ray diffractogram was generated. The minerals identified are summarized at the
bottom of a diffractogram.
For the slope analyses, Morgenstern and Price (1965) method and Bishop (1955) simplified
method were adopted due to their popularity and familiary. The factors of safety (FS) obtained
from these two methods were found to be essentially same as can be seen in the results. The
computations of the slope analysis were performed using the computer program SLOPE/W
(Geo-slope 2004). FS was calculated on different slip surfaces and the minimum FS was
determined. Three types of slip surface could be searched or defined in SLOPE/W, which
included circular, block specified and fully specified slip surface. Circular search of slip surface
was performed by the program with the range of the center and radius for the slip circle being
defined. A block-specified slip surface consisted of several line segments defined by two grids of
intersection points. Slip surfaces were created by connecting each point in the left block with
each point in the right block, and then projecting each point to the surface at specified angles.
This type of slip surface was suitable to “guide” the slip surface passing through a specified soil
range. The fully specified slip surface was most suitable for a known or observed slip surface.
20
Site Conditions
Location
The slope is a fill slope and is located at the north side of Highway 34 MP169.3, Monroe County
(Figure 12).
History
The exact time of failure was unknown. Failure or deformation may mostly have occurred during
2000-2003. No evidence of fresh movement was found when it was investigated in July 2004.
Area Geology
According to the USDA (1984) Soil Survey Report, most of the soils in Monroe County formed
in loess, glacial till, or alluvium. A few of the soils formed in colluvium, eolian sand or shale
residuum. The major Pleistocene deposits are glacial tills ranging from 0 to more than 90m in
thickness. Shale residuum is the oldest parent material in the county. The shale consists of a
series of beds deposited during the Des Moines sedimentary cycle in the Pennsylvanian period.
These beds include shale of different colors and textures, conglomerates, and a few organic
layers such as layers of coal.
Field Investigations
Slope Geometry
The slope (Figure 13) had an overall sloping angle of about 17 degree (H:V = 3.3:1), a maximum
length of 30 m and a maximum height of 9 m (Figure 14). The width of the slope was about 20 m
(along the highway). The top of the slope had filled with a strip of stones as remediation
measures when the slope was investigated. The strip was about 2 m wide and extended along the
slope beside the highway. The depth of the stone fill was unknown. There also existed a gentle
hump at the mid surface of the slope indication the failure of the slope. A slope profile that is
perpendicular to the highway indicating the slip direction was developed (Figure 14)
Site Geology
A 3.0 m deep borehole was drilled manually in the slope. The borehole revealed that the slope
was formed with mixture of backfilled light grey clay shale and brown glacial till. The soils were
generally soft to medium stiff. The boring log is shown in Appendix (Figure A1).
21
Lab Investigations
Basic Properties
Basic properties for representative soil samples were investigated and the results are summarized
in Table 2. The results show that both the shale and glacial till samples have very low sand
content of less than 5%, clay content of less than about 50% and liquid limit around 40%. All the
soils are classified as low plasticity clay (CL) by USCS.
Slope Analysis
Soil Properties
The field and lab test results show that the slope mainly consists of low plasticity clay shale and
glacial till. Based on the field investigation, it appeared that the soils were mixed and could not
be sorted into clear layers. Therefore, a uniform slope was assumed for slope stability analysis. A
unit weight of 18.0 kN/m3 as determined in lab was used.
22
values obtained from ring shear test. The back-calculated shear strength values for FS = 1.0 was
essentially the same as the shear strength parameter values from BST. These results suggested
that the slope was most likely unstable when it was investigated as FS was close to 1.0 based on
the information from BST. The instability was further suggested by the repair work (rip-rap) at
the top of the slope. The instability was due to the relatively low shear strength of the soil as
exhibited by the clay shale; and also possibly due to the relatively high ground water level.
Conclusions
BST was used to obtain the shear strength parameter values of the soil in the slope. The shale in
the slope had a relatively low shear strength with φ’ = 10o and c’ = 8 kPa. These values together
with the measured ground water condition were used to evaluate the stability of the slope. The
factor of safety was found to be close to 1.0, and the potential slip surface was circular passing
through near the top of the slope where failure zone already existed. The analyses indicated the
slope was close to an unstable state when it was investigated, and this was probably mainly due
to the low shear strength of the shale measured and the relatively high ground water table in the
slope.
Slope 1
23
(a) Looking east, showing the rip-rap (photo taken by Thompson, 10/27/03)
(c) Looking east, upward, close view of the slope (photo taken by Yang, 04/01/05)
Figure 13. Photographs for slope 1
24
0 5 10 15 20 25 30 35 40 45 50
0
BH1
Stone fill
Height (m)
-5
-10
-15
Distance (m) N
Figure 14. Cross-section for slope 1
60
h = 2.1m o
(φ' = 10 , c' = 8 kPa)
o
h = 2.1m (Residual) (φ' = 6 , c' = 12 kPa
Shear stress (kPa)
40
y = 0.184x + 8.047
2
R = 0.995
20
y = 0.10x + 11.99
2
R = 0.99
0
0 20 40 60 80 100 120 140 160
Normal stress (kPa)
25
150
Glacial till, h = 2.4m
125 o
φ r' = 22.1 , cr' = 3.2 kPa
Shear stress, τ (kPa)
100
75
50 y = 0.4058x + 3.2022
2
R =1
25
0
0 50 100 150 200 250
300
Normal stress, σ' (kPa)
Figure 16. Ring shear test results for the glacial till in slope 1
1.005
-5
height (m)
-10
-15
-20
0 5 10 15 20 25 30 35 40 45
50
Distance (m)
26
Site Conditions
Location
The slope is a cut slope and is located at the north side of Highway 34 MP171.7, three miles west
of Albia, Monroe County (Figure 18).
History
The exact time of failure was unknown. Failure or deformation may have occurred prior to 2001
based on discussion with a nearby resident. The failure features of scarp and hump of the slide
appeared quite old when it was investigated in July 2004.
Area Geology
According to the USDA (1984) Soil Survey Report, most of the soils in Monroe County formed
in loess, glacial till, or alluvium. A few of the soils formed in colluvium, eolian sand or shale
residuum. The major Pleistocene deposits are glacial tills ranging from 0 to more than 90m in
thickness. Shale residuum is the oldest parent material in the county. The shale consists of a
series of beds deposited during the Des Moines sedimentary cycle in the Pennsylvanian period.
These beds include shale of different colors and textures, conglomerates, and a few organic
layers such as layers of coal.
Field Investigations
Slope Geometry
The slope (Figure 19) had an overall sloping angle of about 11 degrees (H:V = 5.2:1), a
maximum length of 40 m and a maximum height of 8m (Figure 20). It had a curved scarp near
27
the top with a maximum height of 1.5 m. The scarp extended along the two wings of the slope
and ended at the toe of the slope. The width of the slope (at the toe) was about 70 m (along the
highway). There were a few small humps at the surface of the slope. There was also a small ditch
located at the toe of slope. The ditch was parallel to the highway.
Site Geology
A total of four boreholes were drilled manually along the maximum length of the slope, and the
direction of the profile is perpendicular to the highway (Figure 20). The boreholes showed that
the slope was covered with about 0.15 m thick topsoil underlain with brown to grey, highly
weathered shales. The shales were generally medium stiff to stiff, with the lower portion being
soft to medium stiff. A thin layer of coal was found near surface in BH4 (near the toe of the
slope). The boring logs are shown in Appendix (Figures A3 to A6).
Lab Investigations
Basic Properties
Basic properties for representative soil samples were investigated and the results are summarized
in Table 5. The results show that all the shale samples have very low sand content of less than
5%, high clay content of about 50% and liquid limit larger than 50%. All the shales are classified
as high plasticity clay (CH) by USCS.
28
Slope Analysis
Soil Properties
The field and lab test results show that the slope mainly consists of high plasticity weathered clay
shales. Based on the field visual inspections and BST results, the shales are interpreted as three
layers as shown in the Figure 20, with the Layer 2 being relatively weak. Slope stability analysis
is performed based on this interpretation, and the soil properties used in the analysis are shown in
Table 6.
All these results suggest that the slope may have most likely failed under the conditions as in
Analysis 12 in Table 7, i.e. the slope failure took place under a high GWT that was located near
the surface. In this situation, the shear strength of the weak Layer 2 developed or mobilized has
similar φ’ value as measure from BST but with zero c’ value. The BST results may represent the
peak shear strength of the shale; and the back-calculated results, which has same φ’ with that of
BST and near zero c’, indicated the softened shear strength (or mobilized shear strength) of the
shale during the slope failure (Figure 24). The residual shear strength as obtained from the ring
shear test was the ultimate shear strength corresponding to a slide with relatively large
displacement.
Conclusions
BSTs were used to characterize the slope. An underlying relatively weak shale layer was
detected. The shear strength parameter values obtained from BST for each soil layer were used
for the slope analyses to investigate the possible causes of the failure.
29
The slope most likely failed under near surface GWT conditions with the slip surface passing
through the relatively weak shale layer. The weak shale layer has minimum peak shear strength
parameter values of φ' = 11o and c’ = 13 kPa as measured by BST. It has softened shear strength
parameter values of φ' = 10.85o and c’ = 0 kPa during the slide mobilization as obtained-from
back-calculation; and residual shear strength parameter values of φ' = 6.8o and c’ = 1.6 kPa as
indicated by ring shear test. The different shear strength parameter values together with the
location of the slip surface can be considered when slope remediation design is considered.
Slope 2
30
(a) Looking north, overview of the slope (photo taken by Yang, 04/01/05)
(b) Looking north, showing the scarp (photo taken by Thompson, 10/27/03)
(c) Looking east, showing the ditch at the toe of the slope (photo taken by Thompson, 10/27/03)
Figure 19. Photographs for slope 2
31
0 5 10 15 20 25 30 35 40 45 50 55
0
BH1
Layer1
BH2
BH3
Height (m)
-5 Layer2 Hwy34
BH4
Layer 3 ?
-10
-15
N Distance (m) S
80
BH1 (2.3m)
BH1 (2.9m)
BH1 (3.8m)
BH2 (2.0m)
60 BH2 (2.6m)
BH2 (3.2m)
Shear stress (kPa)
BH3 (2.3m)
BH3 (2.6m)
BH4 (1.1m)
40
20
0
0 20 40 60 80 100 120
150
Test 1, BH2, 3.2m (φ'r =6.8o, cr'= 1.6 kPa, R2=0.9999)
Shear stress (kPa)
50
0
0 50 100 150 200 250 300 350 400
32
Shale
-10
Slip surface
-15
-20
0 5 10 15 20 25 30 35 40 45 50 55
Distance (m)
70
Layer 1 (Average)
BH2 (3.2m), BST
60 BH2 (3.2m), Ring shear
Back-calculated (Analysis 3)
Back-calculated (Analysis 6)
50 Back-calculated (Analysis 9)
Shear stress (kPa)
40
30
20
10
0
0 20 40 60 80 100 120
Normal stress (kPa)
Figure 24. Shear strength parameter values for the weak shale layer (layer 2) in slope 2
φ' (deg.)
'
BH Depth (m) c (kPa) R2 Data points
1 2.3 17 18 0.989 5
1 2.9 27 11 0.954 5
1 3.8 31 11 0.998 5
2 2.0 23 22 0.997 4
2 2.6 40 7 0.996 5
2 3.2 11 13 0.999 4
3 2.3 18 19 0.996 4
3 2.6 25 5 0.995 4
4 1.1 21 7 1.000 4
33
Table 6. Soil Properties used for the slope analysis for slope 2
Layer Soil Unit weight (kN/m3) φ' (deg.) c' (kPa) Remark
1 Shale 19.0 25 12 Average
2 Shale 19.0 Varied Varied
3 Shale 22.0 30 100 Assumed
34
Site Conditions
Location
The slope is a fill slope and is located at the south side of Highway 34 MP175.3, Monroe County
(Figure 25).
History
The exact time of failure was unknown. The failure features of scarp and hump of the slide
appeared quite old when it was investigated in July 2004.
Area Geology
According to the USDA (1984) Soil Survey Report, most of the soils in Monroe County formed
in loess, glacial till, or alluvium. A few of the soils formed in colluvium, eolian sand or shale
residuum. The major Pleistocene deposits are glacial tills ranging from 0 to more than 90m in
thickness. Shale residuum is the oldest parent material in the county. The shale consists of a
series of beds deposited during the Des Moines sedimentary cycle in the Pennsylvanian period.
These beds include shale of different colors and textures, conglomerates, and a few organic
layers such as layers of coal.
Field Investigations
Slope Geometry
The slope (Figure 26) and had an overall sloping angle of about 22 degree (H:V = 2.5:1), a
maximum length of 16 m and a maximum height of 6m (Figure 27). The width of the slope (at
the toe) was about 20 m (along the highway). It had a scarp near the top with a maximum height
of 0.8 m. There was a hump at the surface of the slope. There was also a small ditch located at
the toe of slope.
Site Geology
One borehole was drilled manually on the slope (Figure 27). The borehole showed that the slope
was composed of yellowish brown glacial till which was generally soft to medium stiff. The
boring logs are shown in Appendix (Figure A8).
35
Lab Investigations
Basic Properties
Basic properties for representative soil sample were investigated and the results are summarized
in Table 9. The results showed that the glacial till sample comprised 32% sand, 29% silt and
39% of clay. Its liquid limit was smaller than 50%. The soil was classified as low plasticity clay
(CL) by USCS.
Slope Analysis
Soil Properties
The field and lab test results show that the slope mainly consists of low plasticity glacial tills.
Based on the field visual inspections and BST results, the soil was interpreted as two layers of
different shear strengths as shown in the Figure 27. Slope stability analysis was performed based
on this interpretation, and the soil properties used in the analysis are shown in Table 10.
The results suggest that the slope may have most likely failed under the conditions as in the
Analysis 3 (back-calculation). The back-calculated shear strength parameter values indicated the
possible average mobilized (or softened) shear strength during the slope failure. Also, the BST
results in Table 11 may represent the peak shear strength of the till. The residual shear strength
36
as obtained from the ring shear test was the ultimate shear strength corresponding to a slide with
relatively large displacement. Once the slope movement was initiated, the shear strength of the
soil started to drop from the peak value to the softened value, and may eventually drop to the
residual value if the displacement was sufficiently large.
