Chapter 3

Recognition
and
Identif ication
Harold T. Rib and Ta Liang

The adage that half the solution to a problem is the recogni- The use of aerial techniques for evaluating landslides is
tion that a problem exists is especially appropriate to land- emphasized in this chapter because of their proven value
slides. The recognition of the presence or the potential de- and the unique advantages they offer. No other technique
velopment of slope movement and the identification of the can provide a three-dimensional overview of the terrain
type and causes of the movement are important in the de- from which the interrelations existing among slope, drain-
velopment of procedures for-the prevention or correction age, surface cover, rock type and sequence, and human ac-
of a slide. tivities on the landscape can be viewed and evaluated. In
Several basic guidelines developed through years of ex- addition, the availability of new types of aerial imagery,
perience in investigating landslides form the basis for the including satellite, infrared, radar, and microwave radiom-
present-day approach to landslide investigations. etry, extend the advantages of this technique. (In this
chapter, the term imagery is used to describe .those data
Most landslides or potential failures can be predicted collected from sensor systems other than cameras.)
if proper investigations are performed in time.
The cost of preventing landslides is less than the cost TERRAIN EVALUATION FOR
of correcting them except for small slides that can be LANDSLIDE INVESTIGATIONS
handled by normal maintenance procedures.
Massive slides that may cost many times the cost of Basic Factors
the original facility should be avoided in the first place.
The occurrence of the initial slope movement can Recognition and identification of landslides are as complex
lead to additional unstable conditions and movements. as are materials and processes that cause them. The basic
causes of sliding movements are given in Chapter 2. Be-
This chapter discusses techniques for recognizing the cause, as Chapter 2 states,a landslide can seldom be attrib-
presence or potential development of landslides and the ob- uted to a single definite cause, the overall terrain must be
servable features that aid in identifying the types of slope analyzed and the individual factors and the interrelations
movements and their probable causes. The techniques dis- among those factors distinguished before a potential slope
cussed include (a) a review of topographic maps and geo- movement can be recognized and identified. For this pur-
logic, pedologic, and engineering reports and maps; (b) the pose, the factors contributing to slides are more conveniently
analysis of aerial photography and other forms of aerial grouped into categories relating to features that can be de-
images; and (c) preliminary field reconnaissance surveys. lineated on maps and photographsor that can be measured
These three techniques complement one another and to- and quantified, rather than into the mechanisms of failure
getherforni the, basis of the preliminary analysis in a land- listed in Chapter 2. The basic factors considered in evaluat-
slide investigation. The follow-up detailed field investiga- ing the terrain and the major elements included within each
tions required to accurately delineate the landslides or are given in Table 1 and discussed below.
landslide-prone areas, to identify the causative factors, and
to determine the physical and chemical properties required Geologic Factors
for the design of corrective measures are discussed in sub-
sequent chapters. The present-day landscape—including its topography, geo-

34
logic structure, and composition—is the result of millions of susceptible to slope movement because of the nature of
years of development and modification. The topographic their development and the stage of their evolution.
features exposed on the surface are relatively young in age,
but the geologic structures and composition from which Environmental Factors
the features were carved can be quite old. For example,
the structural features that characterize the Rocky Moun-
tains culminated at the close of the Cretaceous period (ap- The development of landforms and the occurrence of slope
proximately 60 million to 70 million years ago), but little movements are greatly affected by environmental factors.
of the topography in that area dates beyond the Pliocene Significantly different landscapes develop from the same
(several million years), and the present canyons and details geologic materials in different climatic zones. For example,
of relief are of Pleistocene or Recent age (less than 1 mil- a limestone bedrock will usually be found as a cliff or ridge
lion years old) (137). Thus, to properly evaluate the in an and area, but will commonly form a low undulating
present-day landscape requires an appreciation of the geo- plain in a humid area. The former situation would be much
logic and climatic changes that occurred during recent geo- more susceptible to landslides than the latter. Similarly,
logic periods (a determinant of topography) as well as an landslides occurring in arid regions are usually distinct and
understanding of the development of the geologic structure easily recognized because of lack of cover, while those oc-
and composition. The science that deals with landscape de- curing in wet tropical climates are weathered, more subdued,
velopment is geomorphology. covered by vegetation, and difficult to discern.
The basic unit of the landscape distinguished in geomor- Variations in microclimate, such as differences in altitude,
phology is the landform. The distinctive development of exposure to moisture-bearing winds, and exposure to sun-
landforms depends on three factors: (a) initial composition light, can cause significant differences in the geomorphic
and structure; (b) processes that act to modify the initial processes. Various observers have reported that some
composition and structure; and (c) stage of development. south-facing slopes of east-west valleys in the northern
A change in any one of these factors will produce a uniquely hemisphere are less steep than adjacent north-facing slopes.
different landform. North-facing slopes have snow cover longer, experience
To evaluate landforms, one must understand their geo- fewer days of freeze and thaw, retain their soil moisture
logic composition and structure including mineralogy and longer, and probably have a better vegetative cover, all of
lithology; physical hardness of the constituent materials which result in less active erosion and steeper slopes (3.37).
and their susceptibility to weathering; mode of deposition Numerous investigators have demonstrated that there is
and subsequent stress history; structural attitude and pres- a relation between slope movement and precipitation.
ence of structural discontinuities and weaknesses such as Záruba and Mend (3.40) evaluated rainfall records for sev-
joints, bedding planes, faults, and folds; and permeability eral different sites in Czechoslovakia covering periods of
of constituent materials or layers. 50 to more than 75 years. They were able to correlate the
The many physical and chemical ways by which the orig- years of heaviest precipitation with the most active periods
inal composition and structure are modified are called geo- of slope movements.
morphic processes. The most important of these processes Sudden changes in the landscape, often represented by
are associated with changes due to (a) actions of water, slope movement, have resulted from catastrophic occur-
wind, or ice; (b) weathering, mass wasting, and erosion; and rences, such as earthquakes, hurricanes, and floods. Figure
(c) diastrophism and vulcanism. 3.1 shows numerous slides that occurred in western Virginia
The modification and eventual destruction of the land- along the path of hurricane Camille in 1969. These slides
forms are considered to occur in stages that geomorpholo- were rapidly documented from the analysis of aerial photo-
gists generally designate as youth, maturity, and old age. graphs taken after the storm (3.39). Other case histories
Qualifying adjectives such as early and late are often used documenting the effect of catastrophic occurrences are
to designate substages. Chronological age is not inferred, referenced in Chapters 1 and 2. Undercutting by stream
but rather a relative stage of development. and wave action and the erosion and mass movement of
The concept of classifying distinct landforms is of prime slopes by seepage, wetting and drying, and frost action are
importance in recognizing and identifying landslides. Ex- also well-known environmental phenomena resulting in
perience has demonstrated that certain landforms are more slope movement.

Table 3.1. Basic factors considered in evaluating terrain.

Factor Element Examples

Geologic Landform Geomorphic history; stage of development

Composition Lithology; stratigraphy; weathering products
Structure Spacing and attitude of faults, ioints, foliation, and bedding surfaces

Environmental Climate and hydrology Rainfall; stream, current, and wave actions; groundwater flow; slope exposure; wetting and
drying; frost action
Catastrophes Earthquakes; volcanic eruptions; hurricanes, typhoons, and tsunamis; flooding; subsidence

Human Human activity Construction; quarrying and mining; stripping of surface cover; over loading, vibrations

Tempora a
aCommon to all categories and factors

35
 Human Factors way to initiate preliminary landslide investigations. Geo-
morphologists have divided various regional areas into phys-
Human activities, such as construction, quarrying, and min- iographic regions, that is, regional areas within which the
ing, have caused drastic landscape changes. Much has been method of deposition of earth materials and soils is approx-
said about the aesthetic damage (wastelands and scarred imately the same, the landforms are similar, and the climate
landscapes resulting from strip mining and open-pit quarry- is approximately identical.
ing) that human activities have caused, but far more damag- Early efforts to rate the landslide severity of the various
ingto life and property have been the instabilities occasioned physiographic regions of the United States were reported
by those activities. Oversteepening of slopes, removal of by Baker and Chieruzzi (3.3). Their ratings of landslide se-
support, removal of protective cover, overloading, blockage verity were based on information gathered for the various
of drainage, increasing moisture levels in the ground, and physiographic regions with regard to frequency of occurrence,
vibrations are some of the human activities that have caused size of moving mass, and dollars expended per year. This
slope movements. information was obtained from responses to a questionnaire
by state highway organizations and from various case his-
Temporal Factor tories and published records. Baker and Chieruzzi further
noted in their development of the correlation of landslide
Time is common to all other factors. It is a basic condition severity to physiographic regions that specific geologic for-
of nature to wear away the high areas by weathering, ero- mations were usually associated with the landslides in par-
sion, and mass wasting and to fill in the low areas in a pro- ticular regions. A listing of some of the more common
cess of leveling. Even the occurrence of landslides by cata- landslide -susceptible formations is included in their report.
strophic events normally can be related to conditions that A map by Radbruch-Hall and others (3.24) is a recent
have been building during a period of time. The time pe- effort at rating the severity of landslides in the United States.
riod of importance is the stage of development rather than This map, a portion of which is illustrated in Figure 3.2,
an absolute time period. For example, for a given geologic shows areas of relative incidence of landslides and areas sus-
material, evolution from youth to old age proceeds more ceptible to landslides. The accompanying text discusses
rapidly under tropical wet conditions than under arid con- the slope stability characteristics of the physiographic re-
ditions. Thus, in the same elapsed time, the landform unit gions of the United States and the geologic formations and
in a tropical wet area will be much further advanced than a geologic conditions that favor landsliding in the various
similar unit in an and area. provinces.
The stage of development achieved in a given area is ex- Figure 3.2, together with the information provided by
pressed by its topographic characteristics. Thornbury Radbruch-Hall and others and by Baker and Chieruzzi, are
(3.37) lists typical landscape characteristics for each stage offered only as a guide for use in preliminary evaluation of
of development for various idealized geomorphic cycles. landslide potential from a regional concept. They are not
The following major topographic features are typical of intended to depict precise boundaries and conditions. The
the respective stages of development in the common fluvial delineation of areas of low incidence means not that exten-
geomorphic cycle. sive landslides do not occur in those areas but that they are
negligible in comparison with occurrences and magnitudes
I. In youth, topography is relatively undissected, and of those in the other areas. It should also be recognized
only a few streams exist. Interstream tracts are extensive that, even though as noted in these reports certain geologic
and poorly drained. Valleys have V-shaped cross sections, formations are normally associated with landslides, no for-
and their depths depend on the altitude of the region. mation has developed slides throughout the entire extent
Waterfalls or rapids exist where streams cross resistant rock, of its outcrop area. A more detailed investigation is re-
In maturity, a well-integrated drainage system de- quired to pinpoint the actual slides or vulnerable locations.
velops. Topography consists mostly of hillsides and valley-
sides. Drainage divides are sharp, and the maximum possi- Landforms Susceptible to
ble relief exists. Vertical cutting ceases, and lateral destruc- Landslides
tion becomes important.
In old age, valleys are extremely broad and gently The designation of a specific landform connotes both a ge-
sloping both laterally and longitudinally. Development of netic classification and a type of landscape. For example,
floodplains is considerable, and stream meandering prevails. a sand dune landform denotes deposits formed by wind
Interstream areas have been reduced in height, and stream movement and sorting, which form unconsolidated, smooth,
divides are not so sharp as in maturity. flowing hills and ridges. An appreciation of the genetic as-
pects of landforms enables one to estimate their potential
According to Sharpe (3.29) and others, landslides are most susceptibility for movement. The type of landscape of each
prevalent during youth and the transition into maturity landform provides a basis for separating the various land-
when valley walls are steepest and down-cutting the most forms and thus recognizing those most prone to sliding.
active. Describing the genetic characteristics of landforms is be-
yond the scope of this presentation; however, excellent de-
Regional Approach to Landslide scriptions can be found in textbooks on geomorphology
Investigations (3.14, 3.37). In this chapter, landscape characteristics of
landforms are used as the basis for recognizing landslides
Analyzing the regional geology and terrain is the appropriate and landslide-prone areas.

I1
Figure 3.1. Catastrophic debris avalanches in Nelson County. Virginia, after torrential rains associated with hurricane Camille dumped
69cm (27 in) of water August 19-20, 1969. Light-toned scars on tree-covered hillsides indicate locations of debris avalanches.
Light-toned bands along stream floors are veneers of rubble and debris slide material.

Figure 3.2 Portion of USGS preliminary landslide overview map of Landslides can occur in almost any land iorm If the con-
the conterminous United States (3.24). ditions are right (e.g., steep slopes, high moisture level, no
vegetative cover). Conversely, landslides may not occur On
the most landslide-susceptible terrain if certain conditions
are not present (e.g., clay shales on flat slopes with low
moisture levels). Experience in observing and working with
various landforms, however, has demonstrated that land-
slides are common in some landforms and rare in others.
Table 3.2 provides a key to landforms and their susceptibil-
ity to landslides. The subdivisions are based on topographic
expression and, in the case of hilly terrains, also on drainage
patterns. This table gives only those landforms in which
landslides are most common and is not meant to be all in-
clusive. Illustrations of some of these landforms and a
brief description of their landscape characteristics are in-
chided later in this chapter in the section on landforms
susceptible to landslides. Almost all landforms rated as
highly susceptible to landslides are composed of alternate
layers of pervious and impervious materials (rock or soil),
a fact that needs to be specifically recognized.

Vulnerable Locations
The discussion of landslide-susceptible terrain to this point
has progressed (a) from a regional concept in which the Se-
verities of landslide occurrence for various physiographic
regions in the United States were indicated, (b) through
the listing of geologic formations in the United States
where landslides are common, and (c) to the rating of in-
AREA OF HIGH
dividual landforms as to their susceptibility to landslides.
4Z AREA OF HIGH
LANDSLIDE INCIDENCE
LANDSLIDE
SUSCEPTIBILITY

AREA OF MODERATE
This progression from the general, regional overview to
specific landforms can be carried one step further. Within
AREA OF MODERATE LANDSLIDE the susceptible landforms are certain natural, vulnerable
I tj LANDSLIDE INCIDENCE SUSCEPTIBILITY
locations that are conducive to sliding. Typical vulnerable
[ IAREA OFLOW
locations include areas of steep slopes, cliffs or banks being
_]
LANDSLIDE INCIDENCE undercut by stream or wave action, areas of drainage con-

37
 centration and seepage zones, areas of hummocky ground, several factors, such as a sudden heavy rainfall (Figure 3.1)
and areas of fracture and fault concentrations. Special at- or an excavation at the toe of the slope (Figure 3.3) may
tention should be directed to those locations when maps or result in sliding of the overlying soil mass. A study of cut-
aerial photographs are examined and field studies are per- slope failures in North Carolina (3.12) reported that about
formed. In addition, areas that have recently slid require two-thirds of the slides occurred in weathered soil mate-
immediate and close scrutiny because additional movement rials and one-third occurred in rock slopes.
may occur.
Cliffs and Banks Undercut by Streams
Steep Slopes or Waves

If slopes are steep enough,movement can occur on any land- Landslides are common in cliffs or banks that are subject
form. However, on landforms highly susceptible to land- to attack by streams or waves. If the banks are made up of
slides, other factors being equal, the steepest slopes are the soil or other unconsolidated materials, the weakest (and
most vulnerable locations. Only slopes of similar materials hence the most favorable) slide position is often located at
should be compared. For example, a slope cut in earth or the point of maximum curvature of the stream. At this
talus should not be compared with a rock cliff in an adja- point, the bank receives the greatest impact from the water.
cent landform, and slopes in bedrock generally are more In areas of rock outcrops, on the other hand, the exposure
stable, even though steeper, than slopes in adjacent soil at and near the point of maximum curvature is often hard
areas. rock, and the weak spots are to be found upstream and
The most common cause of the large number of slides downstream of this point. These conditions are shown in
that occur on steep slopes is residual or colluvial soils slid- Plate 3.1 and Figure 3.23.
ing on a bedrock surface. The loose, unconsolidated soils Many landslides occur along the edges of oceans and
cannot maintainas steep a slope as the underlying rock sur- lakes because of undercutting by waves. Locating the point
face and are, consequently, in a delicate balance. Any of of maximum water impact is more complex and difficult

Table 3.2. Key to landforms and their susceptibility to landslides.

Landform or Geologic Landslide

Topography Materials Potentiala
Level terrain
A. Not elevated Floodplain 3
B. Elevated
1. Uniform tones Terrace, lake bed 2
2. Surface irregularities, sharp cliff Basaltic plateau 1
3. Interbedded—porous over impervious layers
Lake bed, coastal plain, sedi- 1
mentary plateau
II. Hilly terrain
A. Surface drainage not well integrated
1. Disconnected drainage Limestone 3
2. Deranged drainage, overlapping hills, associated with lakes and swamps
(glaciated areas only) Moraine 2
B. Surface drainage well integrated
1. Parallel ridges -
a. Parallel drainage,dark tones Basaltic hills 1
b. Trellis drainage, ridge.and-valley topography, banded hills Tilted sedimentary rocks 1
c. Pinnate drainage, vertical-sided gullies Loess 2
2. Branching ridges, hilltops at common elevation
a. Pinnate drainage, vertical-sided gullies Loess 2
b. Dendritic drainage
(1) Banding on slope Flat-lying sedimentary rocks 2
(2) No banding on slope
Moderately to highly disseáted ridges, uniform slopes Clay shale 1
Low ridges, associated with coastal features Dissected coastal plain 1
Winding ridges connecting conical hills, sparse vegetation Serpentjnite 1
3. Random ridges or hills
a. Dendritic drainage
(1) Low, rounded hills, meandering streams Clay shale 1
(2) Winding ridges connecting conical hills, sparse vegetation Serpentinite 1
(3) Massive, uniform, rounded to A-shaped hills Granite 2
(4) Bumpy topography (glaciated areas only) Moraine 2
Ill. Level to hilly, transitional terrain
Steep slopes Talus, colluvium 1
Moderate to flat slopes Fan, delta 3
C. Hummocky slopes with scarp at head Old slide 1
Note: This table updates Table 2 in the book on landslides published in 1958 by the Highway Research Board (3.10, p.91).
a1 = susceptible to landslides; 2 = susceptible to landslides under certain conditions; and 3 = not susceptible to landslides except in
vulnerable locations.

