Introduction To LRFD For Foundation and Substructure Design - Module 5
Introduction To LRFD For Foundation and Substructure Design - Module 5
Introduction To LRFD For Foundation and Substructure Design - Module 5
Design– Module 5
132010B September 2021
1
Welcome and Introduction
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Module and Lesson Learning Outcome:
Upon completion of this module, you will be able to recognize basic shallow
and deep foundation design by LRFD. This will be accomplished through
the completion of the lessons, where you will be able to:
• Explain limit state checks for shallow and deep foundations; and
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Lessons in this Module
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Lesson 1: Basic Foundation Analysis Techniques
Let’s get started with the first lesson. At the end of this lesson, you will
recognize basic shallow foundation and deep foundation design by LRFD.
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Shallow Foundations
In the first portion of this lesson, you will learn about shallow foundations.
Spread footings are the most common type of shallow foundation. All
shallow foundations must provide adequate resistance against
geotechnical and structural failure.
6
Shallow Foundation Design Considerations
Limit state checks during the design of shallow foundations are correct
when:
• The footing does not slide due to applied lateral loads; and
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• Overturning does not occur.
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Shallow Foundations and Limit States
For the strength limit state, shallow foundation design considers structural
resistance and loss of lateral and vertical support due to scour at the 100
year design flood event, normal bearing resistance, overturning or
excessive loss of contact, sliding at the base of footing, and
constructability.
For the extreme limit state, scour at the 500 year flood event and seismic
activity are checked. Note that scour is checked at the service, strength,
and extreme limit states.
Typically the service limit state design considerations frequently control the
minimum footing dimension for shallow foundations. Therefore, it is
recommended that the spread footing size is designed at the service limit
state, then checked against all remaining applicable limit states.
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10.5.3.1, and 10.5.3.2 for more information.
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Shallow Foundation Minimum Depth Considerations
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Frost Protection
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Unsuitable Foundation Materials
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Subgrades FHWA Publication Nos. FHWA-RD-79-49 and FHWA-RD-
79-50, 1979.
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Scour
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• Hydraulic Engineering Circular No. HEC 23 V2 – Bridge Scour and
Stream Instability Countermeasures Experiences, Selection, and
Design Guidance, third edition, September 2009; and
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Groundwater
17
Frost protection includes which of the following?
b) Proper compaction
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Overall or Global Stability
Let’s move on to shallow foundation limit state checks. The first one is
global stability. Global stability of shallow foundations is to be evaluated
when the footing is placed on an embankment. Slope stability is also to be
evaluated when the footing is located on, near, or within a slope, and the
stress applied by the footing loads is acting to destabilize the slope
geometry.
The figure on the left shows a footing placed at the bottom of a slope and
the figure on the right shows a footing placed at the top of the slope. The
footing placed at the top of the slope generally destabilizes the slope
because of the additional load placed onto the slope by the footing. The
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footing placed at the bottom of the slope generally helps stabilize the slope
because the footing can act to provide support to the slope.
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Overall or Global Stability – Rupture Planes
There are three common rupture planes typically evaluated for overall or
global stability. They are:
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Shallow Circular Rupture Plane
For the shallow circular rupture, the force driving the movement along the
shallow rupture plane is the weight of the soil. The movement of the soil
along the shallow rupture plane, causes a soil bulge to develop and the
wall to rotate backward.
The concept of maximum and minimum load factors is not easily adaptable
to this method of computation. Therefore, global stability is evaluated at the
service limit state with load factors of 1.0.
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Deep Seated Circular Rupture Plane
For a deep seated circular rupture plane, the deep rupture plane forms at a
greater depth than the shallow rupture plane with similar results. The
movement of the soil along the deep rupture plane causes a soil bulge to
develop and the wall to rotate backward. In this case, the soft material with
low shear strength caused the deep rupture plane to develop.
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Deep Seated Sliding Wedge Rupture Plane
A deep seated sliding wedge rupture plane can occur when the foundation
soil contains a thin seam of weak soil or soft material with low shear
strength beneath firm soil. The results are similar to those of the shallow
and deep seated circular rupture planes. The driving force is the weight of
the soil.
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Overall or Global Stability (con.)
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Overall Stability Resistance Factors for Shallow Foundations
This table compares overall stability ASD factors of safety to the overall
stability LRFD resistance factors. The ASD factors of safety for overall
stability were used when designing for overall stability by the ASD method.
For ASD, the factor of safety is commonly designated as FS. The LRFD
resistance factors are used when designing for overall stability by LRFD.