Conclusions
BST was used to investigate the slope and the shear strength parameter values were used for the
slope analysis. The slope most likely failed with a circular slip surface passing the observed
scarp. The relatively weak top layer has an average peak shear strength values of φ' = 200 and c’ =
21 kPa as measured by BST. It has a softened (mobilized) shear strength values of φ' = 120 and c’
= 2 kPa during the slide mobilization; and a residual shear strength of φr' = 10.50 and cr’ = 1.0
kPa.
Slope 3
37
(a) Looking northwest, overview of the slope (photo taken by Yang, 07/18/04)
(b) Looking northwest, close view of the bulge and the scrap (photo taken by Yang, 04/01/05)
Figure 26. Photographs for slope 3
38
0 Hwy34 4 8 12 16 20 24 28
0
Layer 1 BH1
Height (m)
-4
Layer 2
-8
-12
Distance (m) S
80
h = 0.6m
60
h = 1.2m
h = 1.8m
Shear stress (kPa)
h = 2.4m
40
20
0
0 20 40 60 80
Normal stress (kPa)
39
50
Glacial till, h = 0.6m
Shear stress, τ (kPa)
25 y = 0.1858x + 1.0333
R2 = 0.9996
0
0 50 100 150 200
Normal stress, σ' (kPa)
1.003
-5
Height (m)
-10
-15
0 5 10 15 20 25 30
Distance (m)
40
Table 10. Soil properties used for the slope analysis for slope 3
Layer Unit weight φ' (deg.) c' (kPa) Remarks
1 19.0 20 21 Depends on analysis
2 21.0 39 14
41
Site Conditions
Location
The slope is a fill slope and is located at the north side of Highway 34, MP175.5, Monroe
County (Figure 31).
History
There was no apparent evidence of failure for the slope when it was investigated in July 2004.
The purpose of the investigation was to find out what were the conditions for a typical stable
slope along Hwy 34.
Area Geology
According to the USDA (1984) Soil Survey Report, most of the soils in Monroe County formed
in loess, glacial till, or alluvium. A few of the soils formed in colluvium, eolian sand or shale
residuum. The major Pleistocene deposits are glacial tills ranging from 0 to more than 90m in
thickness. Shale residuum is the oldest parent material in the county. The shale consists of a
series of beds deposited during the Des Moines sedimentary cycle in the Pennsylvanian period.
These beds include shale of different colors and textures, conglomerates, and a few organic
layers such as layers of coal.
Field Investigations
Slope Geometry
The slope (Figure 32) had an overall sloping angle of about 22 degrees (H:V = 2.5:1), a
maximum length of 18 m and a maximum height of 7 m (Figure 33). There was a small hump
near the toe of the slope. There was also a small ditch located at the toe of slope which was
parallel to the highway.
Site Geology
One borehole was drilled manually on the slope (Figure 33). The borehole showed that the slope
composed of fill of brown glacial till which was soft to medium stiff. The boring log is shown in
Appendix (Figures A10).
42
Lab Investigations
Basic Properties
Basic properties for representative soil sample were investigated and the results are summarized
in Table 12. The results show that all the soil samples have a relatively low clay content of less
than 40%, and liquid limit is less than 50%. All the glacial tills were classified as low plasticity
clay (CL) by USCS.
Slope Analysis
Soil Properties
The field and lab test results show that the slope mainly consists of low plasticity clay of glacial
till. Based on the field visual inspections and BST results, the soil in the slop is assumed to be
uniform for slope analysis.
The results indicate that the slope was stable under the conditions when it was investigated. The
shear strength parameter values from back-calculation indicate the average shear strength
parameter values that need to be mobilized if the slope movement is initiated. Also, the BST
results in Table 13 may represent the peak shear strength of the shale. The residual shear strength
as obtained from the ring shear test was the ultimate shear strength corresponding to a slide with
43
Conclusions
BST was used to characterize the slope which did not fail when it was investigated. The shear
strength parameter values obtained from BST were used for the slope analysis to investigate the
factor of safety of the slope. The FS was found to be 1.36 for the slope, which had a circular
potential slip surface passing the top and the toe of the slope. The soil had peak shear strength
values of φ' = 180 and c’ = 9 kPa as measured by BST. The average shear strength values of the
soil will be φ' = 180 and c’ = 3.5 kPa in order to mobilize the slope. The residual shear strengths
of soil were φr' = 10.10 and cr’ = 2.3 kPa.
Slope 4
44
0 4 8 12 16 20 24 28
Hwy34 BH1
0
Height (m)
-4
-8
-12
Distance (m) N
80
o
h = 1.5 m (φ' = 18 , c' = 9 kPa)
o
h = 1.5 (Residual) (φ' = 9 , c' = 6 kPa)
Shear stress (kPa)
60
y = 0.3284x + 9.2164
40 2
R = 0.9967
20
y = 0.1613x + 5.8221
2
R = 0.9934
0
0 20 40 60 80 100 120 140 160
Normal stress (kPa)
45
50
Glacial till, h = 1.5 m
o
40 φr' = 10.1 , c r' = 2.3 kPa
.
Shear stress, τ (kPa)
30
20
y = 0.179x + 2.3043
10 R2 = 0.9985
0
0 50 100 150 200
Normal stress, σ' (kPa)
1.366
-5
Height (m)
-10
-15
0 5 10 15 20 25
Distance (m)
46
Analysis
Shear Strength Factor of Safety
No.
φ' c'
Source M-P Bishop
(deg.) (kPa)
47
Site Conditions
Location
The slope is a fill slope and is located at the north side of Highway 34 MP178.3, Wapello County
(Figure 37).
History
The exact time of failure was unknown. The failure features of scarp and hump of the slide
appeared quite old when it was investigated in July 2004.
Area Geology
According to the USDA (1981b) Soil Survey Report, most of soils formed in glacial till, loess
and alluvium. Clayey shale is the oldest parent material forming the bedrock of the project site.
The bedrock surface closely parallels to the existing ground surface.
Field Investigations
Slope Geometry
The slope (Figure 38) had an overall sloping angle of about 18 degree (H:V = 3.0:1), a maximum
length of 20 m and a maximum height of 7 m (Figure 39). The width of the slope (at the toe) was
about 25 m (along the highway). The slope had a scarp near the top with a maximum height of
0.8 m. It had a 2 m wide subsidence filled with stones (rip-rap) at top in some area of the slope.
There also small hump near the toe. A small ditch was located at the toe of slope.
Site Geology
One borehole was drilled manually on the slope (Figure 39). The borehole showed that the slope
was filled with yellowish brown glacial till which was generally soft to medium stiff. The boring
log is shown in the Appendix (Figure A12).
48
Lab Investigations
Basic Properties
Basic properties for representative soil sample were investigated and the results are summarized
in Table 15. The results showed that the glacial till sample comprised 16% sand, 47% silt and
37% of clay. Its liquid limit was smaller than 50%. The soil was classified as low plasticity clay
(CL) by USCS.
Slope Analysis
Soil Properties
The field and lab test results show that the slope mainly consists of fill of low plasticity glacial
till. Based on the field visual inspections and BST results, the soil was assumed to be uniform for
the slope stability analysis.
All these results suggest that the slope may have most likely failed under the conditions as in
Analysis 3 (the back-calculation). The shear strength parameter values from back-calculation
indicate the average mobilized shear strength during the slope failure. Also, the BST results in
Table 16 may represent the average peak shear strength of the shale. The residual shear strength
as obtained from the ring shear test was the ultimate shear strength corresponding to a slide with
relatively large displacement. Thus the actual failure condition would appear to be a softened
condition.
49
Conclusions
BST was used to characterize the slope. The shear strength parameter values obtained from BST
were used for the slope analysis to investigate the possible cause of the failure. The slope most
likely failed with a circular slip surface passing the scarp. The soil has a peak shear strength
values of φ' = 160 and c’ = 25 kPa as measured by BST. It has a softened (mobilized) shear
strength values of φ' = 13.20 and c’ = 2.7 kPa during the slide mobilization; and a residual shear
strength of φr' = 8.40 and cr’ = 2.7 kPa.
Slope 5
50
(a) Looking south, showing the scarp of the slope (photo taken by Thompson, 06/04/04)
(b) Looking southwest, showing field investigation (photo taken by Thompson, 06/04/04)
Figure 38. Photographs for slope 5
51
0 Hwy34 4 8 12 16 20 24 28
0
BH
1
Height (m)
-4
-8
-12
Distance (m) N
60
50 h = 0.6m
h =1.2m
Shear stress (kPa)
40 h = 1.8m
h = 2.4m
30
20
10
0
0 20 40 60 80 100
Normal stress (kPa)
50
40
φr' = 8.4o, c r' = 2.7 kPa
30
20
y = 0.1478x + 2.6994
10
R2 = 0.9993
0
0 50 100 150 200
Normal stress, σ' (kPa)
52
1.818
-5
Height (m)
-10
-15
0 5 10 15 20 25 30
Distance (m)
53
Slope 6 (Hwy34 Mp175.3s)
Site Conditions
Location
The slope is a full slope and is located beside Highway 34 MP178.3 (south side), Wapello
County (Figure 43).
History
The exact time of failure was unknown. The failure features of scarp and hump of the slide
appeared quite old when it was investigated in July 2004.
Area Geology
According to the USDA (1981b) Soil Survey Report, most of soils formed in glacial till, loess
and alluvium. Clayey shale is the oldest parent material forming the bedrock of the project site.
The bedrock surface closely parallels to the existing ground surface.
Field Investigations
Slope Geometry
The slope (Figure 44) had an overall sloping angle of about 14 degrees (H:V = 4.0:1), a
maximum length of 22 m and a maximum height of 6 m (Figure 45). The width of the slope (at
the toe) is about 30 m (along the highway). It had a scarp near the top with a maximum height of
1.0 m. There was a hump at the surface of the mid of the slope.
Site Geology
One borehole was drilled manually on the slope (Figure 45). The borehole showed that the slope
was backfilled with grey and brown shale which was generally soft to medium stiff. The boring
log is shown in Appendix (Figure A14).
54
Lab Investigations
Basic Properties
Basic properties for a representative soil sample were investigated and the results are
summarized in Table 17. The results showed that the shale sample comprised 18% sand, 43% silt
and 39% of clay. Its liquid limit was smaller than 50%. The soil was classified as low plasticity
clay (CL) by USCS.
Slope Analysis
Soil Properties
The field and lab test results show that the slope mainly consists of low plasticity clay shales.
Based on the field visual inspections and BST results, the soil was assumed to be uniform for the
slope stability analysis.
The results suggest that the slope may have most likely failed under the conditions as in back-
calculation (Analysis 3). The shear strength parameter values from back-calculation indicated the
average mobilized shear strength during slope failure. Also, the BST results in Table 18 may
represent the peak shear strength of the shale. The residual shear strength as obtained from the
ring shear test was the ultimate shear strength corresponding to a slide with relatively large
displacement. Thus the actual failure conditions would appear to be a softened condition.
55
Conclusions
BST was used to characterize the slope. The shear strength parameter values obtained from BST
were used for the slope analysis to investigate the possible cause of the failure. The slope most
likely failed with a circular slip surface passing the failure zone near the top of the slope. The
soil in the slope had a peak shear strength values of φ' = 160 and c’ = 25 kPa as measured by
BST. It had a softened (mobilized) shear strength values of φ' = 160 and c’ = 1.1 kPa during the
slide mobilization; and a residual shear strength of φr' = 9.30 and cr’ = 1.1 kPa.
Slope 6
56
(a) Looking northeast, showing the slope failure at top (photo taken by Thompson, 06/04/04)
57
0 4 8 12 16 20 24 28 32
Hwy34
0
BH1
Elevation (m) -4
-8
-12
Distance (m) S
60
Shale, h = 1.8 m
o
Shear stress (kPa)
y = 0.283x + 24.461
2
R = 0.990
20
0
0 20 40 60 80 100
Normal stress (kPa)
50
Shale, h = 0.3 m
Shear stress, τ (kPa)
40 o
φ r' = 9.3 , cr' = 1.1 kPa
30
20
y = 0.1638x + 1.0504
2
10 R = 0.9998
0
0 50 100 150 200
Normal stress, σ' (kPa)
58
3.354
-5
Height (m)
-10
-15
-20
0 5 10 15 20 25 30
Distance (m)
59
Site Conditions
Location
The slope is a cut slope and is located at the west side of Highwy 169, about 3 miles north of
Winterset, Madison County (Figure 49).
History
The slope started to move during 2003, and failed in 2004 thus brought IaDOT's attention. The
scarp of the slide appeared to be newly formed when the slide was first investigated in June
2004. The slope was repaired around November 2004 by cutting.
Area Geology
According to the USDA (1975) Soil Survey Report, the soils of Madison County formed from
loess, glacial till, alluvium, shale, limestone and sandstone. Near Winterset, the soils mainly
formed in moderately well drained loess and glacial till, and slowly permeable shales.
Field Investigations
Slope Geometry
The slope (Figure 50) had an overall sloping angle of about 13 degree (H:V = 4.4:1), a maximum
length of 33 m and a maximum height of 7 m (Figure 51). It has a nearly straight, steep scarp
near the top with a maximum height of 1.7 m. The scarp extends along the side of highway for
about 70 m, which is the maximum the width of the slide. There are a few transverse cracks at
the mid of the slope and a small hump near the toe of the slope.
Site Geology
A total of four boreholes were drilled following the sliding direction (perpendicular to the
highway) of the slope to establish a representative profile for the slope (Figure 51). Two of the
boreholes (BH1 and BH2) were drilled with a rotary drill rig, and two (BH3 and BH4) were
drilled manually using a hand auger. The boreholes show that the slope is covered with about 2
m thick brown silty clay overlying clay shales. The shales are divided into three layers based on
the field visual classifications and the in-situ shear strength of the soil as measured by BST. The
boring logs are shown in the Appendix (Figures A16 to A19).
60
Lab Investigations
Basic Properties
Basic properties for representative soil samples were investigated and the results are summarized
in Table 20. The results show that all the shale samples have very low sand content of less than
3%. The soils have relatively high clay content of about 35% and liquid limit larger than 50%.
All the shales are classified as high plasticity clay (CH) by USCS.
Slope Analysis
Soil Properties
The field and lab test results show that the slope mainly consists of high plastic weathered clayey
shales. Based on the field visual classifications and BST results, the soils were interpreted as four
layers with different shear strength as shown in the Figure 51, with the Layer 2 exhibiting
relatively low shear strength. Slope stability analysis was performed based on this interpretation
of slope profile, and the soil properties used in the analysis were listed in Table 22.