38
Figure 3.3. Slides (1) caused by
undercutting of slopes by coal
stripping operations in Washington
County. Ohio (3.22).

- ;.c

., ,—
1;, '
.-_.I ' - 11•i '-

along lake and ocean shores than along stream banks. Fac- sometimes aided by the identification of near-surface chan-
tors to be considered include shape and slope of the shore- nels, wet areas, tall vegetation on the slope, and displaced
line, direction of wave action, and frequency and magni- or broken roads adjacent to the slope. Delineating the drain-
tude of storms producing large waves. Data obtained at age network, especially the presence of seeps or springs, is
different periods of time are often of value in the analysis extremely important for planning new construction. Many
of these factors.
Figure 3.4. Oblique aerial view of extensive seepage zones (1) on
Areas of Drainage Concentration scarp face of massive landslide in Kittitas County, Washington.
and Seepage Seepage was major cause of slide in these unconsolidated sediments.
Remnant of dry sand flow (2) is seen above scarp face and is re-
sult of seepage at higher level. Landslide disrupted highway at base
A survey conducted by the Federal Highway Administration
of slope and water canal carried through slope.
of major landslides on the federal-aid highway system in the
United States revealed that "water is the controlling or a
major contributing factor in about 95 percent of all land-
slides" (3.5). Thus, careful study of the drainage network
and areas of concentration or outfall of water is extremely
important. Close scrutiny of existing slide scars often indi-
cates that a line connecting the scars points to drainage
channels on higher ground. Such drainage may appear as
seepage water, which is responsible for the damage. An ex-
ample of this condition is shown in Figure 3.4
Seepage with subsequent sliding is likely to occur in
areas below ponded depressions, reservoirs, irrigation canals,
and diverted surface channels (Figure 3.4). Such circum-
stances are sometimes overlooked on the ground because
the water sources may be far above the landslide itself,but
they become obvious in aerial photographs. The importance
of recognizing the potential danger in areas below diverted
surface drainage, especially in fractured and porous rocks,
needs particular emphasis. Extensive field experience has
proved repeatedly that within an unstable area one of the
most dangerous sections is the lower part of an interstream
divide through which surface water seeps from the higher
to the lower stream bed. The recognition of seepage is

39
 Figure 3.5. Landslide in highway
fill section of Ohio-22 in Jefferson
County. Fill slid en masse as
block slide with fill above block
slumping into resultant hole as dNL
aben. Ohio Department of
Transportation staff report
attributed failure to saturation
of colluvial materials on side
slopes beneath fill by springs
outcropping on hillsides. Few
small drainageways commencing
part way down slope (1) and
small slide (2) on natural hillside
indicate presence of seepage
zones and instability of natural
slopes.

highway fills have failed because the natural drainage was ing should not discourage construction unconditionally, be-
blocked by the fill and allowance Was not made for drain- cause the unstable condition of the past may not necessarily
age. Figure 3.5 shows such a situation. exist today. In some parts of the western United States,
for example, railroads built in extensive old landslide areas
Areas of Hummocky Ground
have been stable for a long time. Nevertheless, special care
The presence of huminocky ground whose characteristics should be taken in construction on old slides. Figure 3.6
are inconsistent with those of the general regional slopes shows aerial photographs of the construction of a road on
an existing slide and the consequences.
and the presence of a scan) surface (sometimes not very dis-
tinct) at a higher elevation are often indications of an exist- Areas of Concentration of Fractures
ing landslide. The older the landslide is, the more estab- and Bedding Planes
lished the drainage and vegetation become on the slide mass.
The drainage and vegetation thus help in determining the Movements of slopes may be structurally controlled by sur-
relative age and stability of the slide. faces or planes of weaknesses, such as faults, joints, bedding
Once an old landslide is found, it serves as a warning that planes, and foliation. These structural features can divide a
the general area has been unstable in the past and that new rock mass into a number of individual units, which may act
disturbances may start new slides. However, such a warn- independently of one another. The result can be an incor-

40
Figure 3.6. Before (top) and '
after (bottom) construction -
of Ohio78 in Monroe
County on unstable ground
(3.22). Before construction,
presence of hummocky

unstable a terrain q&t.'

R&ocation of side road to -
meet new grade of highway
r
placed side road on slide
prone slope which resulted // \ -' 4
in small slide of embankment
(2) Crack crossing road
diagonally, seen as faint dark
line (3) is site of possible "\4i.v*
future movement

I AN

.V

,. -.'. -' . q• ,

.--2
Vk

— ?'
--• T /1 it

\
'
4-4
(

rect slope design because the designer considered the rock very steep slope. The scarp face rapidly ret rogrades uphill
to be one continuous mass rather than a series of individual by continued slumping until a more stable condition occurs.
blocks. These planes of weakness also provide egress for Thus, a new landslide should be investigated as soon as pos-
water and vegetation, which further weaken the individual sible not only to determine corrective measures but also to
units by wedging action, frost heave, and reduction of slid- look for evidence of possible continued movement. The
ing friction. A careful search should be made to locate most significant sign of possible further instability is the
areas with close spacing of faults and joints, especially presence of cracks on the crown of the slide. Figure 3.7
where they cross and divide the rock mass into smaller (top) shows some telltale signs at a recent slide. Figure 3.7
blocks. (bottom) shows the slide area 5 months later. Additional
movement occurred in the area where the telltale signs were
Recent Landslides evident in the earlier photograph.

The occurrence of a landslide does not mean that final ad- Procedure for Preliminary
justments to the unstable conditions have occurred and no Investigations of Landslides
further movement will occur. In many cases, especially in
unconsolidated deposits, the materials present in the scarp Not only must the presence or potential development of
face remain in an unstable condition because they are on a landslides be recognized, but the types and causes of

41
Figure 3.7. Sequential
photography for use in I
evaluation of recent slide
development along US-95 in
Idaho County, Idaho, and
4
investigation of possible further
movements. Aerial photography
(top) taken in February 1974
thortly after occurrence of
slide (1) shows features present
at crown of slide that forewarn
of further movement fissures
- IF .
-

above scarp face (2), loose

colluvial materials on steep -.
4
slopes at crown (3), and - 4 _________
-
evidence of water feeding into
\ ' -'
these loose materials (4). . •. -, ,,
Photography obtained 5 months
lat:r in July 1974 (bottom)
shows that additional slide

in photograph above

40

lie

-.. p

I,

' IF

.:.4

movement must be identified so that preventive or correc- A typical procedure, which has been developed over the
tive action can be taken. Chapter 2, and specifically Figure years, for performing terrain investigations is used for a va-
2.1 ,gives the classification of the various types of landslides riety of programs, such as engineering soil surveys, location
based on the dominant types of movement and the types of of construction materials, and landslide investigations. The
materials involved. In this chapter, the emphasis is on how following steps are generally performed. Some excellent
those various types of slides appear on the various data case studies demonstrating the application of this step-by-
sources used in terrain investigations, step procedure are reported by Mintzer and Struble (3.20).

42
 Obtain aerial photography and other special coverage. the terrain as to landslide susceptibility is included.
Small-scale photography for regional overview and large- Maps have certain inherent advantages for landslide
scale coverage for detailed study are obtained from available studies. They are prepared at a uniform scale and, there-
sources. If landslides are already present, photographs taken fore, are subject to direct quantitative measurements. Fur-
both before and after the occurrence should be obtained to thermore, planimetric information is often included, thereby
aid in locating causative factors that may have been obliter- minimizing uncertainties in superimposing other informa-
ated by the slide. Special types of photography should be tion. Their disadvantage is that they are outdated as soon
acquired as needed (the section on use of aerial photog- as they are published. Unless the maps are updated period-
raphy gives details on types and scales of photography ically or no changes occur, the maps will not show the most
needed). recent terrain and cultural features.
Review literature and maps. A review is made of ex- The various maps discussed in this section are published
isting topographic maps, geologic maps and reports, water mainly by governmental and public agencies and may be ob-
well logs, agricultural soil survey reports, and other litera- tained free or at nominal costs. The published maps can
ture to develop an area] concept of the area under investiga- also be viewed at many governmental and university li-
tion. braries and in some large public libraries. Included in the
Analyze the photography and other special coverage. following discussions of the various types of maps is infor-
The patterns on the photographs are analyzed, and land- mation on map sources.
forms are identified and related to the areal concept de-
veloped from the literature review. A careful examination Topographic Maps
is made of all vulnerable locations. Existing and potential
landslide sites are delineated, and a three-dimensional con- Topographic maps show the size, shape, and distribution of
cept of the terrain is developed. In this step, a mosaic is features of the surface of the earth. These maps depict the
usually prepared and landform and drainage maps are de- features of relief, drainage, vegetation, and culture, usually
veloped. Sites that offer the best opportunities for confirm- in a color format. Some of the more recent topographic
ing or extending the information developed are selected for maps prepared in the United States use orthophotographs
field verification. as the base for depicting drainage and culture as they actu-
Perform field reconnaissance. A field reconnaissance ally appear and superimpose contours on the orthophoto-
of the area is performed to verify the three-dimensional con- graphs to depict the relief; these are published as ortho-
cept developed in the earlier steps, to fill in information in photomaps.
questionable areas, and to observe the surface features and Major landslide areas that are clearly evident are some-
details that could not be determined from other data times labeled on topographic maps. On some maps, the
sources. The type of landslide movement is classified from boundaries of the slide and arrows pointing toward the di-
the available data and from a study of the surface features rection of movement are also shown. Small slides, the types
and crack patterns observed in the field. more commonly encountered in highway and other engi-
Conduct final analysis and plan field investigations. neering works, are not usually labeled on such maps. Iden-
A final analysis of the photography is performed based on tification of these smaller slides or unlabeled larger slides
the results obtained from the field reconnaissance. A deter- can be accomplished by noting the following features on
mination is made as to what additional information is topographic maps (Figure 3.8):
needed to fully define the three-dimensional model of the Topographic expression observable, for example,
site and what samples and test data are required to design steep slope (closely spaced contours) at scarp head of slide,
the corrective procedures. Based on those needs, the field hummocky topography in slide mass (irregular nonsymmet-
investigation program is planned (this process is described rical contour patterns with shallow depressions), and pres-
fully in Chapter 4). ence of detached mass and flow characteristics at the lower
end;
MAP TECHNIQUES FOR Wavy contour lines, uneven or broken local roads,
LANDSLIDE DETECTION and other artificial lineaments such as transmission lines;
and
The acquisition and the analysis of various types of maps Minor movements or irregularities at "vulnerable lo-
constitute one of the first steps in landslide investigations. cations," as discussed in the section in this chapter on vul-
Maps depicting topography, geology, agricultural soils, and
nerable locations.
other special terrain and cultural features are available for
many areas of the world. The maps represent, in a two- The potential for identifying landslides on topographic
dimensional format, the authors' interpretations of terrain maps is essentially limited by the scale and contour interval
and cultural features determined from field investigations, of the maps. On small-scale maps, even moderate-sized
a study of available literature and photography, and map- landslides significant to engineering works may be such
making procedures. The nature and quality of the informa- microfeatures that they are not identifiable. The U.S. Geo-
tion relating to the presence of landslides or potential for logical Survey (USGS) publishes topographic maps at various
landslides that can be derived from existing maps depend scales from 1:20 000 to 1:1 000 000. Indexes to the status
on the purpose, type, scale, and detail used in preparing the of topographic mapping for the United States are available
map. From most maps, only a general indication of land- free from USGS. Information on status of mapping in
slide susceptibility can be derived. In recent large-scale other countries can be obtained from similar mapping or-
maps, more detail on locating existing landslides and rating ganizationsin the respective countries.

43
Figure 3.8. Portion of USGS topographic map of Laguna Quadrangle, New Mexico, on which typical landslide characteristics described in
text are evident (1). Figure 3.31, which shows landscape within same region but not same area, illustrates appearance of this landslide
topography on aerial photograph.

C U BE R.ô—
GJANT.

Geologic Maps and Reports particularly when surface and near-surface fo rnial ions are
of significance. Bedrock geology maps show only the bed-
The principal sources of geologic information in the United rock and do not indicate surficial deposits. These maps
States are the maps and publications of USGS and the state must be used with care to avoid erroneous assumptions
geologic surveys. Government geologic maps range from about surface conditions. For landslide investigations, bed-
general, small-scale maps covering en tire continents to rock geology maps are useful when lit tie or no soil overlics
large-scale maps covering counties and quadrangles. Not the bedrock or when slides are deep-seated.
all the geologic literature published annually. however, is Other maps of special importance for landslide investi-
found in government publications. Numerous articles are gations are the USGS Miscellaneous Investigation Series
published by geologic organizations. whose publications and Miscellaneous Field Studies Series. Maps are now being
are referenced in two periodicals: Bibliography ojNorth produced that directly indicate landslides and rate the va-
American Geology, published by USGS, for areas in North riotis terrain features as to their susceptibility to sliding.
America, the 1-lawa iia n I sian (Is. and Guam: and Bibliography Figure 3.10 shows a portion of this type of map. Other
and Index of Geology Exclusive of Non/i A inerica, pub- maps in these series indicate items or features closely re-
lished by the Geological Society of America. for other areas. lated to landslides, such as engineering geology, \vater re-
In addition, USGS publishes iiidexcs of- ologic maps and sources, and groundwater. These maps are also of some
listings of maps and reports for each of the states. value in landslide investigations. Unfortunately, most of
The most extensive fo mi of geologic maps published in the maps in these miscellaneous series are not well catalogued
the United States is the quadrangle map series, which in- and are difficult to locate.
cludes surficial geology maps, bedrock geology maps, and Even though geologic maps and reports may not specifi-
standard geologic maps depicting both surface and bedrock cally indicate the presence of landslides or landslide-
units present in the map area. Although the recent quad- susceptible terrain, this informal ion can be extracted from
rangle maps show landslide deposits, only relatively large the geologic literature by noting the physiographic regions,
slides are shown because of the small scale and large con- the geologic format ions mapped, the topogra hic a it d strati-
tour intervals. An example of this type of map is shown in graphic setting, and the landforms present. As discussed in
Figure 3.9. The surficial geology maps show only the sur- the earlier section on regional approach to landslide invest i-
face materials present and are usually limited to areas ex- gations. certain physiographic regions and geologic forma-
tensively covered by complex surficial deposits, such as in tions are noted for landslides. The presence of these suscep-
glaciated areas. These maps are useful for landslide st udies. tible format ions can be determined from the geologic lite ra-

44
Figure 3.9. Black-andwhite repoduction of a portion of USGS geologic quadrangle map GQ-970, Tyler Peak Quadrangle, Washington.
Slide areas are identified by map symbol 01 and are outlined by dashed line. On original color map, surficial deposits including slide areas
are shown in yellow and are more easily distinguishable.

-
I-

\ •;:\
Qsu Ql 1 f1
Q.l

L QOrn

Surficial deposits
Qsu, undil'ided .uo:ficial deposits
QI, landslde deposits
Qym, young alpine moraine deposit; pattern
denotes morai -ne crest
Qal, alluvium
Qt. talus
Qc, continental glacial deposits
Qom, old alpine moraine deposit; pattern
deiwtes moraine crest

Figure 3.10. Portion of USGS Miscellaneous Field

Studies Map MF-68513, Susceptibility to Landsliding,
Allegheny County, Pennsylvania.

7_F -- -4D'3? 30

SLOPES WITH MODERATE TOSERVERE STEEP SLOPES MOST

I SUSCEPTIBILITY TO LANDSLIDING SUSCEPTIBLE TO ROCK FALL
SLOPES WITH SLIGHT TO MODERATE, LOCALLY
SEVERE, SUSCEPTIBILITY TO LANDSLIDING MAN-MADE FILL

GROUND WITH LITTLE SUSCEPTIBILITY GENERAL DIRECTIONS OF

DTO LANDSLIDING I DIP OF ROCK LAYERING
C
______ PREHISTORIC LANDSLIDES

45
Figure 3.11. Portion of soil survey report for Chilton County, Alabama (3.18). Information on susceptibility to sliding in situ is indicated in column, Soil Features Affecting Highway
Location. Presence of possible seepage zonesis also designated in this column. Some indication of slide susceptibility of soils placed in fill or embankment section is indicated in columns,
Road Fill Material or Farm Ponds, Embankments.

8uitabtbty no o oouree of— Soil f tisree nllnrtint_ -

8oU .rrles and mop nymbolo

Topsosl dand and gravel Rood fill muteal
n Htghy
o'a locut,00
r... ponds
- _____________________________ Agricultural Irrigation Terrore. and Waterways
suduee layer) drainage diversion,
I(r'ervoir sr005 Embankment,

tars-roe: LoB, L.W. InC. LoC2.