For LRFD, the resistance factor is designated by the phi symbol.
The development of the LRFD resistance factors for overall slope stability
analysis were created by generally fitting to ASD factors of safety. When
LRFD load factors, designated by the gamma symbol, are equal to 1.0, the
LRFD resistance factor, phi, is generally the inverse of the ASD factor of
safety, FS.
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If geotechnical parameters are well defined by in-situ or laboratory tests,
and the slope does not support structure, the factor of safety using ASD is
equal to 1.3, and the equivalent LRFD resistance factor phi is equal to 0.75.
The LRFD resistance factors of 0.75 and 0.65 are provided in AASHTO
Article 11.6.2.3.
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Nominal Bearing Resistance
The next shallow foundation limit state check is for the nominal bearing
resistance that is determined at the strength limit state by the basic nominal
equation (AASHTO 10.6.3.1.2a-1). Measured soil parameters by in-situ or
laboratory testing are to be used that are representative of the soil shear
strength under the considered loading and subsurface conditions.
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Settlement
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Elastic Settlement
Elastic settlements are not truly elastic; it’s just an immediate distortion
response to an applied load.
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Primary Consolidation
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Primary Consolidation (con.)
The second graph illustrates the compressibility of the soil that increases
as the soil void volume increases and the soil solids volume decreases.
The magnitude of settlement is directly related to the void volume. It is
important to investigate, through sampling and laboratory testing, soils in
the silt, clay, or organics range because of the high percentage of void
volume.
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Consolidation Settlement (Spring Analogy)
Recall from previous modules, the effective stress analogy shown by the
piston figures.
For this concept, a cylinder filled with moist soil that is covered with a piston
is compared to a spring supported piston in a cylinder filled with water and
a valve that is closed within the piston. The spring represents the mineral
skeleton.
As the valve is opened, the applied load is transferred from the water to the
mineral skeleton. The total stress is composed of both the stress in the
mineral skeleton and the fluid water.
Once the water pressure is relieved, long term support is provided by the
mineral skeleton. This volume change typically leads to higher strength of
the soil.
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Secondary Settlement
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What type of process is elastic settlement?
a) Immediate
b) Long-term
c) Secondary
d) Hydraulic
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Shallow Foundation Settlement Considerations
This table shows the basic relationship of soil type to settlement types.
• Sand and gravel are subject to elastic settlement, but not primary or
secondary settlement.
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Layered Profile Settlement Analysis
In the real world, settlement analysis encounters layers of soil with a variety
of compositions. Settlement is evaluated by a layered profile settlement
analysis.
For layered profile settlement analysis, the soil profile is divided into layers
as shown. The stress increase at the layer’s midpoint is calculated, then
the settlement for each layer is determined and added for total settlement.
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Settlement
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Shallow Foundations on Rock
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Impact of Settlement on Structures
Now that you understand settlement, let’s look at its impact on structures.
In this photo, the superstructure is making contact with the backwall due to
the rotation and settlement of the abutment. In an effort to mitigate this,
notice the large shim below the bearing. A settlement analysis of the soft
clay layer beneath the abutment indicates a potential for an additional 1
foot of settlement over the next 10 years. This example underscores the
importance of conducting a subsurface investigation and laboratory testing
to properly evaluate the subsurface. Note that the contact with the backwall
is only one impact and is not the only impact from settlement.
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Vertical Stress Investigation
When a load is applied to the soil surface, the vertical and lateral stresses
within the soil mass increase. The greatest increase in stress is directly
beneath the area where the load was applied. This increase in stress
dissipates or lessens within the soil as the distance from the applied load
increases. The increase in stress is a function of the distance from the
applied load area.
The two methods used to investigate the change in vertical stress are the
Boussinesq pressure isobar method and the 2-to-1 distribution method.
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Boussinesq Pressure Isobars
From the graph, the stress below and laterally to the shallow foundation
increases from the load of the shallow foundation. The size of the loaded
area or shallow foundation controls the stress distribution. The stress is
greatest beneath the shallow foundation and dissipates both vertically and
laterally with depth. The depth of influence is different between continuous
or strip and square footings of the same width.
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2-to-1 Distribution Method
The equation with this method states that the approximate distribution
estimate is equal to the footing width divided by the sum of the footing
width and the depth all multiplied by the loading.
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Shallow Foundation Failure by Sliding
Sliding failure occurs when load force effect exceeds the factored shear
resistance of either the soil or the interface between the soil and
foundation.