61
lower than the residual shear strength values (Analyses 3, 6, 9 and 12) indicating the average
shear strength along the slip surface mobilized during the failure of the slope is lower than the
measured residual shear strength. This due to two possible reasons: the actual residual shear
strength may be lower than what has been measured using ring shear test due to the spatial soil
variation; the slip surface may not be exactly the one assumed in the analysis.
The Analysis 13, which assumed uniform soils (applying uniform shear strength parameter
values for the slope) and circular slip surface passing near the scarp, gave a back-calculated shear
strength parameter values that is exactly the same as the lower values of residual shear strength
parameter as obtained from the ring shear tests. The ground water level condition was low for
this analysis.
Based on slope analysis results, the slope most likely failed under a high GWT condition with a
relatively low mobilized shear strength (Analysis 12) and flat slip surface; or failed under a low
GWT with a mobilized shear strength close to the residual shear strength measured from ring
shear test and with slip surface that is more close to circular shape (Analysis 13). The uncertainty
of the back-calculated shear strength was due to the conditions involved exactly during the slope
failure which were not observed. The BST results may represent the peak shear strength of the
shale; and the back-calculated results indicate the probable softened shear strength (or mobilized
shear) of the shale during slope failure (Figure 7). The residual shear strength as obtained from
ring shear test is the ultimate shear strength corresponding to a slide with relatively large
displacement.
Conclusions
BST was used to characterize the slope and gave shear strength parameter values for each soil
layer. These values were used for the slope analysis considering various possible geometry and
ground water table conditions. The slope most likely failed under the near surface GWT
conditions with a flat slip surface passing through the relatively weak shale layer. The weak
shale layer has peak shear strength parameter values of φ' = 18o and c’ = 20 kPa as measured by
BST, and residual strength parameter values of φ' = 12.0o and c’ = 3.5 kPa as measured by ring
shear test. The average shear strength mobilized during the slope failure was estimated to be
close to the residual shear strength assuming a circular slip surface.
62
Slope 7
Figure 49. Location for slope 7 (Hwy 169, Winterset, Madison Co.)
63
(a) Looking south, showing the scarp and graben of the slope (photo taken by Thompson,
05/26/04)
(b) Looking north, showing the overview of the slope (photo taken by Thompson, 05/26/04)
(c) Looking northeast, showing the scarp of the slope (photo taken by Thompson, 05/26/04)
Figure 50. Photographs for slope 7
64
0 5 10 15 20 25 30 35 40 45 50 55
0
Hwy169 BH1
Soil layer 1 BH2
BH3
2
height (m)
-5 3
BH4
4
-10
-15
E Distance (m) W
80
60
40
BH,depth,φ'(deg.),c'(kPa),R2
BH1 (2.4m) (27, 45, 0.926)
BH2 (2.7m) (18, 16, 0.982)
BH3 (1.5m) (21, 24, 0.982)
BH3 (2.7m) (19, 24, 0.993)
20
BH3 (3.4m) (34, 15, 0.987)
BH3 (3.7m) (35, 11, 0.999)
BH4 (1.0m) (23, 33, 0.951)
BH4 (1.3m) (25, 32, 0.996)
BH4 (1.7m) (18, 17, 0.997)
0
0 20 40 60 80 100 120
150
50
0
0 50 100 150 200 250 300 350 400
Normal stress (kPa)
65
Original surface
Slip surface
0 BH1 Current surface
BH2
Layer 1 BH3
Layer 2
-5 Layer 3
BH4
Height (m)
Layer 4
-10
-15
Layer 5
-20
0 5 10 15 20 25 30 35 40 45 50 55
Distance (m)
0.999
-5
Height (m)
-10
-15
-20
0 5 10 15 20 25 30 35 40 45 50 55
Distance (m)
Figure 55. Analysis 13 assuming homogeneous soils and circular slip surface for slope 7
66
70
BST Average
BH2, 2.7m, Ring shear test 2
60 BH3, 2.7m, Ring shear test 2
Analysis 3
Shear stress (kPa) 50 Analysis 6
Analysis 9
Analysis 12
40 Analysis 13
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100 110
Normal stress (kPa)
Figure 56. Different shear strength for the weak shale layer 2 in the slope 7
'
BH Depth (m) φ' (deg.) c (kPa) R2 Data points
1 2.4 27 45 0.926 4
2 2.7 18 16 0.982 5
3 1.5 21 24 0.982 5
3 2.7 19 24 0.993 4
3 3.4 34 15 0.987 5
3 3.7 35 11 0.999 4
4 1.0 23 33 0.951 5
4 1.3 25 32 0.996 5
4 1.7 18 17 0.997 4
67
Table 22. Soil properties used for the slope analysis for slope 7
Unit weight
3 φ' (deg.) c' (kPa)
Layer (kN/m ) Remarks
1 18.0 24 35 Average
2 19.0 (18) (20) Various
3 19.5 29 23 Average
4 21.0 35 100 Assumed
68
Site Conditions
Location
The slope is a cut slope and is located at the east side of Highway 169, 2 miles south of Afton,
Union County (Figure 57).
History
The time of the slope failure was unknown. The scarp and the hump generally appeared old in
August 2004 when the slope was investigated.
Area Geology
According to the USDA (1978) Soil Survey Report, most of the soils in Union County formed
from loess, glacial till, alluvium and shale. Glacial tills dominate the area near Afton, ranging to
a depth of 30 m or more.
Field Investigations
Slope Geometry
The slope (Figure 58) had an overall sloping angle of about 22 degree (H:V = 2.5:1), a maximum
length of 27 m and a maximum height of 10 m (Figure 59). The width of the slope (at the toe) is
about 60 m (along the highway). It had a scarp near the middle of the slope surface with a
maximum height of 0.5 m. There was a hump near the toe of the slope. There was also a 2 m
wide shallow ditch located at the toe of slope.
Site Geology
Two boreholes were drilled manually on the slope (Figure 59). The boreholes showed that the
slope was made of yellowish brown glacial till which was generally soft to medium stiff. The
boring logs are shown in Appendix (Figures A.20 and A.21).
69
Lab Investigations
Basic Properties
Basic properties for representative soil sample were investigated and the results were
summarized in Table 25. The results showed that the glacial till sample comprised 34% of sand,
37% of silt and 29% of clay. The liquid limit was lower than 50%. The soil was classified as low
plasticity clay (CL) by USCS.
Slope Analysis
Soil Properties
The field and lab test results show that the slope mainly consists of low plasticity glacial till.
Based on the field visual inspections and BST results, the soil was assumed to be uniform for the
slope stability analysis.
All these results suggest that the slope may have most likely failed under the conditions as in
Analysis 3. The shear strength parameter values from back-calculation indicated the average
mobilized shear strength during slope failure. Also, the BST results in Table 5 may represent the
peak shear strength of the shale. The residual shear strength as obtained from the ring shear test
was the ultimate shear strength corresponding to a slide with large displacement.
Conclusions
BST was used to characterize the slope. The shear strength parameter values obtained from BST
were used for the slope analysis to investigate the possible cause of the failure. The slope most
likely failed with a circular slip surface passing the top of the slope. The soil in the slope has an
average peak shear strength values of φ' = 260 and c’ = 20 kPa as measured by BST. It has a
70
softened (mobilized) shear strength values of φ' = 14.20 and c’ = 11.5 kPa during the slide
mobilization; and a residual shear strength of φr' = 14.20 and c’ = 0 kPa.
Slope 8
Figure 57. Location of slope 8 (Hwy 169, 2 miles south of Afton, Union Co.)
71
(a) Looking east, showing the overview of the slope (photo taken by Thompson, 06/04/04)
(b) Looking east, showing the bulge and scarp of the slope (photo taken by Thompson, 06/04/04)
(c) Looking southwest, showing the overview of the slope (photo taken by Thompson, 06/04/04)
Figure 58. Photographs for slope 8
72
0 5 10 15 20 25 30 35 40 45 50
0
Hwy169
BH2
-10
-15
Distance (m) W
80
60
Shear stress (kPa)
40
20 BH1, 1.8m
BH2, 1.2m
BH2, 1.8m
Bh2, 2.4m
0
0 20 40 60 80 100
Normal stress (kPa)
100
Glacial till, BH1, h = 1.8m
Shear stress, τ (kPa)
60
40
20 y = 0.2527x - 0.0999
2
R =1
0
0 100 200 300
Normal stress, σ' (kPa)
73
1.846
-5
Height (m)
-10
-15
-20
0 5 10 15 20 25 30 35 40 45 50
Distance (m)
74
Site Conditions
Location
The slope is a cut slope and is located at the west side of Highway 169, 2 miles south of Afton,
Union County (Figure 63).
History
The exact time of failure of the slope was unknown. The scarp and the hump generally appeared
old in August 2004 when the slope was investigated.
Area Geology
According to the USDA (1978) Soil Survey Report, most of the soils in Union County formed
from loess, glacial till, alluvium and shale. Glacial tills dominate the area near Afton, ranging to
a depth of 30 m or more.
Field Investigations
Slope Geometry
The slope (Figure 64) had an overall sloping angle of about 23 degree (H:V = 2.4:1), a maximum
length of 33 m and a maximum height of 13 m (Figure 65). The width of the slope (at the toe) is
about 40 m (along the highway). There was a hump near the toe of the slope. There was also a
shallow ditch located at the toe of slope.
Site Geology
One borehole was drilled manually on the slope (Figure 65). The borehole showed that the slope
was composed of 0.6 m thick brown glacial till overlying grey or brown shale. The soils were
generally soft to medium stiff. The boring log is shown in the Appendix (Figures A22).
75
Lab Investigations
Basic Properties
Basic properties for representative soil sample were investigated and the results are summarized
in Table 27. The results showed that the shale sample at the depth of 1.8m comprised 17% of
sand, 41% of silt and 42% of clay. The liquid limit was higher than 50%. The shale was
classified as high plasticity clay (CH) by USCS.
Slope Analysis
Soil Properties
The field and lab test results show that the slope mainly consists of a thin layer of low plasticity
glacial till underlain with high plasticity clay shale. Based on the field visual inspections and
BST results, the clay shale was assumed to be uniform for the slope. The glacial till is also
assumed to have same shear strength with the shale for simplicity. Slope stability analysis was
performed accordingly.
The results suggest that the slope may have most likely failed under the conditions as in Analysis
3 (back-calculation). The shear strength parameter values from back-calculation indicated the
average mobilized shear strength during slope failure. Also, the BST results in Table 28 may
represent the peak shear strength of the shale. The residual shear strength as obtained from the
76
ring shear test was the ultimate shear strength of the soil under large displacement.
Conclusions
BST was used to characterize the slope. The shear strength parameter values obtained from BST
for the soil were used the slope analysis to investigate the possible cause of the failure. The slope
most likely moved with a circular slip surface passing the top of the slope. The soil has an
average peak shear strength values of φ' = 210 and c’ = 20 kPa as measured by BST. It has a
softened (mobilized) shear strength values of φ' = 210 and c’ = 5.1 kPa during the slide
mobilization; and a residual shear strength of φr' = 7.10 and c’ = 1.6 kPa.
Slope 9
Figure 63. Location of slope 9 (Hwy 169, 2 miles south of Afton, Union Co.)
77
(a) Looking south, showing the overview of the slope (photo taken by Thompson, 06/04/04)
(b) Looking north, showing the overview of the slope (photo taken by Yang, 04/16/05)
Figure 64. Photographs for slope 9
78
0 5 10 15 20 25 30 35 40 45 50
-15
-20
Distance (m) E
80
h = 1.8m (φ' = 18o, c' = 20 kPa)
h = 2.4m (φ ' = 23o, c' = 20 kPa)
60
Shear stress (kPa)
y = 0.427x + 19.714
2
40 R = 0.985
y = 0.325x + 19.833
2
20 R = 0.992
0
0 20 40 60 80 100
50
Shale, BH1, h = 1.8m
Shear stress (kPa)
40
o
φr ' = 7.1 , cr' = 1.6 kPa
30
20
y = 0.124x + 1.6236
10
R2 = 0.9996
79
1.547
-5
Height (m)
-10
-15
-20
0 10 20 30 40 50
Distance (m)
80
Site Conditions
Location
The slope is a cut slope and is located at the east side of Highway 169, 4 miles south of Afton,
Union County (Figure 69).
History
The time of the slope failure was unknown. The scarp and the hump generally appeared old in
August 2004 when the slope was investigated.
Area Geology
According to the USDA (1978) Soil Survey Report, most of the soils in Union County formed
from loess, glacial till, alluvium and shale. Glacial tills dominate the area near Afton, ranging to
a depth of 30 m or more.
Field Investigations
Slope Geometry
The slope (Figure 70) had an overall sloping angle of about 20 degree (H:V = 2.8:1), a maximum
length of 21 m and a maximum height of 7 m (Figure 71). The width of the slope (at the toe) is
about 25 m (along the highway). There was a scarp at middle of the slope and a hump near the
toe of the slope. There was also a shallow ditch located at the toe of slope (beside the highway).
Site Geology
One borehole was drilled manually on the slope (Figure 71). The borehole showed that the slope
was composed of grey or brown clay shale. The soils were generally soft to medium stiff. The
boring log is shown in Appendix (Figure A.24).
81
Lab Investigations
Basic Properties
Basic properties for representative soil sample were investigated and the results are summarized
in Table 29. The results showed that the shale sample at the depth of 2.4 m comprised 10% of
sand, 53% of silt and 37% of clay. Its liquid limit was larger than 50%. The shale was classified
as high plasticity clay (CH) by USCS.
Slope Analysis
Soil Properties
The field and lab test results show that the slope mainly consists of high plasticity clay shale.
Based on the field visual inspections and BST results, the clay shale was assumed to be uniform
for the slope stability analysis.
The results suggest that the slope may have most likely failed under the conditions as in Analysis
3. The shear strength parameter values from back-calculation indicated the average mobilized
shear strength during slope failure. Also, the BST results in Table 30 may represent the peak
shear strength of the shale. The residual shear strength as obtained from the ring shear test was
the ultimate shear strength corresponding to a slide with large displacement.
Conclusions
BST was used to characterize the slope. The shear strength parameter values obtained from BST
the soil were used for the slope analysis to investigate the possible cause of the failure. The slope
most likely failed with a circular slip surface passing the scarp of the slope. The slope has an
82
average peak shear strength values of φ' = 110 and c’ = 8 kPa as measured by BST. It has a
softened (mobilized) shear strength values of φ' = 110 and c’ = 4.4 kPa during the slide
mobilization; and a residual shear strength of φr' = 10.60 and c’ = 3.4 kPa. The results indicated a
softening response of the slope movement.