LcD?. L.O. l'n02. L.F.
Fair togood -------- -Poor: no nastier
gravel available.
Fai.t,, Ito,.,: high
clay content; vert'
Sosreptibletoolittlng;
lair tettTic-.ti.ln.rltng
Moderate to sIns'
ovepage.
! low to moderate
these strength;
No drainage needed.. Slopes 01210 11 percent; High erodibilils... High erodibility.
oso,lerato' inlokr;
Fr Ll..,.,v.'ll tart of LwC2, plootie. copocitv; .Iot0s.' poor stability. nedisan available
Lw02, and LwP, refer to bowel outer capacity.
series. -
2fcLau,jn: McB, McC. M.D -------- -Fair --------------- -Fair In pane: sand Good--------------- No lin,ilntions except Itapid veepage, - - ' Moderate shear No drainage needed.. Ste1,e of 2 to tS .eeevot; Slight credibility, Moderate ceodt-
ot a depth .tf 50 elopen. . strength; fair r:,1,i,l a tab,-: no' 1st bility.
orb.-.; fattr' Ito . otalolily. o',linr,,, :ss'ailat,l,
gravel available. seater capacity.
Mdioon: MdB2. MdC2. MdD2 ...... Fair to good -------- - P,n,r: n. sand a- Fair to good -------- - ll,-deoek at a tl,-1sth of 20 lno' to toodrrtte Mod-rate shear No deaionpr needed, SIo,l,'rot,'ietab-e; .504. . StaIn-,of Sto H. 111gb crodihility.
geavel available, to 50 isis-bc,. ala-pope. uo.enth; fair ,'rti' in'roo':ilsiiit; .s'rs'.'t,t;n,oder. I
- oi:slstlity. t',liao,as'silalste ntv to high
Water rapacity. eroslilality.

Masada: MnA --------------------- - Good to lair -------- - Poor: no sand or Fair to good------- - Oecnsii,sal flooding--------elan ,reiage.......' Fair stability------- - No dr:iinage needed; Sloi.,'s 010102 pert-en?; No terraces or No wn(erw&y*.
gravel availobk-. l,edroek at a o,o,l.'r;tle intake; diversions oeodsd,
• depth of 30 In 50 I eeadrratc asnilable seeded,
inches. water capacity.
Myatt: Ml. MpA------------------ Foir---------------- Pane: no sand Sr Fair: high water high seater t5lsle----------.'lOO'St'5lt5ge - - - - Fair to good 111gb n'atvr tattle; : Sit.15,'. of 0 to 2 percent; No ierraeeo ne No o'aterwayn
For Bibb part s.f MyA, refer to gravel available, tattle; th,oding. viability. . e,otivrate to toad. : locate to elan' diversions seeded.
Blbbneeies. ' I i croivlyoioo'
prr.neabrlrty;
.ske; tone soil for
crops.
needed.
outlets difficult
to eatablish.
Ora: OeA.OrB. 0e62. 0cC. OrC2... Good ---- ---------- Poor: no sand oe Good- ------------- - Seepngn in eats aloof Moderate seepage.) Fair nhearseresgth; No drainage nrcdcd.. Slopes of 010 10 inches; Moderate erodi.
gravel available. Modrato erodi.
the lrogspan layer. fate stability. . ssodvraieintcke; bility. bitity; fe*glpso.
t,.enteotsilty; medium
os'ailablo water
capacity.
Rock land: Ru.
No Interpretations; properties too
senolable.
nflo,ton. R.A. RoB. RaC. RsC2. Good ............... Poor: no sand or Good ------------- - No limitation, except Moderate In rapid : Moderate shear No drainage needed,,' Slops's of 010 IS inches; Moderate erodi-
R,D2, RuE. RIP, gravel available, elopes. seepage. , strength; pone to Moderate erodt.
,,ndeeatn to ropid bility. bility.
For Bhubuta nnd Troop to parts of . lose stability, irtalse; i,,cdiam
RIE and RIP, refe, Shabuta : available a'oter
and Troop melee, rapacity. -
Baffell: SuB. SaC. S.D .............. Fair to poor: Fair to poor: us- Good ............... No listilations except .Rapid seepage. .. Poor to fair otability. No drainage needed,,' Slopes of 210 IS inehes;
grave ly. dcrinto b- sand Moderate eeodt. Gravelly; lIttle
. elopes. notlrrate to ropid
and gravel at a I i bility. motstare.
, lislatin; medium to
depth of 4 toO '
feet; grovel gand
for road eon.
I I
-.
lose available .tcr
capacity,
otroctina tnatcriol.
8hubuIn: ShD2 ..................... Fair to good......... Poor: no sand or . Fair: high clay Sosceptible to sliding; Slow seepage.-..... Lose shear strength; No drainage aeedrd. Slopes of 2 to 15 inches' I High crodibility..
gravel available---- contest; very seepage in cttts; . lair to poor Highcoodiblltty.
moderate to slow -
plastic, moderate sheiol.. , ntohility. 'itstake; medium
well poteotiat. ' available macor -
capacity,
ture. In addition, as noted in the sections oil landforms sus- provide lists for the respective states. For copies of the pub-
ceptible to landslides and on vulnerable locations, certain lished reports, the unpublished field sheets, and the most
landlorms and topographic settings are especially suscep- up-to-date information about surveys in progress, one should
tible to landslides. For example, one of the most favorable check with the local soil conservat ion service office in the
settings for landslides is the presence of permeable or solu- county of interest. The Canada Department of Agriculture
ble beds overlying or interbedded with relatively impervious provides lists of Canadian soil survey maps and reports.
beds in an elevated topographic setting. The beds could be Sources of information on soil surveys in foreign countries
either rock or soil. Conditions such as this can be deter- include World Soil Map Office, SCS: libraries in major ag-
mined from the geologic literat ure. ricultural colleges and universities: and departments of agri-
culture in other countries.
Agricultural Soil Survey Reports
Special Maps and Reports
The Soil Conservation Service (SCS). U.S. Department
of Agriculture, in cooperation with state agricultural experi- The susceptibility of the terrain to landslides is also dis-
ment stations and other federal and state agencies, has per- cussed in special maps and reports prepared by various or-
formed and published soil surveys since 1899. The agricul- ganizations. Included in this group are engineering soil maps,
tural areas in the United States are, therefore, covered by engineering soil surveys, geologic reconnaissance surveys, and
fairly detailed soil surveys, and much of the remaining area engineering guides to agricultural soil series.
is mapped in less detail. Similarly, agricultural areas in Engineering soil maps depict the relation of the landforni
other countries are covered by soil surveys with varying and engineering soils. The lanclform - which const it utes the
degrees of detail and are available in published form. foundation of this mapping, is subdivided on the basis of
Although SCS soil survey reports are compiled primar- its engineering characteristics, such as soil textuic, drainage
ily for agricultural purposes, those published since 1957 are conditions, and slope category. Although these reports gen-
of special value for landslide investigations. Most of these erally do not iidicate landslide conditions, estimates of
soil surveys contain information on the engineering uses of landslide susceptibility can be determined by evaluating the
the soils. In addition, most of the soil maps published in landfornl types, soil characteristics, and slope conditions.
these reports are printed on a photomosaic base at a scale In the United States, engineering soil mapping is performed
of I : 15 840 or 1:20 000, and the soil information is super- at a variety of regional levels. For example, New Jersey and
imposed on the mosaic. This provides for easier identifica- Rhode Island are completely covered: Indiana and Tennessee
tion of natural and cultural features. are preparing maps on a county basis: and Kansas, Louisiana.
Soil surveys provide a three-dimensional concept of the and New Mexico prepare maps on a project-by-project basis.
land and contain information about the areal extent and For most engineering projects, preliminary engineering
vertical profile for each soil unit. The insight given of each soil surveys and geologic reconnaissance surveys are per-
soil unit and its relation to adjacent soil units enables one formed. In many of these surveys, landslides or landslide-
to estimate the environmental setting and potential stabil- susceptible areas are depicted prior to construction. Figure
ity. 3.12 shows ,- in example of' such an investigation performed
The engineering sections included in the recent reports for a route location in Kansas. The results of the survey
contain engineering test data for the major soils mapped in influenced the final location of the highway (3.31).
the counties: a table of estimated engineering properties of Since large parts of the United States are covered by ag-
the soils, including depth to water, depth to bedrock, typi- ricultural soil surveys and extensive data are available for
cal profile, and grain-size distribution: a table of engineer- the basic soil units mapped (the soil series), several state
ing interpretation of the soils; and accompanying text dis- highway organizations have (lCvCIoped information on t he
cussing the special engineering features of the soils. Es-
tinlates of susceptibility to landslides for the Various soils
mapped can be made from the data furnished. In some Figure 3.12. Delineation of landslide terrain during preliminary
cases, these estimates are indicated in the table on engineer- route location in Lincoln and Ellsworth counties, Kansas, aided in
ing interpretations. Figure 3.11 shows a portion of such a minimizing damage due to landslides (3.31).

table from the Chil ton Cowi ty, Alabama. soil survey re-
'N
port (3.18). 1-laying identified from this table the soils sus- .
,—i'? J''"-.'
ceptible to sliding, one can then locate those soil units on
the photomosaic and thus delineate the most susceptible
areas.
4L/L TM c
Other county soil surveys and generalized reconnaissance
surveys can also provide useful information for landslide in-
vestigations, even though engineering sections are not in-
cluded. Those surveys provide information on parent ma-
terials and their geologic origin, climate, physiographic set-
ting, and soil profiles.
The status of availability of agricultural soil survey re-
ports in the United States is published annually by SCS in
List of Published Soil Surveys. In addition, those state soil
conservation services that publish their own soil surveys

47
engineering characteristics of the soil series mapped in their special applications, the additional data are not cost-
states. The information generally provided for each soil effective. That being the case, the ensuing discussion on
series includes origin,topographic setting, soil profile, drain- the use of remote-sensing techniques to recognize and iden-
age characteristics, and engineering significance for items tify landslides deals mostly with the use of aerial photog-
such as location, slope stability, foundation and pavement raphy. The special applications in which other forms of
support, and source of construction materials. Several remote-sensing data are useful are also discussed. Some
states, such as Indiana, Michigan, New York, and Ohio, comments are included on systems that have potential
have summarized this information in report form. In many value but have yet to be demonstrated for this application.
areas, and particularly in densely developed areas, existing
reports for geologic and engineering investigations may be Use of Aerial Photography
available and should be acquired for reference. Detailed
boring logs, well logs, and similar valuable soil and geologic The material in this section expands and updates Chapter 5
information can be obtained at little cost. in the book on landslides published by the Highway Re-
search Board in 1958 (3.10).
REMOTE-SENSING TECHNIQUES The interpretation of aerial photography has proved to
FOR LANDSLIDE DETECTION be an effective technique for recognizing and delineating
landslides. No other technique can provide a three-
Remote sensing is the collecting of information about an dimensional overview of the terrain from which the inter-
object or phenomenon by the use of sensing devices not in relations of topography, drainage, surface cover, geologic
physical or intimate contact with the subject under investi- materials, and human activities on the landscape can be
gation. The distance of separation might be as close as a viewed and evaluated. Aerial photographs at suitable scales
few millimeters, as in the case of nuclear moisture-density are available for almost all of the United States and a large
devices, or as far as 800 km (500 miles) or more, as in the part of the world. New photography of various types and
case of satellites. The devices range from cameras to vari- scales can readily be obtained. In addition, new techniques
ous scanners and radiometers. in production and interpretation processes have continued
Most remote-sensing devices collect information from to extend the advantages of aerial photography.
the electromagnetic spectrum (Figure 3.13), which extends The value of aerial photography in landslide investigations
through many decades of wavelength from very long radio has been reported by many investigators (3.13, 3.16, 3.19,
waves to extremely short gamma and cosmic radiation waves. 3.28, 3.31). The effectiveness of this technique was well
As shown in Figure 3.13, various types of sensors are avail- demonstrated by Maruyasu and others (3.16) in analyzing
able for obtaining information in the different regions of a mountainous region where landslides were common in
the spectrum. The Manual of Remote Sensing (3.25) gives Japan. Using photo-interpretation techniques, they iden-
greater detail on the electromagnetic spectrum and sensor tified 365 landslides. After the region was field checked,
systems. an additional 68 landslides were identified, resulting in an
The attractiveness of these systems to various investiga- 84 percent accuracy in overall identification. An accuracy
tors is that they greatly expand the scope of possibilities of 96 percent was reported in identification of landslides in
for uniquely and more accurately identifying subjects of in- areas such as paddy fields not covered by trees and shrubs.
terest. In addition to the sensors that provide information The major advantages of using aerial photography in
on the spectral reflectance phenomena of objects are those landslide investigations include the following:
that provide information on the thermal-emission proper-
ties of objects in the infrared region, the backscatter of Photographs present an overall perspective of a large
radar energy in the microwave region, and the reflectance area (when examined with a pocket or mirror stereoscope,
and fluorescence phenomena in the ultraviolet region. Ana- overlapping aerial photographs provide a three-dimensional
lyzmg spectral regions individually provides special data view);
about the properties of objects in that region (e.g., thermal Boundaries of existing slides can readily be delin-
characteristics or reflectance properties). eated on aerial photographs;
Collecting data simultaneously in several spectral regions Surface and near-surface drainage channels can be
and comparing the various responses in those regions, how- traced;
ever, provide more data than can be obtained in each region Important relations in drainage, topography, and
individually and increase the accuracy of interpretation. other natural and man-made elements that seldom are cor-
This approach is referred to as multisensor analysis and is related properly on the ground become obvious on the
based on the following principle: Although two or more photographs;
objects may have similar responses in one region, they will A moderate vegetative cover seldom blankets de-
not respond similarly in all regions. Thus, evaluating the tails to the photo interpreter as it does to the ground ob-
response of various objects in different regions of the server;
spectrum increases the ability to uniquely identify each Soil and rock formations can be. seen and evaluated
of the objects. in their undisturbed state;
In almost all engineering evaluations of various sensor Continuity or repetition of features is emphasized;
systems, the conclusion has been that photography is the Routes for field investigations and programs for sur-
best sensor system and provides the most information. Al- face and subsurface exploration can be effectively planned;
though other sensor systems provide some useful data that Recent photographs can be compared with old
cannot be obtained from photography alone, except for ones to examine the progressive development of slides;

48
Figure 3.13. Electromagnetic Frequency in cycles /second
spectrum and types of
remote-sensing systems used for 020 iO° 06 io0 02 to'° 08 02

I I I
collecting data in each major I I I I
INFRARED I I
spectral region. I I -j
I UV I I I MICROWAVE VHF I IF
RAYS X RAYS
I MEDIUM ond FAR I
I
I Z

E S SEE E P
P 8
0
Wovelenglh -
o I
-
2 2 0
00—
I I I I
en I I
I
I I

'N

Film in cameras Cameras with Solid-state Radar: rI Electromagnetic

Scintillation IScanners with )
counters: y rays filtered photo-
i infrared
sensitive i
detectors
I in scanners and
receivers in
scanners and
sum
techniques
spectrometers multipliers:
image orfhicons
i l
film: so id- radiometers radiometers
and cameras state detectors
in scanners and
I
with filtered
I I radiometers
I
I infrared film I I I I

Aerial photographs can be studied at any time, in for delineating the presence of water on the surface and for
any place, and by any person; and giving clues to subsurface water conditions by showing the
Through the use of aerial photographs, information vigor of the surface vegetative cover. This has made color
about slides can be transmitted to others with a minimum infrared film especially valuable for locating the presence
of ambiguous description. of seepage zones at or near the surface.
The format commonly used for landslide investigations
Factors Influencing Aerial-Photograph is 23 x 23-cm (9 x 9-in) vertical photographs taken with
Interpretation an aerial mapping camera. Consecutive photographs having
an average overlap of 60 percent are obtained to provide
Success in the application of aerial-photograph interpreta- stereoscopic coverage. In addition to their use for interpre-
tion techniques for landslide investigations depends on four tation purposes, these photographs can be used for prepar-
major items: (a) qualifications of the interpreter; (b) photo- ing maps of the slide areas by photogrammetric methods.
graphic parameters; (c) natural factors; and (d) equipment Details on flight planning and mapping are given in the
and analysis techniques used. Analysis by competent inter- - Manual of Photogrammetry (3.35). Oblique photographs,
preters with suitable equipment will certainly be an impor- such as those shown in Figures 3.1 and 3.4, are also ob-
tant factor in the successful application of this technique. tained for landslide investigations. The view they provide
However, a proper understanding of the photographic param- of the unstable slopes and valley walls is more unobstructed
eters and natural factors that can influence the data is im- than that provided in vertical photographs. In recent years,
portant to the proper use of aerial photography; these two oblique views of landslide areas are being obtained with
items are discussed below. small format, nonmapping cameras in low-flying planes and
helicopters; 35 and 70-mm cameras are commonly used.
Photographic Parameters Scale of photography can be critical to the interpreter's
ability to identify landslides in that it influences his or her
Photographic parameters include types of photography, for- ability to denote topographic features indicative of land-
mat, scale, coverage, quality, and availability.. General infor- slides. In some cases, the landslides are so massive [some-
mation concerning these parameters is given in many stan- times involving more than 4km3 (1 mile3)] of material
dard references, such as those published by the American that they are virtually impossible to detect on the ground
Society of Photogrammetry (3.7, 3.25, 3.30).The following or on large-scale photographs (3.9). Small-scale photographs
discussion covers-only those items pertinent to landslide in- (1:48 000 or smaller) are required in order to view stereo-
vestigations. scopically the full extent of massive slides. However, most
Aerial photographs typically used for landslide studies landslides encountered in engineering construction are small.
include panch.romatic and infrared black-and-white films Cleaves (3.6) stated: "Within the scope of the author's ex-
and natural color and color infrared films. Panchromatic perience, more than 90 percent of all landslides requiring,
black-and-white film is the most common type used because control or correction have been small, averaging less than
it is low cost, convenient to handle, and readily available, 500-feet in width from flank to flank, and 100 to 200 feet
but natural color and color infrared fihns are now being more in length from crown to toe." Hence, many small landslides
extensively used. Color photography is especially valuable would not be readily discernible on normal-scale photography
for outlining differences in moisture, drainage, vegetation (1:15 000 to 1:30 000) and would require large-scale photog-
conditions, and soil and rock contacts. Plates 3.1 through raphy (1:4800 or larger) for identification. At these large
3.6 show the uses and advantages of colorfilms. The strati- scales, however, the area covered in each photograph is
fication in exposed soils and rocks is most easily recognized limited, and the overall perspective is more difficult to grasp.
on natural color film. Color infrared films are most helpful Whenever practicable, it is desirable to examine both small-

49
 Plate 3.1. Slides in Douglas County,
Oregon, caused by undercutting of
slopes by stream. Slides largely :1
...
composed of weathered colluvial
materials are evident at almost every
bend of stream where it undercuts
naturally steep slopes (1). Advantages
provided by use of color film include
distinction of potentially more unstable
colluv jal material from more stable
bedrock surfaces; ease of distinguishing
seepage areas; and indication of
vegetation under stress due to
insufficient moisture, which could
occur because of slope movement.