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Shallow Foundations on Soil – Use of Eccentricity and
Effective Dimensions
This course will only focus on the stress distributions for a rectangular
footing. Other stress distributions, including trapezoidal, triangular, and
circular are not covered in this course.
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Eccentricity of Footings on Soil
The first equation states that the eccentricity of the footing width, referred to
as e sub B, is equal to the moment along the footing width, referred to as M
sub B, divided by the load, referred to as P.
The second equation states that the eccentricity of the footing length,
referred to as e sub L, is equal to the moment along the footing length,
referred to as M sub L, divided by the load.
Image description 1:
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footing length, referred to as M sub L, or both, the load shifts. The
eccentricity or shift in the load is measured along each axis. E sub B is the
eccentricity along the footing width, and e sub L is the eccentricity along the
footing length.
Image description 2:
The variables used in this equation are eccentricity of the footing width,
referred to as e sub B, footing width, referred to as B, overturning moment
along the footing width, referred to as M sub B, and the load, referred to as
P. The equation states that the eccentricity of the footing width is equal to
the moment along the footing width divided by the load.
Image description 3:
The variables used in this equation are eccentricity of the footing length,
referred to as e sub L, footing length, referred to as L, overturning moment
along the footing length, referred to as M sub L, and the load, referred to as
P. The equation states that the eccentricity of the footing length is equal to
the moment along the footing length divided by the load.
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Effective Dimensions for Footings on Soil
Now that eccentricity has been calculated, the effective dimensions for the
soil footing from application of a moment, M, can be calculated. The
effective footing width, referred to as B prime, is equal to the footing width,
B, minus two times the eccentricity along the footing width, referred to as e
sub B. A similar equation to calculate the footing length states that the
effective footing length, referred to as L prime, is equal to the footing
length, L, minus two times the eccentricity along the footing length, referred
to as e sub L.
Image description 1:
Image description 2:
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The variables used in this equation are effective footing width, referred to
as B prime, footing width, referred to as B, and the eccentricity along the
footing width, referred to as e sub B. The equation states that the effective
footing width is equal to the footing width minus two times the eccentricity
along the footing width.
Image description 3:
The variables used in this equation are effective footing length, referred to
as L prime, footing length, referred to as L, and the eccentricity along the
footing length, referred to as e sub L. The equation states that the effective
footing length is equal to the footing length minus two times the eccentricity
along the footing length.
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Applied Stress Beneath Effective Footing Area
The basic equation states that the effective stress, q, is equal to the load,
P, divided by the effective footing width, B prime, and the effective footing
length, L prime.
Image Description 1:
Image Description 2:
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The variables used in this equation are effective stress, referred to as q,
load, referred to as P, effective footing width, referred to as B prime, and
effective footing length, referred to as L prime. The equation states that the
effective stress is equal to the load divided by the effective footing width
and the effective footing length.
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Eccentricity and Shape Correction Factors
Table Description:
AASHTO Table 10.6.3.1.2a-3 Shape Correction Factors sc, sγ, and sq.
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than 0; Cohesion Term: 1 plus B divided by 5 times L, Times N sub q
divided by n sub c; Unit Weight Term: 1 minus 0.4 times B divided by L;
Surcharge Term: 1 plus B divided by L times tangent of theta sub f.
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Evaluation of eccentricity is performed to do what?
d) Prevent overturning
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Deep Foundations on Soil or Rock
Now that you’ve learned about shallow foundations, let’s move on to deep
foundations. Deep foundations are a structural element designed to
transfer substructure loads from the bottom of the substructure to
competent soil or rock materials located at some depth below the bottom of
the substructure. They must provide adequate resistance against
geotechnical and structural failure.
Other special or alternative deep foundation types exist but are beyond the
scope of this lesson.
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Driven Piles
Top-driven piles are long narrow structural elements that have sufficient
strength to withstand installation into the ground by repeated impacts
applied to the top of the pile. The impacts are generated by a pile-driving
hammer consisting of a heavy ram that is raised by various means and
dropped on top of the pile. Top driven piles can be subdivided into low
displacement piles and high displacement piles.
Low displacement piles have a small cross sectional area and do not push
a lot of soil out of the way when being installed. H-piles and open-ended
steel pipe piles are considered low displacement piles.
High displacement piles have a large cross sectional area and push a large
volume of soil when being installed. Precast concrete piles and closed-end
piles are examples of high displacement piles.