Slope 10
Figure 69. Location of slope 10 (Hwy 169, 4 miles south of Afton, Union Co.)
83
(a) Looking east, showing the overview of the slope (photo taken by Yang, 08/11/04)
(b) Looking southeast, showing the overview of the slope (photo taken by Yang, 04/16/05)
Figure 70. Photographs for slope 10
84
0 5 10 15 20 25 30 35 40 45
-5 Hwy169
-10
-15
Distance (m) W
40
Shale, 2.4m
30
o
(φ' = 11 , c' = 8 kPa)
20
y = 0.194x + 7.989
10
2
R = 0.992
0 20 40 60 80 100 120
100
Shale, h = 2.4m
o
φ r' = 10.6 , cr' = 3.4 kPa
Shear stress (kPa)
50
y = 0.1873x + 3.3647
2
R = 0.9991
0
0 50 100 150 200 250 300
Normal stress (kPa)
85
1.674
-5
Height (m)
-10
-15
-20
0 5 10 15 20 25 30 35
Distance (m)
Figure 74. Slope stability analysis for slope 10
86
Site Conditions
Location
The slope is a cut slope that is located at the south side of Highway E57, 0.5 mile west of Des
Moines River, 4.5 miles west of Luther, Boone County (Figure 75). This slope is connected with
Slope 12, and is very close to Slopes 13 and 14. Since Slopes 11 and 12 have quite different
geometries and cross-sections, they are treated separately in this and the following sections.
History
The exact history of the development for the slope was not recorded. The air-photo taken on
1994 did not appear to show apparent evidence of landslide, but the air-photos taken in 2002 and
2004 show the scarp and the image of landslide (Figure 76). The scarp and the humps generally
appeared old in August 2003 when the slope was first investigated. The bushes and vegetations
were well grown on the slope surface (Figure 77). There was also newly repaired pavement near
the toe of the slope.
Area Geology
According to the USDA (1981a) Soil Survey Report, the soils of Boone County formed in glacial
till and sediment from glacial till, glacial outwash and alluvium, etc. Glacial till is the parent
material of most of the soils. Most of the soils formed in glacial till deposited by the most recent,
the Wisconsin Glaciations. Sandstone and shale are the oldest parent materials in the county,
which were deposited during the Pennsylvanian and Permian Periods.
Field Investigations
Slope Geometry
The slope had an overall sloping angle of about 16 degrees (H:V = 3.5:1), a maximum length of
85 m and a maximum height of 23 m (Figure 78). The width of the slope is about 80 m along the
highway. It had a scarp near the top with a maximum height of 5 m. It also had a few cracks near
the middle and the toe of the slope. The maximum widths of the cracks were about 0.3 m. There
was also a hump near the toe of the slope. A 2 m wide shallow ditch was located at the toe of
slope (beside the highway).
Site Geology
Three boreholes were drilled manually on the slope following the direction of the slope
movement (Figure 78). The maximum depth of the boreholes was 4.1 m. The boreholes showed
that the slope was made of yellowish brown glacial till which was generally soft to medium stiff.
Shale was found in some outcrops near the bank of the Des Moines River but was not seen in the
slope. The boring logs are shown in the Appendix (Figures A25 to A27).
87
Lab Investigations
Basic Properties
Basic properties for representative soil samples were investigated and the results are summarized
in Table 32. The results showed that the glacial till sample comprised 48% of sand, 33% of silt
and 19% of clay. Its liquid limit was only 28%. The soil was classified as low plasticity clay
(CL) by USCS.
Slope Analysis
Soil Properties
The field and lab test results show that the slope mainly consists of low plasticity glacial till.
Based on the field visual inspections and BST results, the soil was assumed to be uniform for the
slope stability analysis.
88
All these results suggest that the slope may have most likely failed under the conditions as in
Analysis 3 (back-calculation). The shear strength parameter values from back-calculation
indicated the average mobilized shear strength during slope failure. Also, the BST results in
Table 33 may represent the peak shear strength of the shale. The residual shear strength as
obtained from the ring shear test was the ultimate shear strength corresponding to a slide with
large displacement. This value was higher than the average value obtained from BST and the
values obtained from back-calculation, which may be due to the soil variability. The soil sample
for the ring shear test may not be exactly the same with what BST has been performed. Another
possible reason was that the soil may have exhibited a hardening response during shearing.
Conclusions
BST was used to characterize the slope. The shear strength parameter values obtained from BST
for the soil were used for the slope analysis to investigate the possible cause of the failure. The
slope most likely failed with a circular slip surface passing through the observed scarp of the
slope. The soil in the slope has an average peak shear strength parameter values of φ' = 200 and c’
= 10 kPa as measured by BST. It has a softened (mobilized) shear strength parameter values of φ'
= 17.70 and c’ = 0 kPa during the slide mobilization; and residual shear strength parameter values
of φr' = 25.70 and c’ = 3.8 kPa as measured by ring shear test. The measured residual strength
may be the upper bound strength value for the glacial till in the slope, and the soil may exhibit
hardening response during shearing.
14
11
13 12
Slopes
Figure 75. Location of slopes 11-14 (Hwy E57, 4.5 miles west of Luther, Boone Co.)
89
14
Highway E57
11
12
13
90
(a) Looking southwest, overview of the slopes 11 and 12 (photo taken by Thompson, 08/27/03)
91
(b) Looking southwest, overview of the slopes 11 and 12 (photo taken by Thompson, 03/30/04)
(c) Looking southeast, overview of the slope 11 (photo taken by Thompson, 03/30/04)
92
(d) Looking southwest, scarps of the slope 11 (photo taken by Thompson, 03/30/04)
(e) Looking southwest, overview of the slope 12 (photo taken by Thompson, 03/30/04)
Figure 77. Photographs for slopes 11 and 12
93
0 10 20 30 40 50 60 70 80 90 100
0
BH1
Height (m)
-10 BH2 Hwy E57
-20 BH3
-30
Distance (m) N
100
BH1, h = 3.7m
BH1, h = 4.1m
Shear stress (kPa)
75 BH2, h = 3.8m
BH3, h = 2.7m
50
25
0
0 25 50 75 100 125 150 175 200
Normal stress (kPa)
o
φ r' = 25.7 , cr' = 3.8 kPa
100
y = 0.4807x + 3.7822
50 2
R = 0.9996
0
0 50 100
150 200 250 300 350
Normal stress (kPa)
Figure 80. Ring shear test results for slope 11
94
00
2.8
00
2.0
1.356
1.6 0
00
0
1.400
1.8
0
-5
-10
Height (m)
-15
-20
-25
-30
-35
-40
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
Distance (m)
'
Depth φ' c Data
BH Soil R2
(m) (deg.) (kPa) points
1 3.7 Glacial till 22 11 1.000 5
1 4.1 Glacial till 21 7 0.998 5
2 3.8 Glacial till 15 12 0.990 4
3 2.7 Glacial till 22 9 0.993 5
95
Site Conditions
Location
The slope is a cut slope that is located at the south side of Highway E57, 0.5 mile west of Des
Moines River, 4.5 miles west of Luther, Boone County (Figure 75, see the section for Slope 11).
This slope is connected with Slope 11, and is very close to Slopes 13 and 14. Since Slopes 11
and 12 have quite different geometry in their cross-section, they were treated separately in this
and the following section.
History
The exact history of the development for the slope was not recorded. The air-photo taken on
1994 did not appear to show apparent evidence of landslide, but the air-photos taken in 2002 and
2004 show the scarp and the image of landslide (Figure 76, see the section for Slope 11). The
scarp and the humps generally appeared old in August 2003 when the slope was first
investigated. The bushes and vegetations were well grown on the slope surface (Figure 77, see
the section for Slope 11). There was also newly repaired pavement near the toe of the slope.
Area Geology
According to the USDA (1981a) Soil Survey Report, the soils of Boone County formed in glacial
till and sediment from glacial till, glacial outwash and alluvium, etc. Glacial till is the parent
material of most of the soils. Most of the soils formed in glacial till deposited by the most recent,
the Wisconsin Glaciations. Sandstone and shale are the oldest parent materials in the county,
which were deposited during the Pennsylvanian and Permian Periods.
Field Investigations
Slope Geometry
The slope had an overall sloping angle of about 18 degree (H:V = 3.0:1), a maximum length of
63 m and a maximum height of 20 m (Figure 82). The width of the slope is about 70 m along the
highway. It had a scarp near the top with a maximum height of 2 m. It also had a few cracks and
a hump near the middle of the slope. The maximum widths of the cracks were about 0.3 m. A
small ditch was located at the toe of slope (beside the highway).
Site Geology
Three boreholes were drilled manually on the slope following the direction of the slope
movement (Figure 82). The maximum depth of the boreholes was 4.1 m. The boreholes showed
that the slope was made of yellowish brown glacial till which was generally soft to medium stiff.
Shale was found in some outcrops near the bank of the Des Moines River but was not seen in the
slope. The boring logs are shown in Appendix (Figures A.28 to A.30).
96
Lab Investigations
Basic Properties
Basic properties for representative soil sample were investigated and the results are summarized
in Table 35. The results showed that the glacial till sample comprised 32% of sand, 45% of silt
and 23% of clay. Its liquid limit was only 31%. The soil was classified as low plasticity clay
(CL) by USCS.
Slope Analysis
Soil Properties
The field and lab test results show that the slope mainly consists of low plasticity glacial till.
Based on the field visual inspections and BST results, the soil was assumed to be uniform for the
slope stability analysis.
97
All these results suggest that the slope may have most likely failed under the conditions as in
Analysis 3 (FS = 1.0). The shear strength parameter values from back-calculation indicated the
average mobilized shear strength during slope failure under the assumed ground water
conditions. The back-calculated shear strength was higher than the residual strength, which could
be attributed to two possible reasons: (1) the residual strength was not representative of the
average soil conditions along the slip surface; (2) the ground water level could be higher than the
observed (adopted) water level, thus the shear strength at failure should be higher than the back-
calculated strength. The BST results in Table 36 may represent the average peak shear strength
of the shale. The residual shear strength as obtained from the ring shear test was the ultimate
shear strength corresponding to a slide with large displacement.
Conclusions
BST was used to characterize the slope. The shear strength parameter values obtained from BST
were used for the slope analysis to investigate the possible cause of the failure. The slope most
likely failed with a circular slip surface passing the observed scarp near the top of the slope. The
soil in the slope has an average peak shear strength values of φ' = 240 and c’ = 6 kPa as measured
by BST; and a residual shear strength of φr' = 24.30 and c’ = 0 kPa.
0 10 20 30 40 50 60 70 80 90
0 BH1
Hwy E57
BH2
Height (m)
-10
BH3
-20
-30
Distance (m) N
125
BH1, h = 3.8m
100 BH1, h = 4.1m
Shear stress (kPa)
BH2, h = 3.8m
75 BH3, h = 3.4m
50
25
0
0 25 50 75 100 125 150 175
Normal stress (kPa)
98
200
Glacial till, BH3, h = 3.4m
100
y = 0.4506x - 0.9358
2
50 R = 0.9995
0
0 100 200 300 400
Normal stress (kPa)
1.379
-5
-10
Height (m)
-15
-20
-25
-30
-35
-40
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
Distance (m)
Figure 85. Slope stability analysis for slope 12
99
Depth φ
'
c' 2 Data
BH Soil R
(m) (deg.) (kPa) points
100
Site Conditions
Location
The slope is a cut slope that is located at the south side of Highway E57, 0.5 mile west Des
Moines River, 4.5 miles west of Luther, Boone County (Figure 75, see the section for Slope 11).
This slope is very close to Slopes 11 and 12.
History
The exact history of the development for the slide was not recorded. The air-photo did not appear
to show apparent evidence of landslide (Figure 76, see the section for Slope 11). The scarp and
the hump generally appeared old in August 2003 when the slope was first investigated. The
bushes and vegetations grew well on the slope surface (Figure 86).
Area Geology
According to the USDA (1981a) Soil Survey Report, the soils of Boone County formed in glacial
till and sediment from glacial till, glacial outwash and alluvium, etc. Glacial till is the parent
material of most of the soils. Most of the soils formed in glacial till deposited by the most recent,
the Wisconsin Glaciations. Sandstone and shale are the oldest parent materials in the county,
which were deposited during the Pennsylvanian and Permian Periods.
Field Investigations
Slope Geometry
The slope had an overall sloping angle of about 18 degree (H:V = 3.0:1), a maximum length of
58 m and a maximum height of 16 m (Figure 87). The width of the slope with slide is about 10 m
parallel to the highway. It had a scarp near the center of the slope with a maximum height of 0.5
m. It also had a hump downside of the scarp near the middle of the slope.
Site Geology
One borehole was drilled manually on the slope near the scarp (Figure 87). The maximum depth
of the borehole was 3.1 m. The borehole showed that the slope was made of yellowish brown
glacial till which was generally soft to medium stiff. Shale was found in some outcrops near the
bank of the Des Moines River but was not seen in the slope. The boring log is shown in the
Appendix (Figure A31).
101
Lab Investigations
Basic Properties
Basic properties for representative soil sample were investigated and the results are summarized
in Table 37. The results showed that the glacial till sample comprised 47% of sand, 34% of silt
and 19% of clay. Its liquid limit was only 27%. The soil was classified as low plasticity clay
(CL) by USCS.
Slope Analysis
Soil Properties
The field and lab test results show that the slope mainly consists of low plasticity glacial till.
Based on the field visual inspections and BST results, the soil was assumed to be uniform for the
slope. Slope stability analysis was performed accordingly.
All these results suggest that the slope may have most likely failed under the conditions as in
Analysis 3 (the back-calculation). The shear strength parameter values from back-calculation
indicated the average mobilized shear strength during slope failure. Also, the BST results in
Table 38 may represent the peak shear strength of the shale. The residual shear strength as
obtained from the ring shear test was the ultimate shear strength corresponding to a slide with
large displacement. This value was higher than the average value obtained from BST and the
values obtained from back-calculation, which may be due to the soil variability. The soil sample
102
for the ring shear test may not be exactly the same with what BST has been performed on.
Another possible reason is that the soil may have exhibited ductile or hardening response with
respect to shearing.