Plate 3.2. Color infrared photography

of landslide terrain in Skamania County,
Washington, best shows subtle features
indicative of slide or slide-prone areas:
presence of seepage areas as indicated
by dark-toned streaks or "tails" (1) or
by concentration of lush vegetation (2);
vegetation under stress due to moisture
deficiency as indicated by lighter
reddish tones and by sparser leaf
coverage (3); and presence of leaning
and fallen trees (4).

Plate 3.3. Oblique view of area shown

in Plate 3.2 allows some of subtle
features to be more easily identified.

50
Plate 3.4. Black-and-white
. .. :
photographs taken October 25,
1954, of delta deposits overlying
fr -• - i
-
,
- ,,•
41,
-
.•

lake bed in Tompkins County,

.' • . -
New York. Many landslides in '-'
glaciated areas occur in lake bed
'. r .. -'
r..
materialsoverlain by granular .. '.,, ,.- I -. -- - .
deposits. Slides occur on both • . .
17
sides of creek. Benching and - . ." . •
other slope-stabilizing practices
were incorporated to stop . ... -. - —•- ., .- .. ' .:
..C4 t' t' -
sliding. Large landslide is easily
identified (1), but evidence of
.•. \.
•v. / -
seepage and boundary between
delta and lake-bed deposits is
subtle anddcuto isinguish ,. - -
• -
on this film type. .•i._.' .'i, - '- ' I ._t.:

; s4

'A
Plate 3.5. Natural color
photographs taken April 9.
1976, of area shown in
Plate 3.4. Presence of
seepage at contact between
delta and lake-bed deposits
(2) and change in color tones
of soils of these two
materials make it easier to
distinguish between these
two deposits: Darker brown
tones are seepage zones;
bluish gray tones are lake
bed; and lighter tan tones
are delta deposits.

Plate 3.6. Color infrared

photographs taken April 9,
1976, of area shown in
Plate 34. Seepage zones and
wet areas (2) are best seen on
this film type. Dark tones are :
clearly distinct. It is too early
for presence of luxurious 7-

vegetation, which could aid in

delineating seepage areas.
Boundary between two
deposits is more distinct on
this film than on black-and-
white film, but contrast is
not so sharp as on natural
color film.

51
 and large-scale photographs: the smaller scale (1:20 000 or Agricultural Stabilization and Conservation Service (ASCS),
smaller) to provide the regional setting and landform boun- U.S. Department of Agriculture, periodically publishes the
daries and the larger scale (1:8000 or larger) to provide the ASCS Aerial Photography Status Maps and the Comprehen-
details. sive Listing of Aerial Photography, which indicate the cov-
Sufficient coverage is required in order to determine the erage available from ASCS for each county in each of the
full extent of the slide, the potential areas affected, and the 50 states. The National Archives in Washington, D.C., is
changes occurring with time, In some instances the active the depository of all nitrate-base photography collected
slide is present only at the lower or toe end of a slope while during the years 1936 to 1941. The EROS Data Center
unnoticed far up the mountainside are tension cracks that has USGS and National Aeronautics and Space Administra-
indicate the instability of the entire slope. With time, the tion photography.
slides may retrogress up the slope until the whole mountain-
side is affected. Clearly, the plans for stabilization will dif- National Cartographic Information Center
fer depending on whether one is dealing with a small con- U.S. Geological Survey
fined slide area or with a whole mountainside. A situation 507 National Center
of this type can be dealt with satisfactorily if two scales of Reston, Virginia 22092
photography are obtained, as previously recommended.
Aerial Photography Field Office
Coverage at the larger scale should include the full height
and depth (mountain top to valley bottom) of the area Agricultural Stabilization and Conservation Service
U.S. Department of Agriculture
under investigation and at least three to five times the width
2222 West, 2300 South
of the present unstable area.
Post Office Box 30010
Many areas, particularly in the United States, have been
Salt Lake City, Utah 84125
photographed at various times since the 1930s, and this se-
quential photography is valuable for noting the changes in Forest Service
terrain conditions, such as drainage, slope stability, land use, U.S. Department of Agriculture
and vegetation, that have occurred during a period of time. Washington, D.C. 20250
These changes provide a clue to the origin and irate of move- Headquarters, Defense Mapping Agency
ment of a slide. Besides denoting changes, such photographs U.S. Naval Observatory
were probably taken at different seasons, times of day,,and Building 56
moisture conditions and will often show detailed features Washington, D.C. 20305
that might otherwise have been obscured. These older photo-
graphs can also be of value in planning new coverages. Coastal Mapping Division, C-34 15
The quality of the photography obtained has a direct, in- National Ocean Survey
fluence on the amount of detail that an interpreter can de- National Oceanic and Atmospheric Administration
rive from the photographs. The quality depends on the 6001 Executive Boulevard
products used and the various processes the images pass Rockville, Maryland 20852
through. Proper selection and control of the film-filter EROS Data Center
combinations require close cooperation between a knowl- U.S. Geological Survey
edgeable interpreter who can specify the requirements of Sioux Falls, South Dakota 57198
the end product needed and an experienced contractor who
Audiovisual Department
knows the capabilities and limitations of the various systems
National Archives and Records Service
and can aid in selecting the optimum combination for the
specific job. Eighth Street and Pennsylvania Avenue, NW
Washington, D.C. 20408
Availability and access to existing photography are ex-
cellent in the United States and Canada and in much of the Maps and Surveys Branch
rest of the world. In the United States, most aerial photog- Tennessee Valley Authority
raphy has been taken for federal and state agencies. Many 210 Haney Building
areas in the United States have been covered several times Chattanooga, Tennessee 37401
byone or more agencies. The National Cartographic Infor- National Airphoto Library
mation Center (NCIC) of USGS publishes a catalog, Aerial Canada Department of Energy, Mines and Resources
Photography Summary Record System Catalog, of existing, 601 Booth Street
in-progress, and planned aerial photography of the conter- Ottawa, Canada K1AOE8
minous United States held by several federal agencies. The
catalog includes a series of outline map indexes, indicating Several state highway and transportation departments
coverage within a 75 x 7.5 -min geographic cell, and acorn- similarly have available for public review index maps that in-
puter listing on microfiche film that shows amount of cloud dicate the type and extent of coverage held by state organi-
cover, camera used, specific scale, date of coverage, film zations. Some state agencies publish lists of all the aerial
type and format, and agency holding the photography. The coverage available for their states from all sources (3.2).
NCIC address and other U.S. and Canadian agencies that Using existing photography in a landslide investigation
have large holdings of aerial photography are given below. is of course less expensive than acquiring new coverage.
Several of the government agencies whose holdings are Existing photographs, however, may be several years old,
listed in the NCIC catalog also publish status maps of aerial may not show the present terrain conditions, or may be at
photography held by their organizations. For example, the a scale too small for performing detailed analyses, and new

52
coverage may thus be necessary. The existing photography These optimum conditions are not always present at one
can then be useful in the initial analysis and planning for the time, and a compromise may be required to obtain as many
new coverage. of the ideal conditions as possible. Some detailed investiga-
When the organization performing the investigation does tions may require photographs to be taken several times
not have the facilities for obtaining new photographic cov- during the year.
erage, the best approach is to engage a reputable firm with a
base of operation close to the study site. In some cases, a Principles of Aerial-Photograph
small plane can be hired locally, and a hand-held camera can Interpretation
be used to make oblique photographs that are economical
and suitable for preliminary analysis and planning. The interpretation of aerial photographs involves three major
steps:
Natural Factors Examine the photographs to get a three-dimensional
perception,
Consideration of natural factors is important in planning Identify ground conditions by observing certain ele-
aerial photography for landslide studies. Flight planning is ments appearing in the photographs, and
certainly controlled by meteorological conditions, but con- Using photo-interpretation techniques, analyze spe-
sideration must also be given to time of year and time of cific problems by the association of ground conditions with
day in which the photography is obtained. one's background experiences.
Meteorological conditions include cloud cover, haze, tur-
bulence, and solar angle, all of which affect the quality of The quality and the reliability of any interpretation are, of
definition of images on the film. Suitable conditions for course, enhanced in direct ratio to the interpreter's knowl-
color photography are when the sun angle exceeds 300 from edge of the soils and geology of the area under study. The
the horizontal, cloud cover is less than 10 percent, visibility acquirement of such knowledge, either by field examination
at aircraft altitude when one looks toward the sun exceeds or by study of available maps and reports, should, therefore,
24km (15 miles), and the atmosphere is free of turbulence be considered an essential part of any photo-interpretation
(3.30, p. 43). job.
Time of year is also critical to the quality of aerial photog- The technique used in interpreting aerial photographs is
raphy. Although no clear-cut recommendations can be made referred to as pattern analysis. This approach is based on
to fit all situations or localities, some of the desirable con- the premise that the landforms, the basic units of the land-
ditions are discussed below. scape, have distinct patterns in aerial photographs. Those
landforms developed by the same geologic processes and in
the same environmental setting will have similar patterns,
1. Optimum soil-moisture contrasts. The optimum con- while those developed by different processes will have dif-
trast of soil moisture occurs under two different conditons: ferent patterns. These patterns are composed of several
(a) when the soils are wet but not totally saturated or (b) major elements that are evaluated in the process of interpre-
when the moisture levels are low. The former occurs in the tation: topographic expression,drainage, erosion, soil tones,
spring or early summer because of high water conditions. vegetation, and culture. These elements are discussed briefly
Wet areas and seepage zones present in unstable or poten-
in the following paragraphs; more thorough treatments are
tially unstable areas are directly visible outcropping on hill-
given by Lueder (3.15), Way (3.38), and Colwell (3.7).
sides or are evident by the presence of luxuriant vegetative
growth over those wet areas. The latter occurs during dry
periods in late summer or fall or over disrupted areas. Dur- Topographic Expression
ing dry periods, vegetation in wet areas or seepage zones
has more growth and color than that in areas that have nor- The features of importance in describing topographic expres-
mal moisture levels. Areas disrupted by cracks and fissures sion include topography, shape, and relative relief. Topog-
are better drained and thus drier, and vegetation located raphy represents the three-dimensional aspect of the land,
over those areas shows evidence of stress earlier. Contrast- scape, such as hill, basin, ridge, or mountain. Shape refers
ing vegetation and moisture conditions are best noted on to a unique character of the landscape, such as conical hill,
color infrared photography, as illustrated in Plates 3.2 and sinuous ridge, or A-shaped mountain, that aids in describing
3.3. or separating different forms of topography. Relative relief
Optimum ground cover. A minimal cover of snow refers to the comparative position of the landform in rela-
and tree foliage is desirable. In humid, temperate climates, tion to other landforms in the vicinity, for example, the po-
this usually occurs in early spring after the snow melts and sition (or relative relief) of the sloping fan-shaped plain lying
before the deciduous trees leaf or in late fall after the leaves at the foot of A-shaped hills and sloping downward to a level
fall and before snow falls. basin area. In this example, three different landforms are in-
Optimum shadow conditions. Photographs taken when dicated based on their relative relief: the mountainous area,
the sun angle is high and shadows on the hillsides and slopes the sloping plain area, and the basin area. Topographic ex-
are minimal are the best for interpretation. However, in pression of landforms is easily determined by a stereoscopic
some special cases, such as in areas of low topography, small examination of the photographs. In such an examination,
slope failures are enhanced by the presence of long shadows various landforms can be separated and clues obtained as to
produced when the photography is taken with a low sun the nature and stability of the materials constituting the
angle (either early in the morning or late in the afternoon). landform.

53
 Drainage and Erosion photographs by dark spots or "tails," are a clue to seepage
on slopes. Cultivated fields, as well as natural growths, are
The density and pattern of drainage channels in a given area good indicators of local soil conditions. Thus, an orchard
directly reflect the nature of the underlying soil and rock. is often found on well-drained soils; black ash, tamarack,
The drainage pattern is obvious in some cases, but more of- and white elm are normally found on wet, fine-grained
ten the channels must be traced on a transparent overlay to soils; and vegetation is sparse in nonproductive serpentine
allow the pattern to be successfully studied. Under other- soils, where landslides are common.
wise comparable conditions, a closely spaced drainage sys- The use of color photography, especially color infrared,
tem denotes relatively impervious underlying materials; greatly facilitates vegetation analysis and thus the overall in-
widely spaced drainage, on the other hand, indicates that terpretation. The vigor of vegetative growth and the pres-
the underlying materials are pervious. In general, a treelike ence of vegetation stress due to detrimental soil and mois-
drainage pattern develops in flat-lying beds and relatively ture conditions are best noted on color infrared photography.
uniform material; a parallel stream pattern indicates the pres- Point 2 in Plates 3.2 and 3.3 shows areas of seepage, as in-
ence of a regional slope; rectangular and vinelike patterns dicated by lush vegetation, and point 3 indicates vegetation
composed of many angular drainageways are evidence of con- under stress on the unstable slide debris.
trol by underlying bedrock; and a disordered pattern, inter-
rupted by haphazard deposits, is characteristic of most gla- Culture
ciated areas. Other patterns have develope4 in response to
special circumstances. A radial pattern, for instance, is Cultural patterns reflect how humans adjust to the natural
found in areas where there is a domal structure, and a terrain on which they live and work. An understanding of
featherlike pattern is common in areas where there is severe these patterns and the reasons for their presence can be valu-
erosion in rather uniform silty material, such as bess. able indicators of soil, moisture, and other terrain conditions.
The shapes of, gullies appearing in aerial photographs pro- For example, drainage ditches and tile fields indicate the
vide valuable information regarding the characteristics of presence of a high water table; steep, vertical cuts along
surface and near-surface materials. Typically, long, smoothly highways denote the presence of bedrock; sloping cuts usu-
rounded gullies indicate clays; U-shaped gullies, silts; and ally typify unconsolidated soils; and contour farming or an
short, V-shaped gullies, sands and gravels. irregular wavy plowing pattern characterizes sloping terrain
or erodible soils.
Soil Tones
Interpretation of Landslides on Aerial
Soil tones are recognizable in photographs unless there is a Photographs
heavy vegetative cover. On black-and-white photographs,
the tones are merely shades of gray, ranging from black to As previously described in the section of this chapter on
white. Because gray tones are highly respondent to soil- landforms susceptible to landslides and the section on vul-
moisture conditions on the ground, they are an important nerable locations, landslides are more prevalent in certain
aerial-photographic element in landslide investigations. landforms and occur most frequently at certain vulnerable
A soil normally registers a dark tone if it has a high mois- locations. The interpretation of aerial photographs by the
ture content and a light tone if it has a low moisture con- pattern-analysis technique is a valuable tool for delineating
tent. The moisture condition is the result of the physical landslide-susceptible landforms and pinpointing the vulner-
properties of the soil, its topographic position, and the po- able locations. The identifying pattern elements on the
sition of the groundwater table. The degree of sharpness photographs and significant features about these landforms
of the tonal boundary between dark and light soils aids in are summarized in the following discussion and illustrated
the determination of soil properties. Well-drained, coarse- in the accompanying figures.
textured soils show distinct tonal boundaries whereas poorly
drained, fine-textured soils show irregular, fuzzy tonal Diagnostic Patterns of Landslides
boundaries. on Aerial Photographs
Color photography greatly increases the number of tones
that can be distinguished by the interpreter and improves An investigator already familiar with the appearance of land-
the amount of detail and accuracy of the information de- slides on the ground should become oriented to the aerial
rived. In addition, the use of special visual, photographic, view of landslides by an examination of photographs of
or electronic enhancement techniques increases the effective- some known examples. The difference between an aerial
ness of analysis of the tonal pattern element. view and a ground view results chiefly from the fact that the
former gives a three-dimensional perspective of the entire
Vegetation slide area, but at a rather small scale. Ground photographs,
on the other hand, show only two dimensions, but on a
Vegetative patterns reflect both regional and local climatic larger scale.
conditions. The patterns in different temperature and rain- The following features discernible on aerial photography
fall regions can be recognized in aerial photographs. Locally, are typical of landslides or landslide-susceptible terrain, but
a small difference in soil moisture condition is often detected not all features are evident for each landslide. Most of these
by a corresponding change of vegetation. A detailed study features are shown in Figure 3.14; those features not evident
of such local changes is helpful in landslide investigations. are shown in other figures in this chapter. The numbers in
For instance, areas of wet vegetation, represented on the Figure 3.14 correspond to those in the list below.