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Drilled or Excavated Piles and Shafts
Drilled or excavated piles and drilled shafts are installed by removing soil
and rock using drilling methods or other excavation techniques and
inserting the foundation element into the excavated hole. The structural
element inserted into the excavated hole may be cast-in-place reinforced
concrete (as in drilled shafts), grout (as in auger cast piles), or a
combination of steel sections and grout or concrete (as in micropiles or
soldier beam and lagging wall).
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Deep Foundations – Methods of Support
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Methods of Support
For the end bearing method, low displacement piles such as steel H-piles
or open end pipe piles are most suitable when on hard rock. All generic pile
types are suitable on soft rock or dense soil. Side friction is used in deep
soil deposits without a hard bearing stratum. High displacement piles can
help to densify the soils around the pile and increase the side friction which
makes them well suited to this application. Deep foundations can use both
methods of support in combination. This method is used when piles are
driven a significant distance into a bearing stratum or for drilled foundations
that are socketed into a hard bearing stratum.
Estimation of pile length is simple for hard bearing surfaces as the pile will
stop when it encounters the hard surface, but is more difficult to estimate if
a bearing layer is not present or not strong enough to stop the pile.
Typically there is a high axial resistance achieved with proper driving
equipment.
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Nominal Resistance for Typical Deep Foundation Types
The magnitude of the applied loads will influence the type of deep
foundation selected. Common deep foundation types include:
• Timber pile;
• Concrete pile;
• Steel H-pile;
• Drilled shaft.
The table shown on the screen is used as a guide for typical nominal
resistance values for preliminary selection. The nominal resistance required
will be a function of the loads to be supported, the space available for
foundation elements (which controls how many piles or shafts can be
used), and the resistance factor to be used.
Table Description:
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Typical Range of Nominal Resistance (kips) (column header)
61
What type of support methods transfer the substructure
loads to the bearing stratum for deep foundation elements?
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Deep Foundation Design Considerations
• Batter configurations;
• Downdrag; and
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Pile Group Configurations
With the layout of the piles in a group, the number of rows and the number
of piles in each row can have a large impact on the foundation response.
For Case 1 in the illustration, a single pile or a single row of piles responds
to a horizontal load and overturning moment by rotating about a point
somewhere below the ground surface. For Case 1, the movement of the
pile causes the pile head to move downward. The overturning moment is
resisted by the bending of the pile. For Case 2 and Case 3, if a second row
of piles is added, the response is translational and the overturning moment
is resisted by a difference in axial force between the two rows of piles
forming a couple. If more piles are added, the distribution of forces to each
pile becomes indeterminate and simplifying assumptions are required in
order to analyze the foundation deflections and forces.
Note that for Case 3, the battered piles respond differently than Case 2
vertical piles. As the battered piles bend, the pile head moves along an arc
that tends to move the pile upward.
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Note that a rotating foundation unit will deflect more at the bearing location
than a translating foundation unit, even though the deflection of the pile
head is the same. For this reason, it is important to evaluate the
displacement at a point where it is critical. This is usually at the bearings for
the superstructure but can be at other locations as well. The displacement
at the pile head is rarely of concern with respect to the performance of the
structure.
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Batter Configurations
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Soil and Structure Interaction
If the piles are close together in the direction of horizontal loading, the lead
pile will exhibit a stiffer response than the trailing pile. This means that the
lead pile will resist more of the horizontal load than the trailing pile. This
softened response of the trailing pile will result in larger horizontal
deflections than an equivalent pile group with more widely spaced piles.
The piles that are closer together, as in Case 1, will result in larger
horizontal deflections than an equivalent pile group with the piles widely
spaced as shown in Case 2.
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Pile Head Fixity
The connection of the pile to the pile cap can be fixed such that rotation of
the pile head relative to the cap is not permitted. In Case 1, the piles are
not fixed to the pile cap. To fix the pile head to the cap requires that the pile
be imbedded about two to three pile diameters into the cap or be fitted with
a specially designed connection. If the pile is embedded, as in Case 2, only
a nominal amount into the cap, it behaves as a pinned connection and is
free to rotate relative to the cap, as shown in the figure on the screen. Most
pile installations are somewhere between these two extremes.
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Downdrag
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maximum load factor for combinations in which the downdrag increases the
load effects and with a minimum load factor for cases when the downdrag
decreases the load effects.
70
Time-dependent Effects
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anticipated, it is important to include provisions for waiting and testing in the
construction documents.