Conclusions
BST was used to characterize the slope. The shear strength parameter values obtained from BST
were used for the slope analysis to investigate the possible cause of the failure. The slope most
likely failed with a circular slip surface passing the observed scarp near mid of the slope. The
soil in the slope had an average peak shear strength values of φ' = 160 and c’ = 8 kPa as measured
by BST. It had a mobilized shear strength values of φ' = 14.30 and c’ = 0 kPa during the slide
mobilization. The residual shear strength of the soil was φr' = 26.00 and c’ = 0.1 kPa. The
measured residual strength could be the upper bound strength value for the glacial till in the
slope. The soil could have exhibited strain-hardening response with respect to shearing.
Figure 86. Slope 13, looking southwest (photo taken by Yang, 10/10/04)
103
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
0
BH1
-5
Height (m)
-10 Hwy
E57
-15
-20
-25
Distance (m) N
60
Glacial till, BH1, 3.1m
Shear stress (kPa)
o
(φ' = 16 , c' = 8 kPa)
40
20 y = 0.280x + 8.115
2
R = 0.997
0
0 20 40 60 80 100 120 140 160
125
Glacial till, BH1, h = 3.1m
Shear stress (kPa)
100 o
φ r' = 26.0 , cr' = 0.1 kPa
75
50 y = 0.4868x + 0.125
2
25 R = 0.9994
0
0 50 100 150 200 250
104
1.682
-5
-10
Height (m)
-15
-20
-25
-30
-35
-40
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
Distance (m)
Figure 90. Slope stability analysis for slope 13
105
Site Conditions
Location
The slope is a fill slope and is located at the north side of Highway E57, 0.5 mile west Des
Moines River, 4.5 miles west of Luther, Boone County (Figure75, see the section for Slope 11).
This slope is on the opposite side of the road to Slopes 11 and 12.
History
The exact history of the development for the slope was not recorded. The air-photo did not
appear to show apparent evidence of landslide (Figure 76, see the section for Slope 11). The
scarp and the hump generally appeared old in August 2003 when the slope was first investigated.
The bushes and vegetations were well grown on the slope surface (Figure 91).
Area Geology
According to the USDA (1981a) Soil Survey Report, the soils of Boone County formed in glacial
till and sediment from glacial till, glacial outwash and alluvium, etc. Glacial till is the parent
material of most of the soils. Most of the soils formed in glacial till deposited by the most recent,
the Wisconsin Glaciations. Sandstone and shale are the oldest parent materials in the county,
which were deposited during the Pennsylvanian and Permian Periods.
Field Investigations
Slope Geometry
The slope had an overall sloping angle of about 12 degrees (H:V = 3.0:1), a maximum length of
47 m and a maximum height of 10 m (Figure 92). The width of the slope with slide is about 30
m. It had multiple minor scarps on the surface of the slope with height ranging from 0.1 of 0.3 m.
It also had a hump downside of the scarp near the toe of the slope. The toe of the slope is
partially eroded by the creek.
Site Geology
One borehole was drilled manually on the mid of slope (Figure 92). The depth of the borehole
was 2.7 m. The borehole showed that the slope was made of yellowish brown glacial till which
was generally soft to medium stiff. Shale was found in some outcrops near the bank of the Des
Moines River but was not seen in the slope. The boring log is shown in the Appendix (Figure
A32).
106
Lab Investigations
Basic Properties
Basic properties for representative soil sample were investigated and the results are summarized
in Table 39. The results showed that the glacial till sample comprised 49% of sand, 32% of silt
and 19% of clay. Its liquid limit was only 27%. The soil was classified as low plasticity clay
(CL) by USCS.
Slope Analysis
Soil Properties
The field and lab test results show that the slope mainly consists of low plasticity glacial till.
Based on the field visual inspections and BST results, the soil was assumed to be uniform for the
slope stability analysis.
All these results suggest that the slope could have failed under the conditions as in Analysis 3
(the back-calculation). The shear strength parameter values from back-calculation indicated the
average mobilized shear strength during slope failure. The BST results in Table 40 represented
the in-situ strength of the soil. The residual shear strength as obtained from the ring shear test
was the ultimate shear strength corresponding to large displacement of the soil. This value was
higher than the value obtained from BST and the value obtained from back-calculation, which
may be due to the soil variability. The soil sample for the ring shear test may not be exactly the
107
same as what BST has been performed on. It could also be attributed to the ductile or hardening
behavior that the soil exhibited.
Conclusions
BST was used to characterize the slope. The shear strength parameter values obtained from BST
were used for the slope analysis to investigate the possible cause of the failure. The slope could
have failed with a circular slip surface passing the observed main scarp of the slope. The soil in
the slope had an average in-situ strength values of φ' = 150 and c’ = 10 kPa as measured by BST.
It probably had a mobilized shear strength values of φ' = 150 and c’ = 6.5 kPa during the failure.
The residual shear strength parameter values were φr' = 26.90 and c’ = 4.2 kPa, which could be
the upper bound shear strength value for the glacial till in the slope. The soil could have
exhibited ductile or hardening response with respect to shearing.
Figure 91. Slope 14, looking southeast (photo taken by Yang, 10/10/04)
108
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70
5
Hwy
E57
Height (m) 0 BH1
-5
-10 Creek
-15
-20
0
Distance (m) N60 W
60
Glacial till, BH1, 2.7m
Shear stress (kPa)
o
(φ ' = 15 , c' = 10 kPa)
40
20 y = 0.261x + 10.370
2
R = 0.995
0
0 20 40 60 80 100 120 140 160
Normal stress (kPa)
125
Glacial till, BH1, h = 2.7m
Shear stress (kPa)
100 o
φ r' = 26.9 , cr' = 4.2 kPa
75
50 y = 0.5069x + 4.2038
2
R =1
25
0
0 50 100 150 200 250
Normal stress (kPa)
109
1.297
-5
-10
Height (m)
-15
-20
-25
-30
-35
-40
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70
Distance (m)
Figure 95. Slope stability analysis for slope 14
110
Site Conditions
Location
The project is located at the site of the proposed Highway 63 over Sugar Creek Bridges, Bridge
Design No. 1001, Segment 3, Ottumwa Bypass, east of Ottumwa, Wapello County, Iowa (Figure
96). The project involves embankment fill slopes.
History
Approach embankment fills on both sides of the Sugar Creek with pile-supported abutments
were designed previously to support the Highway US 63 over Sugar Creek Bridge in Wapello
County, Iowa. Slope analyses indicated potential global instability for the slopes in front of the
abutments with slip surface passing through the highly weathered shale, when assuming a shear
strength parameter, effective cohesion (c’) of 10 kPa, in accordance with IaDOT design
guidelines. As a result, ground improvement alternatives and retaining wall alternatives were
proposed by CH2MHill (2004) with the estimated costs ranging 3 to 5 million dollars.
In view of the high costs, a comprehensive supplemental subsurface exploration and testing
program was developed by CH2MHill and executed at the site with the joint effort from Iowa
State University (ISU) in August 2004. The purpose of the program was to verify that the shear
strength parameters for the soils, especially for the highly weathered shale, used in the slope
stability analyses were reasonable; and to possibly develop more realistic and site specific design
parameters to optimize the design, justify and /or possibly reduce the estimated costs of any
measures if required (CH2MHill 2005).
During the initial field investigation program for the project in April 2001, 16 bridge borings
were drilled. Except for the information of soil stratification for the site, the investigation
program only provided shear strength parameters for three samples of alluvial soils. No shear
strength parameters for the shales were obtained (CH2MHill 2005). Therefore, in the
investigation program of August 2004, extensive tests mainly including in-situ borehole shear
tests (BSTs, performed by ISU) were conducted to obtain the shear strength parameters of the
soils, especially for the shales. Based on the BST results, CH2MHill (2005) performed slope
stability analyses for seven slope sections. As a parallel and independent study as presented in
this report, ISU also performed slope stability analyses but through different approaches, which
included parametric study and reliability study, to avoid repetition. The findings in the study will
be used as verifications and supplements to those of CH2MHill (2005).
Area Geology
According to the USDA (1981b) Soil Survey Report, most of the soils in Wapello County
formed in glacial till, loess and alluvium. Clayey shale is the oldest parent material forming the
bedrock of the project site. The shale consists of a series of beds deposited during the Des
Moines sedimentary cycle in the Pennsylvanian period. The bedrock surface closely parallels to
the existing ground surface.
111
Field Investigations
Site Geology
A total of 16 borings and 10 borings were drilled during the investigation programs in the Sugar
Creek project site in 2001 and 2004, respectively. The borings were distributed on both sides of
the Sugar Creek. The site has gentle slope surface overall covered with well grown woods and
vegetation (Figure 97). The boring layout for the 2004 program is shown in Figure 98, and the
boring logs are presented in the Appendix B.
The subsurface of the project site can be generally grouped into 4 layers with increasing depth.
The first layer consists of lean clay with sand and gravel (a thin layer of topsoil and fluvium or
slope wash) underlain by clayey sand and silt (alluvium); or a mixture of clay, silt, sand and
small amount of gravel. The actual compositions vary throughout the site. The layer is referred to
as the Alluvium layer in this report for simplicity. The thickness of the Alluvium layer ranges
from 2.7 to 8.4 m.
The Alluvium layer is underlain by three layers of highly weathered shale (H.W.Sh), moderately
weathered shale (M.W.Sh), and slightly weathered shale (S.W.Sh) in order of increasing depth.
The divisions of the three layers of the shale were based on the field visual inspection and
strength tested using pocket penetrometer. In general, H.W.Sh has a relatively low strength,
S.W.Sh has a relatively high strength, and M.W.Sh represents the transition between H.W.Sh and
S.W.Sh.. The S.W.Sh was close to fresh rock and was identified by split spoon refusal or where
the N-value in standard penetration test was below 50 blows or greater per 6-inch increment
(CH2MHill 2005).
The surface of H.W.Sh generally appears to parallel the existing ground surface. It has a gentle
slope ranging from 12.5:1 (H:V) to 10:1 (H:V) in the north side, and a relatively steeper slope of
3:1 (H;V) in the south side of Sugar Creek. The thickness of H.W.Sh ranges from 0 to 3.7 m in
the north side and 0.5 to 4.7 m in the south side of Sugar Creek, and most of the H.W.Sh is less
than 3 m thick. The H.W.Sh was underlain by M.W.Sh, which has a thickness ranging from 0 to
5.7 m in the north side and 1.5 to 8 m in the south side of Sugar Creek. M.W.Sh was underlain
S.W.Sh, where the boreholes were terminated. In the south side of the creek, a nearly horizontal,
0.5-0.9 m thick limestone seam was also encountered and mainly located in the M.W.Sh. layer.
The boring results indicate that the spatial distributions of the soil layers are highly variable, and
the section of slope changes from place to place. One of the typical sections of slope with the
proposed embankment fill is shown in Figure 99.
112
The BST results indicate that the shear strengths of the soil layers are highly variable (Table 41).
The variability is further indicated by the statistical results as summarized in Table 42. Despite of
the variation of the shear strength parameter values for the soils, the general trend that the shear
strengths increase following the increase in the depth of the shales is still apparent. It is also
note-worthy that the average shear strength of H.W.Sh, which has average shear strength
parameter values of φ’ = 12.8o and c’ = 33.2 kPa, is much higher than that of c’ = 10 kPa as
assigned by IaDOT design guidelines.
Lab Investigations
Basic Properties
Basic properties for representative soil samples with emphasis on the shales were investigated
and the results are summarized in Table 43. The Atterberg limit and clay fraction for the shales
are also plotted in Figures 102 and 103. The results show that clay fraction for the shale ranges
from 30-65%, liquid limit varies between 35% and 75%, and plastic limit varies from 15% to
45%. All the shales are classified as either low plasticity clay (CL) or high plasticity clay (CH)
according to USCS.
113
Slope Analysis
The Idealized section was used to perform systematic parametric study of slope analysis to
investigate the effect of various factors that affect FS of the slope. The parametric study was
focused on the H.W.Sh., which include its φ’, c’, unit weight (γ) and its depth, since H.W.Sh. has
the lowest shear strength and is the critical layer in the project site. Other factors affecting FS
114
were also considered in the parametric study, which included the water table in the creek and the
ground water table (GWT) within the slope. Besides the parametric study, a probabilistic slope
analysis was also carried out on both the Real section and the Idealized section to account for the
large variation of the soil shear strength and the variation of GWT.
In the slope analyses, the soil shear strength parameter values as obtained from BST were
adopted (Table 47). This is because BST gave in-situ shear strength of the undisturbed soil,
which was more reliable and realistic. The average shear strength parameter values and unit
weight for the embankment compacted fill were recommended by IaDOT. The standard
deviations of φ’ and c’ value for the fill and those of the unit weight of all the soils are assumed
considering their variability in engineering practice. The maximum water table level for the
creek was assumed to be the highest water level according to the estimated 500 years flood event
(CH2M 2005); and the minimum water table level was assumed to be the GWT measured in the
soils during the boring investigations. To simplify the GWT conditions, the GWT was assumed
to be flat since the slope for measured GWT was gentle.
In the slope stability analysis, both Morgenstern-Price method and Bishop simplified method
were adopted. Methods of both circular slip surface search and block slip surface search were
used to obtain the potential slip surface and locate the critical slip surface which has minimum
FS in a deterministic stability analysis. Back-calculations were also performed to determine the
average shear strength of the soil giving unity factor of safety (FS =1.00).
The sensitivity of FS with respect to the variation of the water table level in the creek was
studied and the result is shown Figure 111. In the analysis, mean values for all the parameters
were taken; and the GWT in the slope was assumed to be the same as that of the water level in
the river, which is in line with the observations that the GWT within the soils was generally flat
during the filed borings. The result indicates that within the range of the variation of the water
level in the creek, the variation of FS for the slope is insignificant. The difference between the
maximum and minimum FS corresponding to the lowest (-1.6 m) and highest (+1.6 m) water
level in the creek is only 0.13 for various analyses assuming different slip surfaces.
In the previous analyses, GWT in the slope was assumed to be the same as the water level in the
creek. However, it is also possible that GWT conditions within the slope soils be altered
considerably after the embankment is constructed, ant the GWT can be significant different from
the water table level in the creek. The exact future GWT depends on many factors such as the
hydraulic properties of the fill and the existing soils, the local hydrogeological conditions and
115
climatic conditions (precipitation and evaporation); and its evaluation requires complex analysis
coupled with groundwater seepage analysis. Such study is beyond the scope of this report.