54
Figure 3.14. Typical
characteristic features of
a landslide as shown in
aerial photographs (3.6).
Slide occurred in Yakima
County, Washington.
Numbered items are
discussed in text.

- 5'

••-
It.
:Y

Land masses undercut by streams (Plate 3.1): highway is built on unstable soil, the irregular outlines and
Steep slopes having large masses of loose soil and nonuniform tonal patterns of broken or patched pavements
rock: are often visible (Figure 3.15).
Sharp line of break at the scarp (head end) or pres-
ence of tension cracks or both; Identification of Vulnerable Locations
Hummocky surface of the sliding mass below the
scarp; Many slides are too small to be detected readily in photog-
Unnatural topography, such as spoon-shaped trough raphy at the scales normally available (i.e., I : 15 000 to
in the terrain: 1:40 000). Consequently, the photographs should be
Seepage zones; closely examined for signs that indirectly indicate the pres-
Elongated undrained depressions in the area; ence of slides or, if signs are not visible, for the vulnerable
Closely spaced drainage channels; locations where slides usually occur. Typical vulnerable lo-
Accumulation of debris in drainage channels or cations include areas of steep slopes, cliffs or banks being
valleys; undercut by stream or wave action, areas of drainage con-
Appearance of light tones where vegetation and centration, seepage zones, areas of hummocky ground, and
drainage have not been reestablished (Figure 3.1 and Plates areas of fracture and fault concentrations. The characteris-
3.2 and 3.3); tics of these locations have been discussed in an earlier see-
II . Distinctive change in photograph tones from lighter tion of this chapter in which vulnerable locations are de-
to darker, the darker tones indicating higher moisture con- scribed.
tent; Aerial photographs are valuable aids in identifying the
Distinctive changes in vegetation indicative of changes vulnerable locations. The shape and slope of the terrain are
in moisture (Plates 3.2 and 3.3); and readily discernible from the stereoscopic examination of
Inclined trees and displaced fences or walls due to the photographs. In fact, the vertical appearance of the ter-
creep (Plate 3.3). rain is exaggerated when viewed with a lens stereoscope.
Moderate slopes appear steep, and steep slopes appear al-
In sonic cases the slide itself is not discernible, but indirect most vertical, making them easier to delineate. In addition,
evidence of its existence is noted. For example, where a the slopes can be measured on the photographs by using

55
 Figure 3.15. Presence of
limited sections of patched
pavement (1) in generally
unpatched roadway can be
indicative of possible
slope movement in area.
Presence of fissures on
hillside above this patched
section (2) in Washington
County, Ohio, confirms
instability of slope.

"'
V
-4 r : ;.

simple measuring devices, such as an engineer's scale or a Most of the forms susceptible to landslides are readily recog-
parallax bar, and by applying photogrammetric principles. nizable on aerial photographs. The identifying elements on
Details of this technique are described by Ray (3.23). black-and-white photography and significant facts about the
The presence of drainageways, seepage zones, fractures, landforms are summarized and illustrated in the following
and fault zones is also readily evident on aerial photographs. sections. Table 3.2 gives the landslide potential of various
By means of stereoscopic examination, the complete drain- landforms/geologic materials grouped on the basis of their
age network can be mapped, including the intermittent topographic form. The order of presentation in this section
streams and small gullies. The presence of wet zones or follows a sequence based on origin and character of the ma-
seepage areas is evidenced by darker tones caused by the terials.
higher moisture content in the soils or by a more luxuriant
vegetative growth over the wet areas. Areas of drainage or Sedimentary Rocks and Their
water concentration above a slope should be closely ex- Residual Soils
amined because they are vulnerable locations. Subsurface
seepage from these areas can lead to slope failures: Figure The discussion of rocks and their residual soils is combined
3.4 shows such a case. Fracture and fault zones are indi- in this and the following two sections because the recogni-
cated on the photographs by dark linear or curvilinear lines. tion of types of residual soils depends primarily on the rec-
The darker tones are usually due to the better growl h of ognition of the landform developed on the parent rocks.
vegetation along the fracture zones where it is easier for The determination of depth of residual soil requires consid-
the roots of plants to grow and where moisture levels are erable judgment - However, investigators working constantly
usually higher. In delineating fracture zones, care must be in their own regions should have no difficulty in estimating
taken not to interpret man-made features, such as fence the depth once they are familiar with local conditions.. In
lines or field boundaries, as fracture zones. Generally, fea- general, rounded topography, intricate, smoothly curving
tures having straight lines, right-angle intersections, and drainage channels, and heavy vegetation are indicators of
standard geometric patterns are man-made. Several of the probable deep soils: in contrast, sharp, steep, resistant ridges
above features are illustraled in succeeding figures that and rock -cont rolled angular channels are commonly found
show landslides in various landforms. in areas of shallow soils. The local climatic and erosion pat-
terns should be considered in the interpretation.
L.andforms/Geologic Materials Many landslides occur in residual soils and weathered
Susceptible to Landslides rocks; they are usually in the form of slides or flows. For
example, in a study in North Carolina (3i2), two-thirds of
Landslides are rare in some landforms and common in others the slides involved in cut-slope failures occurred in the

56
Figure 3.16. Pierre shale,
Lyman County, South
Dakota, a formation that
is most highly susceptible
to landslides (3.4).
Numerous slides are
evident throughout area,
some due to undercutting
by streams (1) and others
to steep slopes, seepage,
or stress release along joints
(2).

weathered soil material. Of the sedimentary rocks, shales and medium dark tones. They usually are heavily cultivated
are the most susceptible to landslides. A typical example is in humid areas.
shown in Figure 3.16, Shales are especially susceptible when Interbedded sedimentary rocks show a combination
interbedded with pervious sandstones or limestones or other of the characteristics of their component beds. When hori-
pervious rock types. This geologic setting of alternate layers zontally bedded, they are recognized by their uniformly dis-
of pervious and impervious materials on a slope is conducive sected topography, contourlike stratification lines, and tree-
to sliding. like drainage: when tilted, parallel ridge-and-valley topog-
Landslides in massive horizontal sandstones or limestones raphy, inclined but parallel stratification lines, and trellis
are uncommon unless they are interbedded with shales or drainage are evident.
other soft rocks (Figure 3.17). In steeply tilted positions,
any sedimentary rock may fail by sliding (Figure 3.18). De- The identification of land form as a means of detect ing
pending on the (lip angle, joint syste iii, and climate, slides associated landslides is important in the flat-lying sedimen-
may take one or a combination of forms, such as rock falls, tary group because the slides there are often small and,
rock slides, topples, block slides, slumps. debris falls, debris therefore, not very obvious on aerial photographs. This is
slides, and earth flows. River undercutting and artificial ex- particularly true for slides in colluvial deposits formed from
cavation are important factors in initiating landslides in both sedimentary rocks. These conditions are common since sedi-
horizontal and tilted rocks (Figures 3.3 and 3.16). mentary rocks are the most widespread of all surface rocks.
Methods of identification of sedimentary rocks in aerial
photographs are well established. Igneous Rocks and Their Residual Soils

I . I-lard sandstones are evidenced by their high relief, Basaltic lava flows are among the most common extrusive
massive hills, angular and sparse drainage, light tones, and igneous rocks, and they are readily identifiable on aerial
general lack of land use. photographs. Basalts and other volcanic flows are highly
Clay shales are noted for their low rounded hills, well- susceptible to different types of landslides (Figures 3.19 and
integrated treelike drainage system, medium tones, and 3.31). They often form the caprock on plateaus or mesas:
gullies of the gentle swale type. They are generally farmed their sharp,jagged (saw-toothed) cliff lines are clearly visible
in regiuns with sufficient rainfall. on photographs. Surface irregularities or flow marks, sparse-
Soluble limestones are characteriied by low rolling ness of surface drainage and vegetation, and (lark tones are
topography with sinkhole development in temperate, humid con firming characteristics.
areas, by rugged karst topography in some tropical regions, If it basalt flow is underlain by or interbedded with soft
by a general lack of surface drainage, and by mottled light layers, particularly if it occupies the position of a bold es-

57
carpment, a favorable condition for large slumps is present the foot of cliffs. Disturbance of talus slopes during road
The joints and cracks in basalt give rise to springs and seep construction has caused some large slides of talus materials.
age zones and greatly facilitate movement. Rock falls and Old slides and fissures indicating incipient slides often can
rock slides along rim rock are usually favored by vertical be seen on photographs. In areas of relatively deep weather-
jointing of basalt and by undercutting of basaltic cliffs. big, the landscape is somewhat modified. A more rounded
Talus accumulations of various magnitudes are found at topography and heavier vegetation develop, although dark

Figure 3.17. Interbedded . ' . '' •.• • 4.,.,•.- j? er

sedimentary rocks W4
(3.22). Hummocky -. -. . . .'•• ...
topography, heavy ,• -. .... . . •.•. . . :• ..•• . ;. . . . ..
erosion, and extensive . .. . . . . . . . . . . ..
drainage characterize •.• . ....
. 1

slialematerial .. -. E
'.•J, .'
underlying sandstone
cap rock in Morgan ),
County, Ohio Almost
all hillsidt.s show
evidence of instability
(1). Evidence of slides . .• . . ..
in capping sandstone .. . . ' s .•.
due to undermining .. . . '.: . .
..
by sliding out of the . ... ... ...., ... . .•4 &., .... -. / ••.i .,
shask is also apparent 2
- 41

.
. :''••. ...S.....

,.i

k... ." . .
:.)
. '. • ..
S.
. ... .
. .;... ....
.
.•

dr0ck prodes 5te0 ply

dipping slopes ide ii for i
slides (3.8). No large
slide masses are . . . •
..
apparent in this Le
Fiore County,
Oklahoma, area, but

wasting is indicated by
I
oversteepened slopes
I
having light tones (1); . __. .. .
bulging of lower slopes
(2); and accumulation
3
of loose scree materials r• \ '. ' •. .
at base of slopes (3) .

- 3

58
tones still predominate. Slumps of both large and small can be distinguished on aerial photography by an indication
size are common in basaltic soils. of banding or lineation of the topography and drainage net-
Granite and related rocks constitute the most widely oc- work, and landslides, when present, are clearly evident. Fig-
curring intrusive igneous rock types. The landslide poten- ure 3.21 shows an example of landslides in metamorphic
tial of granitic rocks varies widely, depending on the com- terrain (mica schists). The attitude of the foliation seen at
position of the rock and its fracture pattern, the topography, point 3 indicates that the dip of these layers may have been
and the moisture conditions. In granites that are highly re- a significant factor in the presence of numerous slides (point
sistant to weathering or are of low relief, there is generally I) found just across the river where the layers are steeply
no slide problem. In hilly country where the granite is dipping toward the river.
deeply weathered, slumps in cut slopes, as well as in natural Within the metamorphic group, many slides are associated
steep slopes, are common. Fractures in the rock and high with serpentinite areas, which are identified on aerial photo-
moisture conditions undoubtedly are favorable factors in graphs by sinuous ridges, smoothly rounded surfaces, short
producing landslides. Figure 3.20 shows an example of an steep gullies, poor vegetative cover, and dull gray tones.
extensive slide in granitic materials. Granitic masses are Within a general area, local conditions, such as vegetation,
identified on aerial photographs by the rounded (old) to moisture, and slope, may create special, favorable circum-
A-shaped (young), massive hills and by the integrated tree- stances for landslides. In genera!, low relief and low rain-
like drainage pattern with characteristic curved branches. fall are among the factors responsible for the stability of
The presence of criss-cross fracture patterns and light tones some of the serpentinite slopes.
and the absence of stratification and foliation aid in confirm-
ing the material. Glacial Deposits

Metamorphic Rocks and Their Landslides are common in some glacial and glaciofluvial de-
Residual Soils posits. Although most of the distinct glacial forms are
easily identified on aerial photographs, there are complex
As reported by Leith and others (112) and other investiga- areas that require a high degree of skill for their identifica-
tors, the frequency of landslides per unit area is greater in tion. Moraines are found in nearly all glaciated areas. They
metamorphic rocks than in most other rock types. The pres- are identified on aerial photographs by jumbled, strongly
ence and attitude of the foliation and joints in these rock rolling to hilly terrain. Moraines, particularly in semiarid
types greatly influence the stability of the slopes.. Although areas, contain a large proportion of untilled ]and. Deranged
the characteristics of the major types of metamorphic rocks drainage patterns, irregular fields, and winding roads are con-
gneiss, schist, slate, and serpentinite—have been identified on firming clues. Minor slumps, debris slides, and earth flows
aerial photographs for specific areas, they are not sufficiently are common in cut slopes iii moraines as a result of un-
consistent to develop typical pattern elements. Neverthe- drained depressions and seepage zones in the mass. Because
less, the presence and attitude of the foliation and joints morainic hills are usually small, these slides are not exten-

Figure 3.19. Basalt cap rock

overlying impermeable shale
deposits has resulted in
massive landslides in Mesa
County, Colorado. Large
blocks of material have
toppled and stumped off
scarp face (1). Presence of
incipient slide is indicated by
ciacks on crown of slide (2).

_ .,v
,.
ft.. .'.. .

.et!

59
sive. They are, nevertheless, large enough to cause contin- have been oversteepened by glaciation (Figure 3.23). The
ual trouble to many maintenance engineers (Figure 3.22). topography of such areas is basically that of the underlying
In areas subject in part to mountain glaciation, transpor- bedrock with slight local modifications, depending on the
tation routes must follow valleys formerly occupied by thickness of the mantle.
glaciers and be built on their deposits. Under these condi- Slides seldom occur in other glacial or glaciofluvial de-
tions slides in moraines and colluvium are common. Slides posits, such as kames, deltas, eskers, outwash plains, and
also occur in the shallow mantle overlying bedrock and take till plains. They have occurred, however, in places where
the form of slumps, debris slides, and debris falls: they often these materials are underlain by fine-grained impervious
contribute to failures in artificial fills placed on them. The materials, such as lake-bed deposits, when these deposits
landslides most commonly occur along valley walls that are present in elevated positions (Plates 3.4, 3.5, and 3.6).

Figure 3.20. Large

debris slide (1) with
remnant scar (2) in
'anodiorite rocks
on east front of
Sierra Nevada
Mountains just
east of Lake
Tahoe in Washoe
County, Nevada.
An estimated 95
Mm3 (125 million
yd3) was involved;
portions of toe of
slide are buried
under younger
alluvium (3.36).

Ta
Water-Laid Deposits flats, marshes, and swamps; and the presence of broad, shal.
low, tidal stream channels. The upper coastal plain is iden-
Water-deposited landforms most susceptible to landslides tified by its rolling to rugged topography: an integrated
include coastal plain deposits, river terraces, and lake beds. drainage system with wide, vegetation-filled main drainage-
Coastal plain deposits can be subdivided into three dis- ways: and irregular land-use patterns. It is also associated
tinct subforms for analysis by aerial photographic methods: with coastal features and appears on aerial photographs to
the dissected or upper coastal plain, the undissected or be somewhat similar to areas underlain by consolidated sedi-
lower coastal plain, and the beach zone. Landslides are un- mentary rocks. In the lower coastal plains, landslides offer
common in the beach zone. The lower coastal plain is iden- a problem only in the construction of canals or similar struc-
tified by its low, flat topography; its association with tidal tures that require deep excavation in flat lands. In the upper

Figure 3.21. Typical landslide

topography in weathered mica
schists evident on valley walls
on both sides of Little Salmon
River in Idaho County, Idaho.
Old slide scars with hummocky
terrain below can be seen (1).
Slide debris has covered road 04,
many times (2) necessitating
high maintenance costs and
final relocation. Presence and
attitude of foliation are also
apparent (3).

Figure 3.22. Moraine and lake

bed, Tompkins County, New -
York (3. 10). New slumps and pk ,.
-- -

debris slides, as well as old slide 41

scars, are common occurrences
in this glaciated valley where
postglacial erosion has dissected
1
moraines and overlying lake t!
deposits. Morainal deposits (1) ,..-.- "--,! • Sj

are recognized by their 7 P, )J

irregular, jumbled topography.
Overlying lake deposits have
smoothed topographic
- '-1
appearance; their silty clay I .
composition is characterized
by smooth slopes, high degree 1'
of dissection, and gradual
change of gray tones (2). rj
f '
Prominent old slump is visible
(3). Almost all highway
excavations experience slumps
and debris slides (4). Although VEJ I
,_j.. ',;.••• •iv.fl••• • ' '
deep-seated and large-scale slides
are not common in such an area,
continual maintenance work in
clearing sliding material and in - r-
protecting slopes from erosion '.'.' NI...
has been necessary.