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Lesson Review
Let's take a moment to review the concepts you learned during this lesson.
73
What soil types are most susceptible to secondary
settlement or creep?
a) Sand
b) Organic
d) Clay
74
True or False, downdrag is also known as negative skin
friction.
a) True
b) False
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What type of process is scour?
d) Construction process
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Lesson Summary
You learned to define the basic foundation analysis techniques for shallow
and deep foundations.
For deep foundations, the two common types are driven piles and drilled or
excavated shafts or piles. The methods of support for deep foundations are
end bearing, side friction, or a combination of both. For deep foundation
design, pile grouping and configurations, soil and structure interaction, pile
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head fixity, downdrag, and time dependent effects are to be considered
during the design process.
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Lesson Conclusion
If you would like to further review the material covered in this lesson,
please return to the beginning of this lesson.
If you are confident that you understand the learning outcome, please
continue on to the next lesson.
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Lesson 2: Limit State Checks for Foundation Design
Let’s move on to the second lesson, which describes the limit state checks
required for shallow and deep foundations. At the end of this lesson, you
will be able to explain the service, strength, and extreme event limit state
checks used for shallow and deep foundations analysis.
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Limit State Checks for Shallow Foundations
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Shallow Foundations – Service Limit State
For shallow foundations, service limit states affect the function of the
structure under regular service conditions and loads. Some acceptable
measure of structure movement is tolerable by the service limit state
through the structure’s performance life.
The service limit states for shallow foundations can be reached through:
• Excessive settlement;
For example, if the footing movements due to the loads exceed the
tolerable settlement, the service limit state is reached. The design of
shallow foundations is frequently controlled by movement at the service
limit state.
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Shallow Foundations – Service Limit State (con.)
The service limit state checks for shallow foundations include evaluation of:
• Vertical settlement;
• Overall stability.
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Service Limit State Check for Settlement
For cohesive soils, in the service limit state, settlement can be checked
using elastic theory or other conventional methods.
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Presumptive Values AASHTO Table 10.6.2.6.1-1
Table Description:
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Type of Bearing Material: Foliated metamorphic rock: slate schist (sound
condition allows minor cracks)
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Type of Bearing Material: Well-graded mixture of fine and coarse grained
soil, glacial till, hardpan, boulder clay (GW-GC, GC, SC)
Type of Bearing Material: Coarse to medium sand, and with little gravel
(SW, SP)
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Consistency in Place: Medium dense to dense
Type of Bearing Material: Fine sand, silty or clayey medium to find sand
(SP, SM, SC)
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Bearing Resistance Recommended Value of Use: 6 ksf
Type of Bearing Material: Inorganic silt, sandy or clayey silt, varved silt-
clay-fine sand (ML, MH)
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Bearing Resistance Ordinary Range: 4 to 8 ksf
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Service Limit State Check for Horizontal Displacement
For a pier, the rotation can be in either direction and is usually a function of
the direction of the applied horizontal load or the eccentricity of the vertical
load.
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“backwards” if the approach embankment induces large vertical or
horizontal displacements of the soil behind the abutment. Abutments on
battered piles can also rotate backwards due to the geometry of the piles.
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Service Limit State Check for Overall Stability
The next service limit check is for overall stability. Overall stability is
evaluated by LRFD as resistance factors when evaluating bearing
resistance.
93
Limit State Checks for Shallow Foundations
Now that you understand the service limit state checks for shallow
foundations, let's move on to the strength limit state checks.
94
Shallow Foundations – Strength Limit States
• Substructure sliding.
95
Shallow Foundations – Strength Limit State Checks
When the bearing pressure due to the loads exceeds the bearing strength
of the soil, the strength limit state is reached and failure results. The
strength limit state is typically evaluated in terms of shear or bending stress
failure.
• Sliding resistance.
96
Soil Bearing Resistance
Strength limit state check for bearing resistance includes evaluation using
conventional bearing resistance theory.
For the strength limit state, the nominal bearing resistance is limited by
bearing resistance variables, as compared to the service limit state where
nominal bearing resistance is limited by settlement.
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Sliding Resistance
• Nominal resistance and a resistance factor between the soil and the
foundation; and
For the strength limit state sliding failure occurs if the horizontal loads force
effect exceed either the factored nominal bearing resistance of the soil or
the factored resistance between the soil and foundation.
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Limit State Checks for Shallow Foundations
Now that you understand the strength limit state checks for shallow
foundations, let's move on to the extreme limit state checks.