Nevertheless, a simplified parametric slope analysis was performed to evaluate the effect of
future GWTs on the slope stability. The GWT was idealized by assuming it being flat within the
slope, extending to the edge of the slope, following the slope surface and then connecting to the
mean water table of the creek (elevation of 14.6 m). Such a GWT and the slope analysis results
are presented in Figure 112. It shows that when GWT in the slope was raised from the lowest
elevation of 14.6 m to the top of the embankment at elevation of 30.0 m, FS value dropped by
about 0.38 for different potential slip surfaces using different analysis methods. The results
indicate that the GWT within the slope has significant effect on slope stability. However, as the
FS value of about 1.20 was still larger than utility under the extremely unfavorable condition
with GWT located at the top of the embankment, the effect of GWT within the slope can be
released if further study on this factor is not required.
Considering H.W.Sh was found at various elevations at different locations of the project site, the
effect of the elevation of H.W.Sh. on slope stability was also studied. The H.W.Sh was assumed
to be horizontal and located at various depths in a slope section which had similar configuration
with the Idealized section of Figure 107. Thicknesses of both H.W.Sh and M.W.Sh were
maintained as 3.0 m, and the geometry of the slope remained the same. When the elevations of
the shales were “raised up”, the overlying Alluvium and the compacted fill might have to be
partially or totally “cut off” to maintain the geometry of the slope. Also, GWT was assumed to
be at the mean level of elevation 14.6 m. Soil parameters were assumed to be of their mean
values. The results of the analysis are presented in Figure 113. It indicates that FS value
significantly increased following the increase in the elevation of the H.W.Sh. A slope section
comprising shallow H.W.Sh. has a considerably larger FS value. When the top of H.W.Sh was at
elevation of 20 m or above, the potential slip surface will pass through the Alluvium or
compacted fill, i.e. the location of H.W.Sh has no more effect on the slope stability. In addition,
under the extremely unfavorable condition assumed with H.W.Sh located at elevation of 5.0 -
8.0, which was 3.0 m lower than that in the Idealized section, the FS for the slope is still larger
than 1.42. These results suggest that slope comprising relatively shallow H.W.Sh. can be
released for the stability concern.
The probability density of FS and the probability distribution of FS are presented in Figures 118
and 119, respectively, and also summarized in Table 48. The peak FS values in the probability
density curves indicate the mean FS values, which are also the same corresponding to the 50% of
probability of FS in the probability distribution curves. The probability of failure, or P(FS<1.00),
can be obtained from the curves in Figure 119 where FS=1.00, which has the maximum value of
116
about 6% for the 8 different cases. From the details of the results as summarized in Table 48, it
can be seen that the maximum value of P(FS<1.00) is 5.8% for the 8 different cases. The mean
FS values range from 1.571 to 1.789 assuming different slip surface for the two different sections
(Idealized and Real sections). These results suggest that the slopes analyzed are generally safe,
with probability of failure less than 6%.
The probability of failure as analyzed by CH2MHill (2005) was generally smaller than 0.1%,
which was significantly lower than the results of this study. This is mainly due to the reason that
the shear strength parameter values adopted by CH2MHill were generally larger than those
adopted in this study. CH2MHill proposed to excavate and replace the Alluvium and H.W.Sh.
with compacted fill for the south abutment area, and the BST results tested in the soils to be
excavated were not used for the statistical calculation and the input for the probabilistic analysis.
For example, CH2MHill obtained average φ’ = 16o with a standard deviation of 5o, and average
c’ = 42 kPa with a standard deviation of 17 kPa. for the H.W.Sh. layer. These values are
considerably higher than those used in this study (Table 47). In this study, no excavation of the
weak soils is considered. Nevertheless, the mean FS values are still comparable with those of
CH2MHill, in which mean values of FS of 1.8 and 1.9 were reported with similar slope section.
Geotechnical investigation and characterization was conducted on the Sugar Creek project site
with the emphasis on the determination of the shear strengths of the soils, particularly those of
the relatively weak layer of the highly weathered shale, as there was potential global slope
instability for the proposed embankment slopes with slip surface passing through the highly
117
weathered shale. A substantial number of tests were performed for the soil or soil samples at
different elevations of different borings. The tests included in-situ borehole shear test (BST), and
laboratory direct shear, triaxial compression and ring shear test. The shear strength parameter
values as obtained from BST, which gave the shear strength of in-situ, undisturbed soils, were
used for slope stability analysis.
Slope analyses were performed basically on two sections, the Real section and the Idealized
section. The Real section was the section comprising highly weathered shale at the lowest
elevation with relatively large thickness. This section was representative of the most unfavorable
slope section. The Idealized section assumed horizontal soil layers and idealized soil
configurations in order to accommodate the great variety of soil stratifications, and was also used
for parametric slope analysis. The parametric analysis was performed to investigate the
sensitivity of factor of safety (FS) with respect to the changes of various factors affecting FS of
the slope, particularly in line with the dramatic variation of the shear strength parameter values
of the soils. Probabilistic slope analyses were also performed using the statistical results of the
shear strength parameter values and the different water table conditions.
The study shows that the shear strength of the highly weathered shale has the most effect on FS
of the slope. Variation of soil unit weight does affect the FS of slope appreciably. Variation of
water table level in the creek slightly affects the FS, and variation of water table level within the
slope moderately affects the FS of slope. In addition, the elevation of highly weathered shale
significantly affects FS of the slope. All the analyses using mean values of the various soil
parameter and water table level resulted in FS values larger than unity indicating the slopes are
generally safe.
The probabilistic analyses show that the results for the Real section and the Idealized section are
very close to each other for all cases using analysis methods of Morgenstern-Price Method and
Bishop Simplified Method by assuming both circular slip surface and block slip surface. The
values of probability of failure ranges from 2.77% to 5.80%, and the mean FS values range from
1.57 to 1.79 for the 8 cases analyzed.
Different combinations of φ’ and c’ values for the highly weathered shale required for giving FS
=1.00 for the slope were obtained by back-calculations. The results were compared with the
shear strength parameter values of the highly weathered shale as measured from Borehole Shear
Tests. It is found that all the measured shear strength parameter values are located above the
curves of c’-φ’ plot for giving FS =1.00 for the Idealized slope section, indicating the H.W.Sh
has sufficient strength for the slope to remain safe (FS>1.00).
118
Slope 15
Figure 96. Location of slope 15 (Sugar Creek Project, Ottumwa, Wapello Co.)
119
(a) Looking north, overview of the north side of the site (photo taken by Thompson, 07/27/04)
(b) Looking north, investigation of the north side of the site (photo taken by Thompson,
07/26/04)
Figure 97. Photographs for slope 15
120
40
Sugar Creek
BH CH1003 BH CH1005 BH CH1006 North Abutment (SBL)
30
Sugar Creek
Elevation (m)
20 Mean
Max.
WT
Alluvium WT
Distance (m)
121
300
Sugar Creek CH1009
BST, 1.04m, clay
BST, 3.00m, h.w.sh.
250
BST, 5.03m, m.w.sh.
BST, 5.03m (residual)
BST (High pressure plate), 8.90m, s.w.sh.
200
Shear stress (kPa)
150
100
50
0
0 50 100 150 200 250 300 350
8
BH7, 7.80m, S.W.Sh (φ' = 9o, c' =3.97 MPa) (RBST)
BH8, 11.43m, S.W.Sh (φ' =19o, c' =0.55 MPa) (RBST)
BH9, 8.90m, S.W.Sh (φ' =27o, c' =0.086 MPa) (BST-H)
6
4
y = 0.15x + 3.9667
2
R = 0.9643
y = 0.3369x + 0.5478
2
R = 0.9817
2
y = 0.5185x + 0.0857
2
R = 0.9914
0
0 2 4 6 8 10 12 14 16 18 20
122
50
Highly weathered shale
Moderately weathered shale
Slightly weathered shale
40
U-line:
Plasticiy index, PI (%)
30 PI = 0.9(LL-8)
A-line:
20 PI = 0.73(LL-20)
10
y = 0.77x - 11.37
2
R = 0.95
0
0 10 20 30 40 50 60 70 80
Figure 102. Plastic limit versus liquid limit for the shales at slope 15
80
Highly weathered shale
Moderately weathered shale
Liquid limit, LL (%) or Plasticity index, PI (%)
40
PI
30
y = 0.66x - 3.03
20 2
R = 0.77
10
0
20 30 40 50 60 70
Clay fraction, CF (% < 0.002 mm)
Figure 103. Atterberg limits versus clay fraction for the shales at slope 15
123
200
Sugar Creek Project Borehole CH1009
DS 2.40-2.70m (h.w.sh)
DS 2.70-3.00m (h.w.sh)
DS 4.50-5.25m (h.w.sh)
150 DS 4.50-5.25m (h.w.sh) (Res.)
DS 4.65-4.95m (m.w.sh)
Shear stress (kPa)
100
50
0
0 50 100 150 200 250 300 350
400
Normal stress (kPa)
Figure 104. Direct shear test results for samples of boring CH1009 at slope 15
200
σ3 ' o
φ' = 27.6
33.0 kPa c' = 20.6
150 70.0 kPa
shaer stress (kPa)
105.6 kPa
100
50
0
0 50 100 150 200 250 300 350 400
Figure 105. Consolidated drained triaxial test for highly weathered shale at 0.6-1.2m in
124
120
Test 1 (φ'r =12.8o, cr'= 3.9 kPa )
60
y = 0.2269x + 3.9048
40
R2 = 0.9997
20
0
0 50 100 150 200 250 300 350 400
Normal stress, σ' (kPa)
Figure 106. Results of ring shear test for the highly weathered shale at 0.6-1.2m in boring
CH1010 at slope 15
35
30
water table
20
(El. = 14.6m)
15
Alluvium
10 Highly weathered shale
Moderately weathered shale
5
Slightly weathered shale
0
0 10 20 30 40 50 60 70 80 90 100
Distance (m)
Figure 107. Idealized section for slope stability analysis for slope 15
2.0
1.8
Factor of safety
1.6
Figure 108. Sensitivity of FS on effective friction angle of the highly weathered shale for the
125
2.0
1.8
Factor of safety
1.6
1.4
Circular (MP)
1.2 Circular (Bishop)
Block (MP)
Block (Bishop)
1.0
0 10 20 30 40 50 60
Effective cohesion, c' (kPa)
Figure 109. Sensitivity of FS on effective cohesion of the highly weathered shale for the
1.70
Circular (MP)
Circular (Bishop)
1.65 Block (MP)
Block (Bishop)
Factor of safety
1.60
1.55
1.50
1.45
15 17 19 21 23 25
3
Unit weight (kN/m )
Figure 110. Sensitivity of FS on unit weight of the highly weathered shale for the Idealized
section for slope 15
126
1.75
Circular (MP)
1.70 Circular (Bishop)
Block (MP)
1.65 Block (Bishop)
Factor of safety
1.60
1.55
1.50
1.45
1.40
-2.0 -1.0 0.0 1.0 2.0
Water table in river (m)
Figure 111. Sensitivity of FS on water table level in the river for the Idealized section for
slope 15
1.80 35
30
Water table in slope
25
1.70
Mean river
Elevation (m)
20 water table
(El. = 14.6m)
15
1.60
10 Highly weathered shale
Factor of safety
5
0
1.50
15 25 35 45 55 65 75 85 95
Distance (m)
1.40
1.30
Circular (MP)
Circular (Bishop)
Block (Bishop)
1.10
14 16 18 20 22 24 26 28 30
Water table in slope (m)
Figure 112. Sensitivity of FS on the water table level within the slope for the Idealized
section for slope 15
127
2.4
Circular (MP)
2.2 Circular (Bishop)
Block (MP)
Factor of safety Block (Bishop)
2.0
1.8
1.6 (Thickness of
h.w.sh = 3.0m)
1.4
6 8 10 12 14 16 18 20
Mid elevation of highly weathered shale (m)
Figure 113. Sensitivity of FS on the elevation of the highly weathered shale for the Idealized
section for slope 15
1.584
30
25
Height (m)
20
15
10
0
0 10 20 30 40 50 60 70 80 90 100 110
Distance (m)
Figure 114. Slope analysis assuming circular slip surface for the Real section for slope 15
128
1.732
30
25
Height (m)
20
15
10
0
0 10 20 30 40 50 60 70 80 90 100 110
Distance (m)
Figure 115. Slope analysis assuming block slip surface for the Real section for slope 15
1.559
30
25
Height (m)
20
15
10
0 10 20 30 40 50 60 70 80 90 100 110
Distance (m)
Figure 116. Slope analysis assuming circular slip surface for Idealized section for slope 15
129
1.619
30
25
Height (m)
20
15
10
0
0 10 20 30 40 50 60 70 80 90 100 110
Distance (m)
Figure 117. Slope analysis assuming block slip surface for the Idealized section for slope 15
1.4
Circular (MP)(Real)
Circular (Bishop)(Real)
Block (MP)(Real)
1.2 Block (Bishop)(Real)
Circular (MP)(Ideal)
Circular (Bishop)(Ideal)
1.0 Block (MP)(Ideal)
Block (Bishop)(Ideal)
Frequency (%)
0.8
0.6
0.4
0.2
0.0
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50
Factor of safety (FS)
130
100
90
80
Probability,P(FS<x) (%)
70
60
50
40
Circular (MP)(Real)
30 Circular (Bishop)(Real)
Block (MP)(Real)
Block (Bishop)(Real)
20 Circular (MP)(Ideal)
Circular (Bishop)(Ideal)
10 Block (MP)(Ideal)
Block (Bishop)(Ideal)
0
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50
Factor of safety, FS
120
Circular (MP), idealized section
60
40
20
0 3 6 9 12 15 18 21 24
Figure 120. Shear strength parameter values required for giving FS of unity for slope 15
131
132
133
Table 45. Summary of triaxial and unconfined compression test results for slope 15
BH Depth (m) Soil CU CD UC
' '
φ (deg.)
'
c (kPa) φ (deg.)