61
coastal plains, however, slumps in natural hill slopes, as well ral equilibrium is disturbed by artificial installations, slides
as in road cuts, are common (Figure 3.24). The stratified will occur. Slides in terraces naturally start on unsupported
and unconsolidated nature of the sands, silts, and clays that slopes facing the low land (Figure 3.25).
characterize most coastal plains provides a favorable situa- Lake-bed deposits generally display flat topography un-
tion for landslides. less they are dissected. Although generally composed of
Terraces are easily recognized on aerial photographs as clays, lake beds have little chance to slide except when ex-
elevated flat land along major or minor valleys. Terraces of posed in valley walls or cuts (Figure 3.26). Slides of con-
gravel and sand are usually stable, maintaining clean slopes siderable magnitude have occurred in lake clays under each
on their faces. However, where terraces are composed of of the following circumstances: (a) where lake clays are in-
fine sands or interbedded silts and clays or where the natu- terbedded with or, especially, are overlain by granular de-

Figure 3.23. Thin till deposit overlying bedrock in glaciated valley in Broome County, New York. Highway cuts in hillsides oversteepened
by glaciation create slides (A and B). Slides generally extend to full depth of soil material until competent rock bed (in this case sandstone)
is encountered. Between A and B, section of highway entirely in rock has not experienced any slides. In this respect, landslides serve as
indicators of depth of soil to rock. On other side of river, site of future slide is indicated at C. an area of Concentration of drainage, loose
fill, and lack of bedrock control.

0 ..

2/-:. '
: .
I.
- t- ...

62
posits and (b) where lake clays overlie bedrock at shallow Eolian or Wind Deposits
depth and the base level of erosion of the area is generally
lowered. Examples of flow slides in these types of deposits Loess, or wind-deposited silt, can be identified unmistakably
are shown in Figures 3.27, 3.33, and 3.34. Undissected lake in aerial photographs by its vertical-sided gullies, which are
clays are easily identified in aerial photographs by their char- evenly spaced along wide, flat-bottomed tributaries to show
acteristic broad level tracts, dark gray tones, and artificial a featherlike drainage pattern. Such a landform is confirmed
drainage practices. Dissected and complex lake-bed areas are by equal slopes on hills and valleys (an indication of uniform
relatively difficult to identify, particularly by those not fa- material), heavy vegetative cover in dissected areas, extensive
miliar with the local geologic conditions. Again, the presence farming in undissected areas, and soft gray tones (Figure 3.28).
of existing slides is the most reliable warning signal. Earth flows and minor slumps, generally referred to as cat

Figure 3.24. Dissected coastal

plain in Prince Georges County,
Maryland, is identified by low,
soft hills and associated tidal
channels (more evident when
larger, adjacent areas are
;: .•t.%. "•
4i; t
examined) (3.10). Most
railroad (1) and highway cut
slopes in this region have
experienced slide problems.
Cut slopes steeper than
natural slopes are susceptible
to slides unless adequate
precautions are taken. For
highway locations, grade line
would better be set below
critical clay layers if at all
possible so that, even if slide 1•
occurs, foundation of road
would not be affected.
Presence of amphitheater
scarp hillsides (2) with highly
eroded and hummocky
topography below indicates
natural erodibility and
instability of materials in
this area.

Figure 3.25. Sandy terrace (1) ..._,.,...-r Lb......

overlios basalt flow along -
Snake River in Elmore '..
County. Idaho. Several dry '
sand flows (2) indicating * • ,

1
:
1 ) ci "' ?•
( •t

I -
t 1• J t
63
steps, are commonly found in bess. The cat steps appear Gravity Deposits
as fine, roughly parallel, light tone contours on the aerial
photographs. Because of their small size, they are not al- Loose, unconsolidated talus and colluvial materials formed
ways evident. The individual steps of these small slumps by weathering of parent soil and rock materials and moved
are commonly about a meter wide and several centimeters downslope by gravity are found on steep slopes. These de-
to a meter high. These subtle features are shown in Figure posits are easily identified on aerial photographs as bare
3.29. slopes in mountainous areas, but they are not so obvious on

Figure 3.26. Sewage lagoons

(1) at Harwood Rest Area
along 1-29, Cass County,
North Dakota, may be
endangered by slides
occurring along stream bank
just below (2). Lake-bed
deposits in this area are
highly susceptible to slides
as shown by presence of
other slides along stream
bank (3).

Figure 3.27. Example of

earth flow in sensitive
r -•

clays along Ottawa River

east of Ottawa. Canada
(3.17).

I
' \t

64
Figure 3.28. Highly dissected
nature of the bess terrain in
Lincoln County, Nebraska,
illustrates great erodibility of t kn~
these materials (3.10). -

Presence of earth flows (1) Ilk

is indicated by broad
amphitheater scarp faces
at crown, steep side slopes
of gullies, and light-toned
deposits in gulley bottoms. '
&q ....
Numerous little erosional
scars and slumps can be
seen on hillsides (2).
'it
got II 9.
Section (3) is shown 4t. OXN
enlarged in Figure 3.29. 5: L;

Ov '1
- ' -"-

41 -,
<
AO
.14 AN

ij
i

Figure 3.29. Cat steps can be

seen on this enlarged section
of area shown in Figure 3.28.
Because of their small size,
they appear as thin, wavy
contours on hillside.

-[;;
-
:::
j :
" •' - -
- I
y '1% -

65
vegetated lower slopes. Excavation into these materials usu- investigations have been performed by several investigators,
ally creates stability problems. Old landslide deposits are including Alfäldi (3.1), Gagnon (3.11), Rib (3.26), Stallard
themselves part of this group (Figure 3.30). and Myers (3.32), and Tanguay and Chagnon (3.33). The
general consensus of those studies is that large-scale aerial
Complex Forms photography (most preferred color) provides the most in-
formation on terrain conditions, specifically for landslide
Most of the landforms previously described can be called detection. Several of the reports did indicate, however,
simple forms because they consist predominantly of one that satellite and infrared imagery offered some unique in-
type of material in each unit. In nature, however, complex formation that would prove useful for landslide investiga-
or superimposed forms are numerous and of common oc- tions.
currence, especially in glaciated areas, as mentioned pre-
viously. They are further emphasized here because of their Satellite Imagery
significance in landslide studies. A change of material ver-
tically or horizontally in complex areas often affects the in- Since July 1972, multispectral satellite coverage has been
ternal drainage characteristics and creates slope stability obtained for the United States, most of Canada, and many
problems. The common situation most favorable to slides other areas of the world. Data are collected in four bands
is one in which pervious formations are underlain by rela- onan 18-d cycle:
tively impervious beds.
Band Color Wavelength (nm)
Procedure for Landslide Investigation
Using Aerial Photographs Green 0.5 to 0.6
Red 0.6 to 0.7
Near infrared 0.7 to 0.8
The following procedure is recommended for photographic Near infrared 0.8 to 1.1
studies of landslides.
A second satellite launched in 1975, as well as others sched-
Lay out sites of planned facility on photographs. uled for launching, provides data on a shorter time cycle.
Delineate areas that show consistent characteristics Each satellite scene covers an area of approximately 33 000
of topography, drainage, and other natural elements and 2 (10 000 miles2) and has a resolution of about 80 m
classify into landform types. Large and obvious slides are (250 ft). The satellite scene can be provided as a 70-mm
identified at this stage. film at a scale of 1:3 369 000; as a 185 x 185-mm (7.3 x
Evaluate the general landslide potential of the land- 7.3-in) black-and-white print of each band; as a color com-
form types. Use Table 3.2 and the section on landforms posite at a scale of I : 1 000 000; and on computer-compatible
susceptible to landslides as a guide. tapes for computer processing. The scenes can also be pro-
Make a detailed study of cliffs or banks adjacent to vided on 35-mm slides.
river bends and all steep slopes. Compare slopes within the In addition to the LANDSAT images provided by these
same landform type: For instance, slopes in bedrock land- two satellites, the National Aeronautics and Space Admin-
forms are more stable, even though steeper, than slopes in istration (NASA) has obtained photographic and image coy-
adjacent soil areas. Because slides are usually small in size erages of considerable areas of the United States and
on photographs, look carefully and inspect slopes in minute scattered areas of many foreign countries during the manned
detail. Give particular attention to the following features: satellite SKYLAB missions (May 1973 through February
1974). SKYLAB data of special interest to landslide inves-
Slide Feature tigators are those collected by (a) the multispectral camera
Existing Hillside scarps and hummocky topography
(S-190A) at a scale of 1:2 850 000 with 30 to 79-m (100 to
Parallel spoon-shaped dark patches on hillsides 260-ft) resolution and (b) the earth terrain camera (S-190B)
Irregular outlines of highways and random cracks or at a scale of 1:950 000 with 17 to 30-m (55 to 100-ft) reso-
patches on existing pavements. lution. Black and white, black-and-white infrared, color,
Potential Ponded depressions and diverted drainageways
and color infrared products are included. Detailed informa-
Seepage areas suggested by faint dark lines, which may
mean near-surface channels, and fan-shaped dark tion about film types, spectral characteristics, and areal coy-
patches, probably reflecting wet vegetation erages has been published by NASA (3.21). Compared with
LANDSAT products, SKYLAB products provide better reso-
Relatively new slides appear in light tones because vegeta- lution but lack universal coverage and repeated sequence.
tion and drainage are not well established. The parallel At the small scale of satellite imagery, only extremely
spoon-shaped dark patches on hillsides are likely to reflect large landslides can be identified directly. Figure 3.31
vegetation in minor depressions. Lines drawn through the shows a typical stereographic aerial photograph coverage
axes of the scarps in the slides often point to drainageways of a large landslide zone. Figure 3.32 shows the satellite
on higher ground that contribute to landslide movement. scene covering this landslide zone. The scalloped edges of
Ground check all suspected slides. the scarp slopes and the hummocky topography of this
large slide are evident on the satellite scene.
Use of Other Remote-Sensing Since most landslides are much smaller than the land-
Systems slide shown in Figure 3.31,they are not directly identifiable
on satellite imagery. However, the value of satellite imagery,
Multisensor investigations for terrain analysis and landslide as noted by Alföldi (3.1) and Gagnon (3.11), is that the

66
Figure 3.30. Unstable talus
slopes in Yakima County,
Washington, are indicated
by steep slopes either bare of
vegetation or with streaks of
vegetation remnants (1). Some
debris avalanches (2) are also
evident on steep valley walls.
Instability of slopes was
recognized at time of location
of road. Alternate location on
opposite side of valley was
considered worse because of
numerous deep drainage
channels in addition to talus
slides.

landslide susceptibility of an area can be determined indi- most beneficial for landslide investigations.
rectly from some of the features that are identifiable at
those scales. Regional physiography, geologic structure, Infrared Imagery
and most landfornis as well as land-use practices and distri-
bution of vegetation are evident on the satellite imagery. The infrared region extends from approximately 0.7 pm to
These features in conjunction with the tonal patterns pres- I mm (Figure 3.13). Atmospheric interference, however,
ent on the imagery provide clues to the types of surface ma- limits the areas within the infrared region available for inves-
terials present, the surface moisture conditions, and the pos- tigation to only certain clear zones or "windows." Some of
sible presence of buried valleys. Correlating these factors the common windows used in infrared surveys are in the
to geology and topography and using local experience in a following bands: 1.0 to 1.4 pm, 1.5 to 1.8 pm, 2.0 to 2.6
region make it possible to rate the susceptibility of various pm, 3.0 to 5.0 pm, and 8 to 14 pm. Daytime infrared sur-
areas to sliding. For example, Alföldi noted in his study of veys collecting data in the region below 3.0 to 3.5 pm re-
landsliding in eastern Ontario that on the satellite image the cord infrared reflectance phenomena from various objects
clay plains were easy to spot because they are almost 100 in the scene. Daytime and nighttime surveys collecting data
percent cultivated; the till plains were recognizable because in the region above 3.0 to 3.5 pm record infrared heat-
they form a poorer agricultural area and field and forest sec- emission phenomena.
tions are intermixed; and the elevated sand plains of the old Since two basically different phenomena are being re-
Ottawa River delta (which overlay the clay plains) are kept corded in the infrared region (i.e., reflectance and heat emis-
mostly in forest. sion), a distinction is made in the terminology used to indi-
An additional advantage noted by Alföldi for satellite cate these phenomena. Images recorded in the bands below
imagery was the frequent coverage available. Seasonal 3.0 to 3.5 pm are referred to as infrared reflectance and
changes in vegetative cover and moisture levels—as indicated those recorded in the bands above as infrared imagery. The
by tonal changes—can be evaluated to increase the accuracy most useful window for terrain analysis is the 8 to 14-pm
of interpretation of terrain conditions. Also, any changes band. Further discussions on the characteristics of infrared
noted during the year in the landslide-susceptible zones, surveys are given by Reeves (3.25) and by Rib (3.26).
such as urban expansion, clear-cutting of forest, forest fires, Infrared imagery offers some unique information that
and draining of swamps, might presage renewed or new land- cannot be obtained directly from the analysis of aerial
slide activities. This could alert the interpreter to the neces- photography. The combination of aerial photography and
sity for a more detailed investigation in these areas. infrared imagery provides a more accurate and complete
Satellite photography and imagery for large parts of the portrayal of terrain conditions than can be obtained from
world are available from the EROS Data Center of USGS. either system alone. Infrared imagery provides the follow-
They can be ordered by providing the geographic coordi- ing types of supplemental information that is valuable for
nates (latitude and longitude) of the area of interest or by evaluatingexisting landslide and landslide-susceptible terrain:
indicating particular frame numbers from the Single Land-
sat Coverage Map available from the above-noted organiza- Surface and near-surface moisture and drainage con-
tion. Canadian imagery can be obtained from the Canada ditions;
Centre for Remote Sensing, Department of Energy, Mines Indication of the presence of massive bedrock or bed-
and Resources. Addresses of these organizations are given rock at shallow depths;
earlier in this chapter. Experience has indicated that band Distinction between loose colluvial materials that are
5 (0.6 to 0.7 pm) or the infrared color composite is the present on steep slopes and are susceptible to landslides

67
and the massive bedrock that is more stable on steep slopes; seepage areas, runoff, and standing water are dark on both
and images. On the predawn infrared imagery in Figure 3.34,
4. Diurnal temperature changes that occur in soil masses standing water (points a and f) is warmer and has light
(these provide clues to the soil-water mass conditions). tones while seepage zones and runoff (points c and d) are
cooler and have darker tones. On the photography, all of
Tanguay and Chagnon (3.33) demonstrated the value of these areas have dark tones. Vegetated areas occurring be-
infrared imagery and aerial photography for evaluating the tween the areas of standing water in this figure have a me-
moisture and drainage regime associated with a landslide. dium tone on the imagery and appear as white specks on a
A flow slide had occurred within the crater of a former dark background on the photography.
slide in clay lake beds in the vicinity of Saint-Jean-Vianney, Figure 3.35 shows a nighttime (predawn) infrared image
Quebec. To plan a drilling program to evaluate the poten- of an area along a railroad line being investigated for locat-
tial of further movements required that the areas of seepage, ing potential areas for landslides. The railroad had been
water runoff, and wet soils be identified. By means of plagued for years with landslide problems. The circled
photography alone, these features could not be uniquely darker areas were interpreted as zones of seepage and high
separated from areas of standing water, topographic shad- moisture levels -potential for landslides. Based on this anal-
ows, and dense vegetation (brush and forested zones) be- ysis, the circled areas were drained, and no further slides
cause they all produced similar dark tonal patterns. 1-low- have occurred in those areas.
ever, the combination of photography and daytime and Another example of the value of infrared imagery is
nighttime imagery made it possible to separate these va- shown in Figure 3.36, which illustrates nighttime infrared
rious features and identify the critical items for planning imagery of an area of tilted sedimentary rocks. The mas-
the drilling program. sive bedrock areas (point 2) are indicated by light tones.
Figures 3.33 and 334 show some of the results reported The fractured rock zones and colluvial slopes, which are
by Tanguay and Chagnon. Figure 3.33 shows daytime ther- more susceptible to landslides, are indicated by medium
mal infrared imagery obtained in the 8 to 14-jim band and dark tones (point 3). These types of data would be useful
black-and-white infrared photographs of a portion of the in conjunction with aerial photography for rating the land-
area shown in the imagery. Several of the seepage areas slide susceptibility of the terrain.
(points a and b) interpreted from this figure were drilled Other remote-sensing techniques, such as multispectral
and indicated very deep and soft clay starting from the sur- imagery and microwave radiometry, offer potential for in-
face. The c points indicate both seepage and runoff in a dicating the presence of landslides. Multispectral systems
farm field: the d points show standing water coming from obtain simultaneous coverage of data in several spectral re-
snowmelt and surface drainage: and the e points depict the gions- usually a portion of the ultraviolet, the visible, and
boundary of the recent slide. Point f indicates the upper- both infrared reflectance and infrared emission regions.
most boundary of an older slide surface. In this figure, The data are collected on magnetic tape, which makes them

Figure 3.31. Old, massive landslide

terrain in Rio Arriba County, New
Mexico. Landslides (1) are of such
magnitude that they are easily
identifiable on common 1:20 000
aerial photographs as well as on
smaller scaled photo-index sheets
or even 1:1 000 000 satellite imagery
(Figure 3.32). Most slides encountered
in engineering projects are of smaller
magnitude and difficult to identify
at scales of 1:20 000.

6
amenable to the use of computer analysis techniques. Micro- time either further development is needed or the systems
wave radiometers collect radiomet nc (temperature varying) are too costly in comparison to the level of information
data in the microwave or radar regions. The particular ad- furnished. However, they do offer some future potential
vantage of this system is that at the longer wavelengths in- for landslide investigations.
formation from subsurface layers is included in the data.
Rib and others (3.27) have demonstrated that, under cer- FIELD RECONNAISSANCE
tain condit ions, informal ion on subsurface moist tire condi- TECHNIQUES
tions (i.e., presence of zones of high moisture level) can be
determined by comparative evaluation of photography, The discussion in this section is based to a large extent on
nighttime infrared imagery, and microwave radiometry. informal ion in Chapter 4 in the book on landslides published
These techniques are not described in detail because at this by the Highway Research Board in 1958 (3.10).