'
c (kPa) su (kPa)
4 5.8-6.4 h.w.sh 24 7
5 4.8-5.4 s.clay 20 13
5 9.2-10.7 s.w.sh 321
6 9.9-11.4 s.w.sh 239
7 7.4-8.9 s.w.sh 215
8 1.2-1.8 h.w.sh 21 1
9 0.9-1.4 s.clay 30 0
9 7.5-9.0 s.w.sh 181
10 0.6-1.2 h.w.sh 34 0
10 0.6-1.2 h.w.sh 28 21
10 0.6-1.2 h.w.sh 28 1
10 0.6-1.2 h.w.sh 34 10
CU = Consolidated undrained triaxial; CD = Consolidated drained triaxial;
134
Table 46. Summary of ring shear test results for slope 15
' Data
BH Depth (m) Soil φr' (deg.) cr (kPa) R2 φr (c'=0)
'
points
3 5.6-5.9 h.w.sh 8.2 0 0.9994 3 8.2
4 8.2-8.65 m.w.sh 8.5 0 1.0000 3 8.5
5 5.5-6.1 h.w.sh 6.7 2.1 0.9997 3 7.1
5 7.15-7.3 h.w.sh 8.0 1.7 1.0000 3 8.4
6 8.85-9.45 m.w.sh 9.8 0.9 1.0000 3 10.0
7 2.0-2.6 h.w.sh 7.2 1.2 1.0000 3 7.4
8 2.7-3.0 m.w.sh 7.3 1.9 0.9999 3 8.0
9 1.4-1.5 h.w.sh 6.4 1.0 0.9995 3 6.6
9 2.7-3.0 h.w.sh 5.7 2.9 0.9999 3 6.4
9 4.65-4.95 m.w.sh 7.6 1.0 0.9991 3 8.0
10 0.6-1.2 h.w.sh 12.8 3.9 0.9997 3 15.4
Table 47. Parameters used for the slope analysis for slope 15
'
φ' (deg.) c (kPa) γ' (kN/m3)
Layer Soil
Ave. S.D. Ave. S.D. Ave. S.D.
Table 48. Summary of the results of probabilistic analysis for the Real section and the
Idealized section for slope 15
Real Section Circular (MP) Circular (Bishop) Block (MP) Block (Bishop)
Mean FS 1.604 1.606 1.789 1.738
Reliability Index 1.87 1.89 1.91 1.89
P (Failure) (%) 3.06 2.95 2.77 2.94
Standard Dev. 0.323 0.321 0.412 0.391
Min FS 0.583 0.583 0.592 0.599
Max FS 2.610 2.584 3.266 3.102
# of Trials 5000 5000 5000 5000
Idealized Section Circular (MP) Circular (Bishop) Block (MP) Block (Bishop)
Mean FS 1.571 1.574 1.637 1.587
Reliability Index 1.83 1.85 1.60 1.57
P (Failure) (%) 3.33 3.24 5.46 5.80
Standard Dev. 0.311 0.311 0.398 0.374
Min FS 0.454 0.446 0.366 0.307
Max FS 2.784 2.788 3.160 2.959
# of Trials 5000 5000 5000 5000
135
Additional Discussion
Basic Properties and Shear Strengths of the Soils from a Regional Scale
The basic properties of the glacial tills and the clay shales have revealed some tendency from a
regional scale. This can be shown from Figures 121 and 122. The figures show that the glacial
tills generally have lower clay fraction and lower plasticity index (PI) than the clay shales.
Accordingly, the residual friction angles for the glacial tills are also generally higher than those
for the clay shales (Figure 123). These results may be a reflection of the different origins and
formations of the tills and shales. Corresponding to the basic properties, all the shales are
classified as low plasticity clay (CL) by USCS; while most of the shales are classified as high
plasticity clay (CH), and a few of the shales are classified as CL.
For the clay shales in Slope 15 (Sugar Creek slope), the residual frictions were also generally
low (Figure 124), ranging from 6 to 13o. The spatial variation of the residual friction angle was
not promising, as no tendency or correlation between the residual friction angles and the depths
of the shales (weathering degree) could be found. However, the residual friction angles were
generally decreased following the increase in PI of the shales.
For the in-situ shear strength parameter values of the soils as measured from the BSTs, no clear
tendency was noted on a regional basis. However, statistics of the BST measurements on the
glacial tills and clay shales for Slopes 1 to 14 (Table 49) show that, the glacial tills have a similar
average value of friction angle with the clay shales, but have a considerably lower average value
of cohesion. These results suggest that glacial till slopes may be generally less stable than clay
shale slopes on a regional scale under similar conditions.
50
40
U-line:
PI = 0.9(LL-8)
Plasticiy index, PI (%)
30
A-line:
PI = 0.73(LL-20)
20
10
136
80
y = 0.97x + 9.19
2
70 R = 0.81
50
40 y = 0.68x - 0.99
R2 = 0.76
30
20
LL (glacial till, slopes 1-14)
LL (clay shale, slopes 1-14)
10 PI (glacial till, slopes 1-14)
PI (clay shale, slopes 1-14)
0
0 10 20 30 40 50 60 70 80 90
Clay fraction, CF (%)
Figure 122. Atterberg limits versus clay fraction for all the soils in slopes 1 to 14
30
25
glacial till (slopes 1-14)
clay shale (slopes 1-14)
Kanji (1974)
20
Residual friction angle
15
10
0
0 10 20 30 40 50 60 70
Plasticity index (%)
Figure 123. Residual friction angel versus plasticity for soils in slopes 1 to 14
137
25
Highly weathered shale
Moderately weathered shale
20
Slightly weathered shale
Residual friction angle, φr'
Kanji (1974)
15
φr' = 46.6(PI)
-0.446
10
-0.49
5 y = 39.44x
2
R = 0.69
0
0 10 20 30 40 50 60 70
Plasticity index, PI(%)
Figure 124. Residual friction angel versus plasticity for soils in slope 15
Table 49. Statistics of the shear strength parameter values from the BST for slopes 1 to 14
Total No.
φ (deg.)
' '
Soil c (kPa)
of Test
Max. Min. Ave. S.D. Max. Min. Ave. S.D.
Glacial till 23 39 12 22.5 7.2 35 1 11.6 7.6
Clay shale 23 40 10 22.1 7.7 45 5 17.7 9.8
138
Summary
A total of 15 slopes along highways in Iowa were investigated, which included 13 slides (failed
slopes), one unfailed slope and one proposed embankment slope (the Sugar Creek Project). The
slopes are mainly comprised of either clay shale or glacial till, which are commonly encountered
in Iowa. The slopes are generally gentle and of small scale with slope angle ranging from 11o to
23o and height ranging from 6 to 23m
Extensive field investigations and laboratory tests were performed for the slopes. Field
investigations included survey of slope geometry, borehole drilling, soil sampling, in-situ
Borehole Shear Testing (BST) and ground water table measurement. Laboratory investigations
mainly comprised of ring shear tests, soil basic property tests (grain size analysis and Atterberg
limits test), mineralogy analyses, soil classifications, natural water contents and density
measurements on the representative soil samples from each slope. Extensive direct shear tests
and a few triaxial compression tests, unconfined compression tests were also performed on
undisturbed soil samples for the Sugar Creek Project.
Based on the results of field and lab investigations, slope stability analysis was performed on
each of the slopes to determine the possible factors resulting in the slope failures, or to evaluate
the potential slope instabilities using limit equilibrium methods. Deterministic slope analyses
were performed for all the slopes. Probabilistic slope analysis and sensitivity study were also
performed for the slope of Sugar Creek Project.
Conclusions
BSTs are competent to characterize the slopes, especially to obtain the soil shear strength
parameter values that are essential for the slope stability analysis. The shear strength parameter
values obtained from BSTs have the advantages in that they gave direct, in-situ measurements of
soil shear strength in a relatively quick manner.
BSTs appeared to have measured the peak shear strength parameter values of the soils for almost
all the cases, and the slopes have factor of safety (FS) larger than one all the time indicating the
slopes were stable under the conditions (especially the ground water table conditions) when they
were investigated.
The ring shear tests gave the residual shear strength parameter values of the soils. These values
were normally lower than those values obtained from BSTs since they corresponded to larger
displacement of the soil and represented the ultimate shear strength of the soils. FS based on the
soil residual shear strength are generally smaller than one, indicating the soils in the slopes may
not have reached the residual state.
The ring shear tests occasionally gave shear strength parameter values that were larger than those
obtained from BST. The reason may be due to the soil variability, i.e. the ring shear test and the
BST may not have tested exactly the same soils. In this case, the residual shear strength
139
parameter values may indicate the upper bound of the soil shear strength. Another reason could
be that the soils exhibited ductile or hardening response to shearing.
The back-calculated shear strength of the soils for the slope to give unity FS were generally
between the shear strength measured by BST and by ring shear test, indicating that the slopes
failed or could fail when the soil shear strength become softened. This situation occurred or will
occur once the slope movement was initialized. Most of the slope failures may have been
associated with relatively high ground water table conditions.
The slope analysis assuming slip surface passing through the observed scarp or failure zone on
the slope surface indicated the most probable slip surface of failure for the slope. The
determination of the slip surface together with the soil shear strength will be useful for slope
remediation design.
For a site involving great variability of both soil stratification and soil shear strength and various
ground water table conditions as the case of Sugar Creek Project, sensitivity study of slope
analysis and probabilistic slope analysis were proven to be useful and effective. Sensitivity
analysis showed that shear strength of the soil is the most sensitive parameter affecting FS.
Effect of unit weight on FS is negligible. Water table in the slope has significant effect on FS,
while water table in the river has moderate effect on FS.
Probabilistic slope analysis was useful when a relatively large amount of input parameters are
available, such as the shear strength parameter values as obtained from BST for the Sugar Creek
Project. Probability of failure for the slope was evaluated based on the statistical distribution of
the soil shear strength. The results are useful for further evaluation of the slope design.
140
RECOMMENDATIONS
Borehole shear testing can be performed more on the shearing zone of a failed slope or the
potential slip zone of the proposed slope as long as the site investigation program is permitted.
This may give better information to determine or to predict the controlling factors resulting in the
slope failure or the potential instability. The shearing zone can be estimated by trial slope
analysis in conjunction with the failure features such as the scarp for a failed slope.
To establish long term slope monitoring, including ground water table variation and slope
deformation, for some selected slopes in order to collect relatively complete information, which
will result in improved slope analysis together with the soil shear strengths obtained from
Borehole Shear Test. This may be especially suitable for those newly constructed slopes
susceptible to slope instability.
To establish detailed landslide inventory for the state as long as the resources is available. This
will be helpful to overview the slope instability problems from a regional prospect.
Provide pore water pressure measurement for the Borehole Shear Test (BST) so that the
measurement of the effective stress can be monitored and verified, especially for clayey soils due
to their low permeability. This may improve the BST measurements.
Perform quantitative mineralogical analysis for the weathered shales to investigate the possible
correlation of the mineralogical compositions with the weathering grades.
The research findings are expected to benefit civil and geotechnical engineers of government
transportation agencies, consultants, and contractors dealing with slope stability, slope
remediation, and geotechnical testing in Iowa. In-situ BST measurements provide reliable, site-
specific soil parameters for design applications which can lead to substantial cost savings over
using empirical estimations for critical soil properties.
As the BST is an alternative to expensive and time-consuming laboratory testing, the device is
particularly useful in obtaining relatively large amounts of data necessary for probabilistic
analyses. Procedures for incorporating Borehole Shear tests into practice are documented in
Volume 2 of this report. Nevertheless, some training may be required for effective and
appropriate use.
The BST is intended to test soils such as clays, silts, and sands. The device can produce
erroneous results in gravelly soils. Additionally, the quality of boreholes affects test results, and
disturbance to borehole walls should be minimized before test performance. A final limitation of
widespread Borehole Shear testing may be its limited availability, as only five test devices are
currently being used in Iowa.
141
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146
APPENDIX
Boring Log
Slope 1 Date: 8.12.04
Location: Hwy34 MP169.3 Drilling method: Hand auger
Boring No. BH1 Driller: HY
Plasticity Index
Ground Water
Total density
Content (%)
Level / Date
Descriptions
Sample No.
Shear Test
Depth (m)
Depth (ft)
Moisture
Borehole
(kN/m3)
(%)
Soil
No.
0 0.0 Fill material: yellowish brown
1
0.3 clay (glacial till) mixed with light 1 45 21 28.0 17.0
grey clay (shale), soft to
2 0.6 medium stiff, with trace of fine
gravel
4 1.2
6 1.8
7 2.1 1
8.12.04
10 3.0
(End of borehole)
Symbol Sand Clay
Figure A2. X-Ray Diffraction results for the shale at depth of 0.3 m for slope 1
147
Boring Log
Slope 2 Date: 7.20.04
Location: Hwy34 MP171.7 Drilling method: Hand auger
Boring No.: BH1 Driller: HY
Plasticity Index
Liquid limit (%)
Profile Symbol
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Shear Test
Depth (m)
Depth (ft)
Borehole
Moisture
(kN/m3)
Soil
(%)
0 0.0 Light brown silty clay (shale)
with trace of sand covered
2 0.6 with 6 inches of topsoil
4 1.2
8.12.04
6 1.8
Dark brown or grey silty clay
(highly weathered shale), wet,
7.5 2.3 medium stiff to stiff 1 1 13.2
Boring Log
Slope 2 Date: 8.12.04
Location: Hwy34 MP171.7 Drilling method: Hand auger
Boring No.: BH2 Driller: HY
Plasticity Index
Liquid limit (%)
Profile Symbol
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Shear Test
Depth (m)
Depth (ft)
Borehole
Moisture
(kN/m3)
Soil
(%)
0 0.0
Light brown silty clay (shale)
with trace of sand covered with
2 0.6 6 inches of topsoil
8.12.04
4 1.2
9.5 2.9
10.5 3.2 Grey clay (highly weathered 3 3 64 40 25.2 19.0
11 shale), soft to medium stiff
(End of borehole)
Soft to medium
Symbol Sand Clay stiff clay
148
Boring Log
Slope 2 Date: 8.12.04
Location: Hwy34 MP171.7 Drilling method: Hand auger
Boring No.: BH3 Driller: HY
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Shear Test
Depth (m)
Index (%)
Depth (ft)
Borehole
Plasticity
Moisture
(kN/m3)
Soil
0 0.0
Light brown silty clay (shale)
with trace of sand covered with
2 0.6 6 inches of topsoil
4 1.2
Dark brown or grey silty clay 8.12.04
(highly weathered shale), wet,
6 1.8 medium stiff to stiff
7.5 2.3 1
8.5 2.6 2 1 23.5 18.9
Grey clay (highly weathered shale),
9 2.7 soft to medium stiff
(End of borehole)
Soft to medium stiff
Symbol Sand Clay clay
Boring Log
Slope 2 Date: 8.12.04
Location: Hwy34 MP171.7 Drilling method: Hand auger
Boring No.: BH4 Driller: HY Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Shear Test
Liquid limit
Depth (m)
Index (%)
Depth (ft)
Borehole
Plasticity
Moisture
(kN/m3)
Symbol
Profile
Soil
(%)
149
Figure A7. X-Ray Diffraction results for the shale at depth of 0.6m in BH4 for slope 2
Boring Log
Slope 3 Date: 7.18.04
Location: Hwy34 MP175.3 Drilling method: Hand auger
Boring No BH1 Driller: HY
Shear Test No.
Plasticity Index
Liquid limit (%)
Profile Symbol
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Depth (m)
Depth (ft)
Borehole
Moisture
(kN/m3)
Soil
(%)
4 1.2 2 2
(7.19.05)
6 1.8 3 3
7 2.1
8 2.4 4 4
(End of borehole)
Symbol Sand Clay
150
Figure A9. X-Ray Diffraction results for the till at depth of 0.6m in BH2 for slope 3
Boring Log
Slope 4 Date: 7.18.04
Location: Hwy34 MP175.5 Drilling method: Hand auger
Boring No BH1 Driller: HY
Liquid limit (%)
Profile Symbol
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Shear Test
Depth (m)
Index (%)
Depth (ft)
Borehole
Plasticity
Moisture
(kN/m3)
Soil
4 1.2
6 1.8
(7.19.05)
7 2.1
(End of borehole)
Symbol Sand Clay
151
Figure A11. X-Ray Diffraction results for the till at depth of 0.6m in BH2 for slope 4
Boring Log
Slope 5 Date: 7.15.04
Location: Hwy34 MP178.3 (North) Drilling method: Hand auger
Boring No. BH1 Driller: HY
Shear Test No.
Plasticity Index
Liquid limit (%)
Profile Symbol
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Depth (m)
Depth (ft)
Borehole
Moisture
(kN/m3)
Soil
(%)
6 1.8 3 3
(7.19.05)
8 2.4 4 4 17.9
(End of borehole)
Symbol Sand Clay
152
Figure A13. X-Ray diffraction results for the till at depth of 0.6m in BH2 for slope 5
Boring Log
Slope 6 Date: 7.14.04
Location: Hwy34 MP178.3 (South) Drilling method: Hand auger
Boring No. BH1 Driller: HY
Plasticity Index
Liquid limit (%)
Profile Symbol
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Shear Test
Depth (m)
Depth (ft)
Borehole
Moisture
(kN/m3)
Soil
(%)
No.
4 1.2
153
Figure A15. X-Ray diffraction results for the shale at depth of 0.3m for slope 6
154
Boring Log
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Shear Test
Depth (m)
Index (%)
Depth (ft)
Borehole
Plasticity
Moisture
(kN/m3)
Soil
0.0 0.0 Grey and brown
mixed silty clay,
2.0 0.6 wet, medium stiff.
4.0 1.2
11.5 3.5
Redish brown clay
shale, wet,
14.0 4.2 medium stiff to
stiff. Occasionally
16.0 4.8 seen limestone
pieces.
18.0 5.4
19.0 5.7
Brown clay shale, wet,
20.0 6.0 stiff
(End of borehole)
Note: Borelog below 11.5ft was projected from adjacent borehole drilled by IaDOT.
155
Boring Log
Slope 7 Date: 7.16.04
Location: Hwy169 Winterset Drilling method: rotary drillrig
Boring No BH2 Driller: DJW, MS
Water Level /
Total density
Descriptions
Content (%)
Sample No.
Shear Test
Liquid limit
Depth (m)
Index (%)
Depth (ft)
Borehole
Plasticity
Moisture
(kN/m3)
Ground
Symbol
Profile
Date
Soil
(%)
0.0 0.0 Grey and brown
mixed silty clay,
2.0 0.6 wet, medium stiff.
7.19.04
4.0 1.2
6.0 1.8
7.0 2.1 1 16.7 19.0
Brown clay shale,
wet, soft to medium
9.0 2.7 stiff 1 2 50 27 16.2 18.9
(End of borehole)
Symbol Sand Clay Stiff clay
Boring Log
Slope 7 Date: 7.19.04
Location: Hwy169 Winterset Drilling method: Hand auger
Boring No. BH3 Driller: HY
Water Level /
Total density
Descriptions
Content (%)
Sample No.
Shear Test
Liquid limit
Depth (m)
Index (%)
Depth (ft)
Borehole
Plasticity
Moisture
(kN/m3)
Ground
Symbol
Profile
Date
Soil
(%)
4.0 1.2
6.0 1.8
7.0 2.1
Brown clay shale, wet,
8.0 2.4 soft to medium stiff
10.5 3.2
11.0 3.3 Redish brown clay shale, 3 3
wet, medium to stiff.
12.0 3.6 4 4
(End of borehole)
Symbol Sand Clay Stiff clay
156
Boring Log
Slope 7 Date: 7.19.04
Location: Hwy169 Winterset Drilling method: Hand auger
Boring No. BH4 Driller: HY
Water Level /
Total density
Descriptions
Content (%)
Sample No.
Shear Test
Liquid limit
Depth (m)
Index (%)
Depth (ft)
Borehole
Plasticity
Moisture
(kN/m3)
Ground
Symbol
Profile
Date
Soil
(%)
0.0 0.0 Brown clay shale,
wet, soft to medium
2.0 0.6 stiff
7.19.04
3.7
3.4 1.0 Redish brown clay 1 1 20.3 19.3
4.4 1.3 shale, wet, medium 2 2 22.2
stiff to stiff.
5.7 1.7
(End of borehole)
Symbol Sand Clay Stiff clay
Boring Log
Slope 8 (East side) Date: 8.11.04
Location: Hwy169 Afton (2 mi South) Drilling method: Hand auger
Boring No BH1 Driller: HY
Shear Test No.
Plasticity Index
Liquid limit (%)
Profile Symbol
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Depth (m)
Depth (ft)
Borehole
Moisture
(kN/m3)
Soil
(%)
4 1.2
(8.12.04)
157
Boring Log
Slope 8 (East side) Date: 8.11.04
Location: Hwy169 Afton (2 mile South) Drilling method: Hand auger
Boring No. BH2 Driller: HY
Plasticity Index
Liquid limit (%)
Profile Symbol
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Depth (m)
Depth (ft)
Borehole
Moisture
(kN/m3)
Soil
(%)
0 0.0 Yellowish brown
glacial till, soft to (8.12.04)
medium stiff, with
2 0.6 trace of fine
gravel
4 1.2 1 1 16.7
6 1.8 2 2 16.7
Boring Log
Slope 9 (West side) Date: 8.11.04
Location: Hwy169 Afton (2 mile South) Drilling method: Hand auger
Boring No. BH1 Driller: HY
Shear Test No.
Plasticity Index
Liquid limit (%)
Profile Symbol
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Depth (m)
Depth (ft)
Borehole
Moisture
(kN/m3)
Soil
(%)
8 2.4 2 4 21.7
(End of borehole)
Symbol Sand Clay
158
Figure A23. X-Ray Diffraction results for the shale at depth of 0.6m for slope 9
Boring Log
Slope 10 (Eest side) Date: 8.11.04
Location: Hwy169 Afton (4 mile South) Drilling method: Hand auger
Boring No. BH1 Driller: HY
Shear Test No.
Plasticity Index
Liquid limit (%)
Profile Symbol
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Depth (m)
Depth (ft)
Borehole
Moisture
(kN/m3)
Soil
(%)
6 1.8 (8.12.04)
159
Boring Log
Slope 11 Date: 10.09.04
Location: HwyE57 Luther (Slope A) Drilling method: Hand auger
Boring No. BH1 Driller: HY
Plasticity Index
Liquid limit (%)
Profile Symbol
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Depth (m)
Depth (ft)
Borehole
Moisture
(kN/m3)
Soil
(%)
0 0.0 Brown glacial till, soft
2 0.6 to medium stiff, with
trace of fine gravel
4 1.2
6 1.8
8 2.4
10 3.0
Boring Log
Slope 11 Date: 10.09.04
Location: HwyE57 Luther (Slope A) Drilling method: Hand auger
Boring No. BH2 Driller: HY
Shear Test No.
Plasticity Index
Liquid limit (%)
Profile Symbol
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Depth (m)
Depth (ft)
Borehole
Moisture
(kN/m3)
Soil
(%)
6 1.8
8 2.4
10 3.0
(10.10.04)
12.5 3.8 1 1 13.1
(End of borehole)
Symbol Sand Clay
160
Boring Log
Slope 11 Date: 10.09.04
Location: HwyE57 Luther (Slope A) Drilling method: Hand auger
Boring No. BH3 Driller: HY
Plasticity Index
Liquid limit (%)
Profile Symbol
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Depth (m)
Depth (ft)
Borehole
Moisture
(kN/m3)
Soil
(%)
0 0.0 Brown glacial till, soft
2 0.6 to medium stiff, with
trace of fine gravel
4 1.2
6 1.8
8 2.4 (10.10.04)
9 2.7 1 1 15.0 19.4
(End of borehole)
Symbol Sand Clay
Boring Log
Slope 12 Date: 10.09.04
Location: HwyE57 Luther (Slope B) Drilling method: Hand auger
Boring No. BH1 Driller: HY
Shear Test No.
Plasticity Index
Liquid limit (%)
Profile Symbol
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Depth (m)
Depth (ft)
Borehole
Moisture
(kN/m3)
Soil
(%)
6 1.8
8 2.4
10 3.0
161
Boring Log
Slope 12 Date: 10.09.04
Location: HwyE57 Luther (Slope B) Drilling method: Hand auger
Boring No. BH2 Driller: HY
Plasticity Index
Liquid limit (%)
Profile Symbol
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Depth (m)
Depth (ft)
Borehole
Moisture
(kN/m3)
Soil
(%)
0 0.0 Brown glacial till, soft
2 0.6 to medium stiff, with
trace of fine gravel
4 1.2
6 1.8
8 2.4
10 3.0
Boring Log
Slope 12 Date: 10.09.04
Location: HwyE57 Luther (Slope B) Drilling method: Hand auger
Boring No. BH3 Driller: HY
Shear Test No.
Plasticity Index
Liquid limit (%)
Profile Symbol
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Depth (m)
Depth (ft)
Borehole
Moisture
(kN/m3)
Soil
(%)
6 1.8
8 2.4
10 3.0
11 3.3 1 1 31 17 15.0
(End of borehole)
Symbol Sand Clay
162
Boring Log
Plasticity Index
Liquid limit (%)
Profile Symbol
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Depth (m)
Depth (ft)
Borehole
Moisture
(kN/m3)
Soil
(%)
0 0.0 Brown glacial till, soft
2 0.6 to medium stiff, with
trace of fine gravel
4 1.2
6 1.8
8 2.4 (10.11.04)
Boring Log
Slope 14 Date: 10.10.04
Location: HwyE57 Luther (Slope D) Drilling method: Hand auger
Boring No. BH1 Driller: HY
Shear Test No.
Plasticity Index
Liquid limit (%)
Profile Symbol
Ground Water
Total density
Descriptions
Content (%)
Level / Date
Sample No.
Depth (m)
Depth (ft)
Borehole
Moisture
(kN/m3)
Soil
(%)
6 1.8
8 2.4
9 2.7 1 1 27 14 10.6 18.0
(End of borehole) GWT not observed on 10.11.04
Symbol Sand Clay
163
1600
600
400
200
0 200 400 600 800 1000 1200 1400 1600 1800 2000
150
100
Shear stress (kPa)
164
100
50
0
0 100 200 300 400
280
BH9, 1.04m, clay
BH9, 3.00m, h.w.sh.
240 BH9, 5.03m, m.w.sh.
BH9, 5.03m, m.w.sh. (Res.)
BH10, 1.07m, h.w.sh.
200
BH10, 1.07m, h.w.sh. (Res.)
Shear stress (kPa)
120
80
40
0
0 40 80 120 160 200 240 280 320
165
200
CH1001, 6.50-7.10m, Clay
CH1002, 1.20-1.80m, Clay
CH1003, 5.60-5.90m, h.w.sh
150
CH1004, 3.60-4.20m, Clay
Shear stress (kPa)
100
50
0
0 50 100 150 200 250 300 350
200
CH1004, 8.20-8.65m, m.w.sh
CH1005, 5.50-6.10m, h.w.sh
CH1005, 7.15-7.30m, h.w.sh
150 CH1006, 8.85-9.45 m, m.w.sh
Shear stress (kPa)
100
50
0
0 50 100 150 200 250 300 350
166
100
50
0
0 50 100 150 200 250 300 350
100
50
0
0 50 100 150 200 250 300 350
167
60
CH1003, 5.6-5.9m
CH1004, 8.20-8.65m
50 CH1005, 5.5-6.1m
CH1005, 7.15-7.3m
CH1006, 8.85-9.45m
40
CH1007, 2.0-2.6m
Shear stress (kPa)
30
20
10
0
0 50 100 150 200 250 300 350 400
Normal stress (kPa)
Figure A41. Ring shear test results for the shales at slope 15 (part 1 of 2)
100
CH1008, 2.7-3.0m
CH1009, 1.4-1.5m
80 CH1009, 2.4-2.7m
CH1009, 4.6-4.9m
CH1010, 0.7-0.9m
Shear stress (kPa)
60 CH1010, 0.7-0.9m
40
20
0
0 50 100 150 200 250 300 350 400
Normal stress (kPa)
Figure A42. Ring shear test results for the shales at slope 15 (part 2 of 2)
168
Figure A43. XRD result (1 of 10) (CH1003, 5.6-5.9m, highly weathered shale)
Figure A44. XRD result (2 of 10) (CH1003, 12.75m, slightly weathered shale)
169
Figure A45. XRD result (3 of 10) (CH1004, 8.2-8.65m, moderately weathered shale)
Figure A46. XRD result (4 of 10) (CH1004, 11.22m, slightly weathered shale)
170
Figure A47. XRD result (5 of 10) (CH1005, 5.5-6.1m, highly weathered shale)
Figure A48. XRD result (6 of 10) (CH1005, 7.15-7.3m, highly weathered shale)
171
Figure A49. XRD result (7 of 10) (CH1005, 9.2-10.7m, slightly weathered shale)
Figure A50. XRD result (8 of 10) (CH1007, 2.0-2.6m, highly weathered shale)
172
Figure A51. XRD result (9 of 10) (CH1009, 2.4-2.7m, highly weathered shale)
Figure A52. XRD result (10 of 10) (CH1010, 0.6-1.2m, highly weathered shale)
173