Figure 3.32. LANDSAT-1 satellite scene 1657-17031, band 5, May 11, 1974, north central New Mexico. Arrow pointsto edge of small
basalt remnant shown in Figure 3.31. Massive landslides are evident on satellite scene by irregular, scalloped edges. Hummocky
appearance of slide material can be discerned by close examination of scarp edges.

-
'r
.
- • .

-- .

'•.i

69
Figure 3.33. Daytime thermal infrared imagery in 8 to 14-11m band (above) and mosaic prepared from black-and-white
infrared photography (below) taken in Ontario, Canada (3.33). Lines AB and AB' represent equivalent coverages on
the two image formats. Lettered areas are discussed in text.

B,

IA'

C ,,

b-

Figure 3.34. Predawn infrared imagery in 410 5-,m band (above) and mosiic prepared from black•and•white infrared
photography (below) taken in Ontario, Canada (3.33). Line c-c represents common line on both Images. Lettered
areas are discussed in text.

dbb
: Ak Al-

P. 1kIJ e

70
Figure 3.35. Nighttime infrared imagery.
0645 h, station 560 to 580, Muskingum
Mine Railroad in Ohio. Dark-toned regions
within circled areas are interpreted as zones
of seepage and high moisture levels and
have been associated with slides along
railroad right-of-way. Light-toned areas
(1) represent standing water on surface.

Figure 3.36. Nighttime infrared imagery of

tilted sedimentary rocks in Schuylkill
County, Pennsylvania. Lightest tones (1)
are water or roads; next lightest (2) are
shallow soils over continuous bedrock;
medium gray tones (3) in bedrock area
generally indicate more fractured rock
and colluvial slopes.

An important phase in the preliminary investigation is mnent. Evidence may also be found of landslide movement
field reconnaissance to verify the three-dimensional concept that has not yet affected the highway but that may do SO in
of the terrain developed from the literature review and the time: Minor failure in an embankment, material that falls
analysis of remote-sensing data. Many of the fine details and on the roadway from an upper slope, or even the progres-
more subtle evidences of slope movement cannot be identi- sive failure of the region below a fill may presage a larger
fied at the scales of photographs and maps analyzed and can landslide that will endanger the road itself. Other evidences
be detected only in a field survey. Until that survey is made, of movement are to be found in broken pipe or power lines,
part, or even most, of the concepts developed remains con- spalling or other signs of distress in concrete structures,
jecture. Performing a field survey at the time corrective closure of expansion joints in bridge plates or rigid pave-
measures are being planned is especially important. Many merits (Figures 3.37 and 3.38), or loss of alignment of build-
landslides are complex and as time passes frequently change ing foundations. In many eases, arcuate cracks and minor
their physical characteristics and marks of identification. scarps in the soil give advance notice of serious failure.
For instance, a landslide examined a year after its occurrence Table 3.3 gives the chief evidence of movement in the va-
may have changed remarkably from the conditions immedi- rious parts of each type of landslide. The following discus-
ately following the original movement. If a landslide de- sion elaborates on the significance of cracks in recognizing
veloped as a slump slide and eventually turned into a flow, and classifying slides and on features that aid in identifying
the orinal report on the slide would be invalid as a basis the various types of landslides.
for planning a correction of the slide. The identification of The ability to recognize small cracks and displacements
type should be made at the same time the slide is to be cor- in the surface soils and to understand their meaning de-
re c ted. serves cultivation because it can produce accurate knowl-
edge of the cause and character of movement that is pre-
Field Evidence of Movement requisite to correction. Surface cracks are not necessarily
normal to the direction of ground movement, as is com-
The term landslide, by definition, implies that movement nionly assumed by some. For example, cracks near the
has taken place: hence, an analysis of the kind and amount head of a slump are indeed normal to the direction of hori-
of movement becomes a key to the nature of an active slide. zontal movement, but the cracks along its flank are nearly
It is imperative to learn to recognize the features that first parallel to it.
indicate the onset of ground movement and to learn to rec- Small en echelon cracks commonly develop in the stir-
ognize the kind of slide that already exists. Quite com- face soil before other signs of rupture take place: thus, they
monly the first visible sign of ground movement is recorded are particularly important in the recognition of potential
by settlement of the roadway or, depending on the location or incipient slides. They result from a force couple in which
of the roadway within the moving mass, a bulge of the pave- the angle between the direction of motion and that of the

71
cracks is a function of the location within the landslide area ample, in a slump the walls of cracks are slightly curved in
Thus, in many cases a map of the en echelon cracks will de- the vertical plane and concave toward the direction of move-
lineate the slide accurately, even though no other visible ment; if the rotating slump block has an appreciable vertical
movement has taken place (Figure 3.39). offset, the curved cracks wedge shut at depth. In block
In addition to indicating incipient or actual movement, slides, on the other hand, the cracks are nearly equal in
cracks in surface soils are locally useful in helping to deter- width from top to bottom and do not wedge out at depth
mine the type of slide with which one is dealing. For cx- because failure in a block slide begins with tension at the
base of the block and progresses upward toward the surface.
Figure 3.37. Early signs of impending debris slide along Clear Creek,
The presence of a few major breaks in the upper parts of a
Colorado (3. 10). Displacement of fence, bulging of pavement, and block slide (Figure 3.40) distinguish it from lateral spreading,
distress in bridge abutment (Figure 3.38) all give early indications which is characterized by a maze of intersecting cracks. The
of movement at toe of incipient slide. In several places, now inclination of cracks is commonly almost vertical in block
covered by patching, centerline stripe was offset along cracks.
slides in cohesive soils, regardless of the dip of the slip plane,
but depends on the joint systems in the rock in block slides
q
.k
t;-
of rock.
Distinguishing between incipient block slides and slumps
is one of the most helpful applications of a study of cracks.
If the outline of the crack pattern is horseshoe-shaped in
plan, with or without concentric cracks within it, a slump
is almost certainly indicated. If, on the other hand, most
of the surface cracks are essentially parallel to the slope or
ft' cliff face, a block slide is probably in the making. In either
..iI..
; ,. case, additional cracks may develop as major movement gets
under way, but these will generally conform to the earlier
crack pattern (Figures 2.14 and 2.15).
Evidence of soil creep and of "stretching" of the ground
surface should also be sought. Stretching indicates compar-
atively deep-seated movement whereas soil creep is of sur-
ticial origin. The phenomenon of stretching is most corn-
.;
nionly observed in noncohesive materials that do not form
or retain minor cracks readily. The best evidence of stretch-
ing consists of small cracks that surround or touch some
Figure 3.38. Right wing wall shown in lower center of Figure 3.37 rigid body, such as a root or boulder, in otherwise homno-
(3. 10). Distress in bridge abutment indicates incipient slide. In
addition to offset of wing wall shown here, rockers beneath bridge
gencous materials: these cracks form because the tension
girders were tipped. forces tend to concentrate at or near the rigid bodies.

Field Iden tifica tb ml of Slope-

Movement Types

Once it has been established that slope movement has taken

Figure 3.39. Tension

cracks that typically
develop in slump slide in
cohesive materials Q. 10, - -
3.34). /
DEPRESSION

/
EN ECHELON
/ CRACES

ZONE
OF UPLIFT

-
FOOT

'\

ME

72
place or is in progress, the next essential step is to identify rather than concave, cracks. Lateral spreading with few, if
the type of movement. Table 3.3 gives the surface features any, cracks, on the other hand, is characteristic of earth
that aid in the field identification of slope movements. flows.
This table gives only the most common types and not all Translational rock slides are generally easy to recognize
of the types shown in Figure 2.1 . Further generalizations because they are composed wholly of rock, boulders, or
are given in the following paragraphs. rock fragments. Individual fragments may be very large and
may move great distances from their source. Rock slides
Falls and Topples
Figure 3.40. Block slide in cohesive materials, near Portage.
Although the initial mecha ii isms of falls and topples are dif-
Montana (3. 10). The slide, in alluvium, colluvium, and some wind-
ferent (Chapter 2), their final appearances after movement deposited silt, is moving out over surface of alluvium-filled stream
are similar. Falls and topples are best recognized by the ac- channel with little or no rotation of block and without developing
cumulation of material that is not derived trorn the under- zone of uplift at foot. Parallel step scarps along main scarp and
drag effects along flanks are both characteristic of block slides.
lying slope and that is foreign to normal processes of erosion.
Arrows indicate overbreak cracks that develop after main scarp is
In most cases, this material consists of blocks of rock, de- formed; possibly because of abrupt break in slope, these are more
bris. or earth scattered over the surface or forming a talus sharply curved than are those above most slump slides.
slope. If undercutting by lake or stream waters has caused
the fall, the rate of failure is proportional to the ability of
the water to remove the fallen material. A fast-moving
stream may remove material almost as fast as it falls, thus
removing the evidence but encouraging continuing falls. :
On the other hand, a lake or some parts of an ocean shore
must depend only on wave action to disintegrate and re- .--- - - ' '
move the fallen material: hence, the evidence tends to re-
main in sight but continued falls tend to be inhibited. Most
of the material yielded by rock topples and rock falls is
necessarily close to the steep slopes from which it came,
but some may bound down the slope and come to rest far
from its present source.
In active or very recent slides, the parent cliff is com-
mon!)' marked by a fresh irregular scar that lacks the cres-
cent shape that is characteristic of slumps. Instead, the ir- Figure 3.41. Rock slide and rock fall along Penn-105
regularity of its surface is controlled by the joints and bed- (3. 10). Beds are nearly flat; slide is controlled by joints
ding planes of the parent material (Figures 2.3 and 3.41). dipping at steep angle toward road.
Some idea of the intensity and state of activity of a rock
fall or rock topple can be inferred from the presence or ab-
sence of vegetation on the scarp and by the damage done to
trees by the falling rocks. In active areas, the trees are
scarred and debarked or show evidence of healed wounds.
If the rock fall or rock topple is severe or of long duration -
conifers and other long-lived trees are absent: their places
may or may not be taken by aspen or other fast-growing
species. Many rock falls follow chutes or dry canyons that
can usually be differentiated from normal watercourses or
paths cut by snow avalanches (Figure 3.42).
Some debris and earth falls and topples exhibit most of
the characteristics of rock falls and rock topples. Others
proceed by mere spallirig of the surface, but even this activ-
ity, it' long continued, can lead to removal of considerable
quantities of material. Any fall or topple may • of course.
presage major landslide movement in the near future.

Slides

Slides, as distinct from falls, topples, lateral spreads, and

flows, are characterized by a host of features that are obser-
vable at the surface. These features are related to the kind
of material in which the slide occurs and to the amount
and direction of motion. Rotational slides are character-
ized by rotation of the block or blocks of which they are
composed whereas translational slides are marked by lateral
separation with little vertical displacement and by vertical,

73
Table 3.3. Features that aid in recognition of common types of slope movements.

Parts Surrounding Slide

Kind of
Type of Motion Material Crown Main Scarp Flanks

Falls, topples Rock Consists of loose rock; probably Is usually almost vertical, irregu- Are mostly bare edges of rock
- has cracks behind scarp; has ir- lar, bare, and fresh; usually con-
regular shape controlled by local sists of joint or fault surfaces
joint system

Soil Has cracks behind scarp Is nearly vertical, fresh, active, Are often nearly vertical
and spalling on surface

Slides
Rotational
Slump Soil Has numerous cracks that are Is steep, bare, concave toward Have striae with strong vertical
mostly curved concave toward slide, and commonly high; may component near head and
slide show striae and furrows on sur- strong horizontal component
face running from crown to near foot; have scarp height
head; may be vertical in upper that decreases toward foot;
part may be higher than original
ground surface between foot
and toe; have en echelon
cracks that outline slide in
early stages
Rock Has cracks that tend to follow Is steep, bare, concave toward Have striae with strong vertical
fracture pattern in original rock slide, and commonly high; may component near head and
show striae and furrows on sur- strong horizontal component
face running from crown to near foot; have scarp height
head; may be vertical in upper that decreases toward foot;
part may be higher than original
ground surface between foot
• and toe; have en echelon
cracks that outline slide in
early stages
Translational
Block Rock Has cracks most of which are Is nearly vertical in upper part Have low scarps with vertical
or nearly vertical and tend to fol- and nearly plane and gently to cracks that usually diverge
Soil low contour of slope steeply inclined in lower part downhill

Rock Rock Contains ioose rock; has cracks Is usually stepped according to Are irregular
between blocks spacing of joints or bedding
planes; has irregular surface in
upper part and is gently to
steeply inclined in lower part;
may be nearly planar.or com-
posed of rock chutes
Flows
Dry Rock Consists of loose rock: probably Is usually almost vertical, irregu- Are mostly bare edges of rock
has cracks behind scarp; has ir- lar, bare, and fresh: usually con-
regular shape controlled by local sists of joint or fault surfaces
joint system

Soil Has no cracks Is funnel shaped at angle of repose Have Continuous curve into main
scarp
Wet
Debrisavalanche Soil Has few cracks Typically has serrated or V-shaped Are §1eep and irregular in upper
Debris flow upper part: is long and narrow, part; may have Ieveesbuilt up
bare, and commonly striated in lower parts

Earth flow Soil May have a few cracks Is concave toward slide; in some Are curved; have steep sides
types is nearly circular and slide
issues through narrow orifice

Sand flow • Soil Has few cracks Is steep and concave toward slide; Commonly diverge in direction
Silt flow may have variety of shapes in of movement
outline: nearly straight, gentle
arc, circular, or bottle shaped

Note: This table updates Table 1 in book on landslides published in 1958 by Highway Research Board (3.10, chap. 4 by A. M. Ritchie, pp. 56.57)

74
Parts That Have Moved

Head Body Foot Toe

Is usually not well defined; con- Falls:. Has irregular surface of Is commonly buried; if visible, Is irregular pile of debris or talus
sists of fallen material that jumbled rock that slopes away generally shows evidence of rea- if slide is small; may have
forms heap of rock next to from scarp and that, if large son for failure, such as promi- rounded outline and Consist of
scarp and if trees or material of Con- nent joint or bedding surface, broad, curved transverse ridge
trasting colors are included, underlying weak rock, or banks if slide is large
may show direction of move- undercut by water
ment radial from scarp; may
contain depressions
Topples: Consists of unit or
units tilted away from crown
Is usually not well defined; con Is irregular Is commonly buried; if visible, Is irregular
sists of fallen material that generally shows evidence of rea-
forms heap of rock next to son for failure, such as promi-
scarp nent joint or bedding surface,
underlying weak rock, or banks
undercut by water

Has remnants of land surface flatter Consists of original slump blocks Commonly has transverse cracks Is often a zone of earth flow of
than original slope or even tilted generally broken into smaller developing over foot line and lobate form in which material
into hill, creating at base of masses; has longitudinal cracks, transverse pressure ridges de- is rolled over and buried: has
main scarp depressions in which pressure ridges, and occasional veloping below foot line; has trees that lie flat or at various
perimeter ponds form; has trans- overthrusting; commonly de- zone of uplift, no large individ- angles and are mixed into toe
verse cracks, minor scarps, velops small pond just above ual blocks, and trees that lean material
grabens, fault blocks, bedding foot downhill
attitude different from surround-
ing area, and trees that lean up-
hill
Has remnants of land surface flatter Consists of original slump blocks Commonly has transverse cracks Has little or no earth flow; is often
than original slope or even tilted somewhat broken up; has little developing over foot line and nearly straight and close to foot:
into hill, creating at base of plastic deformation: has longi- transverse pressure ridges de- may have steep front
main scarp depressions in which tudinal cracks, pressure ridges, veloping below foot line: has
perimeter ponds form; has trans- and occasional overthrusting: zone of uplift, no large individ-
verse cracks, minor scarps, commonly develops small pond ual blocks, and trees that lean
grabens, fault blocks, bedding just above foot downhill
attitude different from surround-
ing area, and trees that lean up-
hill

Is relatively undisturbed and has Is usually composed of single or Has none and no zone of uplift Plows or overrides ground surface
no rotation few units; is undisturbed ex-
cept for common tension cracks
that show little or no vertical
displacement
Has many blocks of rock Has rough surface of many lisually has none Consists of accumulation of rock
blocks, some of which may be fragments
in approximately their original
attitude but lower if movement
was slow translation

Has none Has irregular surface of jumbled Has none Composed of tongues; may over-
rock fragments sloping down ride low ridges in valley
from source region and gener-
ally extending far out on valley
floor: shows lobate transverse
ridges and valleys
Usually has none Is conical heap of soil, equal in Has none
volume to head region

May have none Consists of large blocks pushed Is absent or buried in debris Spreads laterally in lobes; if dry,
along in a matrix of finer mate- may have a steep front about a
rial: has flow lines: follows drain- meter high
ageways and can make sharp
turns; is very long compared to
breadth
Commonly Consists of a slump Is broken into many small pieces: Has none Is spreading and lobate; consists
block shows flow structure of material rolled over and
buried; has trees that lie flat
or at various angles and are
mixed into toe material
Is generally under water Spreads out on underwater floor Has none lsspreading and lobate

75
 commonly occur only on steep slopes. The numerous small Figure 3.42. Rock fall and rock slide near Skihist, British
block units with random rotation are mixed in a matrix of Columbia, on Canadian National Railroad (3.10). Note bare
active slopes, closely spaced jointing of rocks, rock chutes, and
finer grained material; the large rock fragments tend to float
absence of water. Wooden and concrete sheds bypass debris
on or in the matrix. Rock slides have translational failure falling over tracks from above.
surfaces rather than the concave surfaces of slumps, and
they do not move as unrotated multiple units, like block
slides. Most rock slides are controlled by the spacing of
joints and bedding planes in the original rock. There is par-
ticular danger of forming a rock slide of serious proportions
if construction is undertaken in an area marked by a system
of strongly developed joints or bedding planes that dip
steeply outward toward the natural slope. This is especially
true if the natural slope angle is steeper than the angle of re-
pose of the broken rock. Water is not so important a factor
in causing rock slides as in causing slides in soil. In sonic in-
stances, however, water helps to weaken bedding or joint
planes that otherwise would offer high frictional resistance.
Any seepage that is apparent after a rock slide has taken
place is most likely to be visible in the scarp region or, per-
haps, in the slide material itself.
Many rock slides are thinner than either slumps or block
slides because they commonly are restricted to the weathered
zone in bedrock or to surficial talus. The shape of the shear
zone, therefore, conforms to the unweathered bedrock sur-
face and is not controlled to any large extent by joints or even so they may be indicated by evidence of surface stretch-
bedding planes in the bedrock. Many talus slopes produce ing, by lines of rock fragments that have been displaced, or
rock slides by failure within the body of talus. even by blades of grass that have been pulled down into the
Rotational slumps rarely form from solid. hard rocks, cracks by sand or other loose surface material as it sifted
although special combinations of factors have been known into them. The head region of a slump or an earth block
to produce them. Slumps are widespread, however, in slide with surficial slump characteristics may also be recog-
sands, silts, and clays and in the weaker bedded rocks. nized by the presence of slump grabens that have experi-
There they can be identified readily from surface indica- enced some rotation (Figures 3.19 and 3.43). These are de-
tions, though only after considerable movement has taken pressed fault blocks of soil or rock, caused in part by de-
place. The head region of a slump is characterized by steep crease in curvature of the shear plane. This produces ten-
escarpments and by visible offsets between separate blocks sion and ultimate failure in the main slump block because
of material (Figure 3.19). The highest escarpment is com- of lack of support on its uphill side.
monly just below the crown: because the crests of the flanks The amount and direction of rotation that any slump
are lower than the crown, they can be recognized as flanks, block has undergone can usually be ascertained by deter-
even if the escarpment on one of them happens to be larger mining the slope of its surface as compared with that of the
than the one below the crown. If the landslide is active or original slope. Comparison of the (lip and strike of bedded
has been active recently, the scarp is bare of vegetation and material within the slump block with the original attitude
may be marked by striations or grooves that indicate the di- of the unslumped material is an even more exact and fool-
rection of movement that has taken place. Striations at the proof method of determining the amount of rotation, be-
head tend to reflect downward movement whereas striations cause erosional alternation of the surfaces may give an er-
along the flanks may be nearly horizontal. If the slump is roneous impression. In a general way, the amount of ro-
compound, its several crescent-shaped scarps will appear as tation is a measure of the amount of displacement.
a scalloped edge in plain view (Figure 2.21). The part just above the foot of a slump is a zone of com-
Undrained depressions and perimeter lakes, bounded on pression. The slumped material is con fined by the foot and
the uphill side by the main scarp, characterize the head re by the flanks of the main scarp SO that it is compressed by
gions of many slumps: even if internal drainage prevents the load above into the bot torn part of the howl-shaped sur-
such ponds from holding water for long periods, their de- face of rupture. In this region, therefore, there are no open
pressions may be evident. In humid regions, the head area cracks. The foot region is marked by a zone of tension and
may remain greener than surrounding areas because of the uplifl because the slumped material is required to stretch
swampy conditions. In the San Juan region of south- over the foot before it passes farther downhill. This stretch-
western Colorado. for instance, groves of aspen trees are ing destroys any remaining slump blocks because of the
commonly good indications of wet ground conditions and change in direction of the forces. The small blocks and frag-
hence of slides and unstable ground. In northern West Vir- ments that result tend to weather to rounded forms, produc-
ginia, the swampy areas in the head remain green during ing hummocky ground. Pavement uplifts and cracks, so
the winter whereas the well-drained low areas are brown. disconcerting to the motorist and highway engineer. are
The tension cracks near the head of a slump are generally also most likely to take place in the foot zone. Seeps,
concentric and parallel to the main scarp. Many such cracks springs, and marshy conditions commonly mark the foot
are obscured by rubble or other noncohesive materials, but and toe of a slump. Moreover, trees tend to be tilted down.

76
Figure 3.43. Graben on slump slide (3. 10). Rotation of slump without rotation, into the space left by removal of the
block is uphill, resulting in flattening of original ground surface, plastic layer.
but graben block, which breaks off from slump block, rotates
downhill. Grabens do not form if slump block has sheared on surface
Dry flows are not difficult to recognize after they have
that approaches arc of circle; instead, they form on slump blocks that occurred, but it is virtually impossible to predict them in ad-
slide over principal surface of rupture having marked decrease in its vance. They are commonly very rapid and short-lived. Dry
curvature, causing greater horizontal movement for each unit of flows are rarely composed of rock fragments; they usually
vertical offset in center portion of slide than in head region.
consist of uniformly sized silt or sand. They exhibit no
cracks above the main scarp and have poorly developed or
nonexistent flow lines. Except for sand runs, they have no
HEAVY LINE INOICATES
GRV8EN / ORIGINAL SURFACE well-defined foot. Dry bess may become fluid and flow
down a slope because of an earthquake or other external
vibrations. Sand runs also behave somewhat as fluids, but
MP J// in the latter part of their course the sand particles are more
likely to slide than to flow.
Wet flows occur when fine-grained soils, with or without
coarser debris, become mobilized by an excess amount of
water. Most of them behave like wet concrete in a chute
(differences are due to water content), but the flow of
some wet silts and fine sands is triggered by shocks. "Quick"
Figure 3.44. General orientation of trees on slump landslide (3. 10). or sensitive clays may be liquefied by the leaching of salts
As a result of rotation, trees on blocks are bowed uphill because or by other causes that are not completely understood.
tree tops tend to grow vertically while stump portion changes with Wet flows are generally characterized by their great
rotating land surface. Contrast head and toe regions and compare
lengths, generally even gradients and surfaces, absence of
with Plates 3.2 and 3.3.
tension cracks, and lack of blocky units and minor scarps.
If tension cracks are present, they are bowed in the direc-
tion of movement (Figure 3.45), showing the effects of
movement of wetter materials at depth beneath the drier
crust. An older flow that has had time to dry may show
large shrinkage cracks or flow lines. In many cases, the
main scarp area is emptied by the removal of all flow ma-
terial and resembles a glacial cirque. In other cases, the gra-
dation may be imperceptible downward from soil creep to
mud flow.
The rate of flow is dependent on the total amount of
material that feeds the slide; the accumulated debris im-
hill rather than uphill as they near the head (Figure 3.44 poses a load on the entire mass below it and tends to main-
and Plate 3.3). tain a constant rate of movement. The flow is under pres-
The approximate age of some slumps can be inferred by sure everywhere from the material above it; consequently,
study of the bent trees. If older trees are bent but younger the mass shows few, if any, cracks over the foot. Flows can
ones are straight, for instance, the slide probably has not and do make sharp turns and move around firmly fixed ob-
moved during the life of the younger trees. On the other stacles that appear in their paths. A very wet flow moving
hand, the sizes of the tree trunks at the points where the at high velocity may leave debris marks high on trees or
bends occur give a running history of the rotation. other objects and ridges of debris called torrent levees along
Just below the foot of a slump the ground is commonly its sides.
marked by long transverse ridges that are separated from
one another by open tension cracks. These cracks seldom Hidden Landslides
remain open for long periods of time, and they do not form
scarps or other evidence of displacement because the mate- Among the most difficult kinds of slides to recognize and
rial is no longer confined but spreads out laterally and de- guard against are old landslides that have been covered by
velops radial cracks at the toe. glacial till or other more recent sediments. To predict, in
detail, the existence of old buried slides or the effect that
Lateral Spreads and Flows they may have on new construction work that exposes
them is probably impossible. One who knows the recent
In lateral spreads, a coherent upper block is broken into geologic history of the region intimately, however, may be
strips or blocks and extended or stretched by plastic flow able to make some educated guesses as to the probable ex-
or liquefaction of the lower layer. The mechanism of fail- istence of such slides and even as to where they are most
ure can involve not only rotation and translation, but also likely to be found. One example is shown in Figure 3.46;
flow, and thus surface characteristics will contain elements the indicated unstable body of soft shale and chalk is
of both slides and flows. One form of lateral spread, in clearly related to a fault in the bedrock, but it also has the
which a plastic layer is squeezed out by the weight of an characteristics of a surface landslide. Since this slide took
overlying rigid layer, is here termed a piston slide. In place, the area was covered by two or more layers of bess,
this type, a part of the upper layer may drop vertically, which completely obscured the landslide until the rising

VIA
Figure 3.45. Crack pattern in slump on south
side of Reservoir Hill, Dunbar, West Virginia,
indicates flowage at depth beneath harder
material at surface (3. 10). Broken pipes from
reservoir at top of hill dumped large amount EDGE OF
of water into old slide and reactivated it. Elm 49m
Horseshoe-shaped scarp is imperfect,
differentiating it from that of true slump.
Greatest movement is near center of slide,
as indicated by arrangement of cracks and
of standing water. Presence of convex
cracks is indicative of flow movement at
depth.
Etev. 28 m
Elev. 23 m

Elev, 23 m

EI~
lm

,Etev. 17 m

oo

EIev. 12 m

30 m
EIev. 9 m
,
/ bOlt
/

Figure 3.46. Hidden landslide exposed when overburden of bess one is most likely to occur. The evidence for distressed con-
was removed by bank-cutting along shore of Fort Randall Reservoir
ditions that may be present or induced lies chiefly in evi-
in South Dakota (3. 10). Buried soil profiles indicate two periods
of bess deposition after faulting and landsliding took place in
dence of movements, minor or major, that have already
underlying shale and chalk of Cretaceous age. All surface evidence taken place or of geologic, soil, and hydrologic conditions
of landsliding was obliterated by bess until lake waters cut new that are likely to cause movement in the future. Once the
face. fact of land movement, actual or potential, has been estab-
lished, the next essential step is to identify the type of land-
slide. One would not apply the same corrective procedure
to a rock fall and a block slide any more than one would at-
tempt to prevent a slide without knowing the kind of slide
expected. If maximum benefit is to be accrued from the
preventive or corrective measures finally employed, it is
imperative to learn to recognize the kind of slide that ex-
-
- --- -
ists or is expected.
One final point needs emphasizing. Even if the prelim-
*---
inary examination of the general environment has indicated
that no landslide movements have yet taken place or are
imminent, the investigator must still determine whether
the ground to be disturbed by the proposed construction
will prove reasonably stable. No one is capable, nor is
money available, of studying in detail and of guaranteeing
waters of the lake undercut the bank and exposed the old the stability of all slopes for most large construction proj-
slide. ects. As a general rule, the amount of investigation that is
warranted is a function of the landslide susceptibility of the
CONCLUSIONS surrounding country and also of the degree of damage that
might be expected to occur to persons or installations if a
All landslide investigations must start with either the recog- slide should occur. In other words, the more serious the
nition of a distressed condition on the natural or artificial consequences of a landslide are, the more detailed the
slope or the determination of the vulnerable locations where search for potential slides should be.

78
REFERENCES eastern Ohio. Ohio Department of Highways, Columbus,
Vols. 1 and 2, 1965.
3.23 Ray, R. G. Aerial Photographs in Geologic Interpretation
and Mapping. U.S. Geological Survey, Professional Paper
3.1 Alföldi, T. T. Regional Study of Landsliding in Eastern 373, 1960, 230 pp.
Ontario by Remote Sensing. Department of Civil Engi- 3.24 Radbruch-Hall, D. H., Colton, R. B., Davies, W. E., Skipp,
neering, Univ. of Toronto, MS thesis, 1974,79 pp. B. A., Lucchitta, I., and Vanes, D. J. Preliminary Land-
3.2 Anderson, R. R., Hoyer, B. E., and Taranik, J. V. Guide slide Overview Map of the Conterminous United States.
to Aerial Imagery of Iowa. Iowa Geological Survey, U.S. Geological Survey, Miscellaneous Field Studies Map
Public Information Circular 8, Sept. 1974, 142 pp. MF-771, 1976.
3.3 Baker, R. F., and Chieruzzi, R. Regional Concept of 3.25 Reeves, R. G., ed. Manual of Remote Sensing. American
Landslide Occurrence. Highway Research Board, Bulletin
Society of Photogrammetry, Falls Church, Va., Vols. 1
216, 1959, pp. 1-16. and 2, 1975, 2144 pp.
3.4 Bruce, R. L., and Scully, J. Manual of Landslide Recog- 3.26 Rib, H. T. An Optimum Multisensor Approach for De-
nition in Pierre Shale, South Dakota. South Dakota De-
tailed Engineering Soils Mapping. Purdue Univ., Lafayette,
partment of Highways, Pierre, Final Rept. of Research
Ind., Joint Highway Research Project 22, Vols. 1 and 2,
Project 615(64), Dec. 1966, 70 pp. 1966, 406 pp.
3.5 Chassie, R. G., and Goughnour, R. D. National Highway 3.27 Rib, H. T., Spencer, J. M., Jr., Falls, C. P., and Koca, J. F.
Landslide Experience. Highway Focus, Vol. 8, No. 1,
Evaluation of Aerial Remote Sensing Systems for Detect-
Jan. 1976, pp. 1-9.
ing Subsurface Cavities in Kansas. Federal Highway Ad-
3.6 Cleaves, A. B. Landslide Investigations: A Field Hand-
ministration, Rept. FHWA RD-75-119, 1977.
book for Use in Highway Locations and Design. U.S. 3.28 Scully, J. Landslides in Pierre Shale in Central South
Bureau of Public Roads, 1961,67 pp.
Dakota. South Dakota Department of Transportation,
3.7 Colwell, R. N., ed. Manual of Photographic Interpreta-
Pierre, Final Rept. of Project 635(67), Dec. 1973, 707
tion. American Society of Photogrammetry, Falls
pp.; Federal Highway Administration.
Church, Va., 1960, 868 pp. 3.29 Sharpe, C. F. S. Landslides and Related Phenomena: A
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Quick Clays of Eastern Canada. Proc., 10th International 3.32 Stallard, A. H., and Myers, L. D. Soil Identification by
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terpretation in Highway Engineering Practice. Materials Hutchinson and Ross, Stroudsburg, Penn., 1973, 329 pp.
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196, Oct. 1966, 213 pp. tion
3.20 Mintzer, 0. W., and Struble, R. A. Manual of Terrain In- Plate 3.2 Courtesy of Washington State Highway Commission
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196-2 and Appendix I, Oct. 1965, 384 pp. Plate 3.5 Courtesy of National Aeronautics and Space Adminis-
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Space Administration, Houston, 1974, 359 pp. tration
3.22 Norell, W. F. Air Photo Patterns of Landslides in South- Figure 3.1 Courtesy of Virginia Division of Mineral Resources

79
Figure 3.3 Courtesy of Ohio Department of Transportation Figure 3.27 Courtesy of Ontario Ministry of Transportation and
Figure 3.4 Courtesy of Washington State Highway Commission Communications
Figure 3.5 Courtesy of Ohio Department of Transportation Figure 3.28 Courtesy of U.S. Department of Agriculture
Figure 3.6 Courtesy of Ohio Department of Transportation Figure 3.30 Courtesy of U.S. Department of Agriculture
Figure 3.7 Courtesy of Idaho Transportation Department Figure 3.31 Courtesy of U.S. Geological Survey
Figure 3.12 Courtesy of Kansas Department of Transportation Figure 3.32 Courtesy of National Aeronautics and Space Adminis-
Figure 3.14 Courtesy of U.S. Department of Agriculture tration
Figure 3.15 Courtesy of Ohio Department of Transportation Figure3.33 Courtesy of M. G. Tanguay
Figure 3.16 Courtesy of South Dakota Department of Transporta- Figure 3.34 Courtesy of M. G. Tanguay
tion Figure 3.35 Courtesy of D. Rubin, American Power Service Corpo-
Figure 3.17 Courtesy of Ohio Department of Transportation ration, and F. J. Buckmeier, Texas Instruments, Inc.
Figure 3.18 Courtesy of U.S. Geological Survey Figure 3.36 Courtesy of Federal Highway Administration
Figure 3.19 Courtesy of U.S. Department of Agriculture Figure 3.37 D. J. Varnes, U.S. Geological Survey
Figure 3.20 Courtesy of U.S. Geological Survey Figure 3.38 D. J. Varnes, U.S. Geological Survey
Figure 3.21 Courtesy of U.S. Department of Agriculture Figure 3.40 E.K. Maughan, U.S. Geological Survey
Figure 3.22 Courtesy of U.S. Department of Agriculture Figure 3.41 Courtesy of Pennsylvania Department of Transporta-
Figure 3.23 Courtesy of U.S. Department of Agriculture tion
Figure 3.24 Courtesy of U.S. Department of Agriculture Figure 3.42 F. 0. Jones, U.S. Geological Survey
Figure 3.25 Courtesy of U.S. Department of Agriculture Figure 3.45 Robert C. Lafferty, consulting geologist
Figure 3.26 Courtesy of North Dakota State Highway Department Figure 3.46 C. F. Erskine, U.S. Geological Survey

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