99
Shallow Foundations – Extreme Event Limit State
The final check we’ll look at is the extreme event limit state check. For the
extreme event limit state, loading combinations that result in an excessive
or improbable condition are evaluated. The Extreme Event I limit state is
used to evaluate seismic loadings and its effect on the bridge. The Extreme
Event II limit state is used to evaluate vessel impact or vehicle impact on
the bridge structure, and ice loads and scour by the check flood 500 year
event. The Extreme Event I limit state may control the design of
foundations in seismically active areas while the Extreme Event II limit state
may control the design of foundations and piers that may be exposed to
vehicle or vessel impacts.
• Overall stability;
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• Scour by check flood 500 year flood event.
101
Service limit state checks for shallow foundations include
which of the following:
Service limit state checks for shallow foundations include which of the
following:
a) Vertical settlement
b) Overall stability
c) Horizontal displacement
102
Limit State Checks for Deep Foundations
Now that you have learned about shallow foundations, let’s move on to
deep foundations. The design of deep foundations also involves
consideration of the following three limit states.
Once again, the fatigue limit state is typically not considered in foundations.
103
Deep Foundations – Service Limit State Checks
Service limit state checks are the same for both driven piles and drilled
shafts and include:
• Overall stability;
• Horizontal displacement.
104
Horizontal and Vertical Displacement
105
Overall Stability
Overall stability is a service limit state check. This type of failure can
destroy a substructure, even with deep foundations. The superstructure,
substructure, and backfill weight must all be included in stability analysis to
prevent this type of failure.
106
Limit State Checks for Deep Foundations
Now that you understand the service limit state checks for deep
foundations, let's move on to the strength limit state checks.
107
Deep Foundations – Strength Limit State Checks
Drivability does not need to be checked for drilled shafts because they are
drilled and not driven.
108
Deep Foundations – Strength Limit State Check – Structural
Resistance
Let’s begin by looking at the strength limit state check for structural
resistance. Structural resistance includes axial, flexure, and shear
resistances.
109
Deep Foundations – Strength Limit State Check – Structural
Resistance
110
Deep Foundations – Strength Limit State Check – Structural
Resistance
111
Deep Foundations – Strength Limit State Check – Structural
Resistance
Structural failure can occur as a result of shear. The image on the screen
shows piles with structural shear failure. Shear failure of the structure is
typically due to excessive bending in the pile to a point that shear failure
occurs and the pile becomes detached from the pile cap.
112
Service limit checks for deep foundations include which of
the following?
Service limit checks for deep foundations include which of the following?
a) Overall stability
b) Vertical displacement
c) Horizontal displacement
d) Axial resistance
113
Deep Foundations – Axial Geotechnical Resistance
The next strength limit state check you’ll review is axial geotechnical
resistance. The deep foundation must transmit the load to subsurface
materials. If the subsurface materials are not strong enough to support the
loading, or the load transfer mechanism is not sufficient, geotechnical
failure, either through axial geotechnical resistance at the front of the deep
foundation or tension pull out at the rear of the deep foundation will occur.
114
Deep Foundations – Pile Drivability
And the final strength limit state check is for pile drivability. A pile drivability
analysis is performed during design and then while driving the pile during
construction to ensure that the pile achieves the required nominal
resistance or minimum penetration with acceptable stresses to avoid
damage to the pile. Driving analysis is also performed to evaluate a
hammer and driving system in other cases.
This animation sequence on the screen shows the stress wave generated
by the hammer’s impact traveling to the pile tip where contact with hard
rock results in excessive stresses. Tensile stress also develops in the
region behind the stress wave. Structural failure of driven piles can also
result from the stress required to drive the pile into the ground.
115
Deep Foundations – Pile Drivability (con.)
Remember, pile drivability only applies to driven piles and not drilled shafts.
Pile drivability denotes the ability of a pile to be driven to a desired
penetration depth, capacity, or both depth and capacity.
116
Drivability can often be the controlling strength limit state check for a pile
foundation. This is especially true for high capacity piles driven to refusal
on rock.
117
Drivability Performance Limit
Though the damage by any one impact may be small, extensive damage
can result from repeated impacts as in the case for the piles shown in the
photo on the screen.
118
Driven Performance Limit
This photo shows an exposed pile head that has deformed. In this
example, the drivability limit check at the pile head has been exceeded.
Drivability analysis not only helps prevent pile damage, but verifies if
geotechnical resistance can be mobilized by common driving techniques.
119
Limit State Checks for Deep Foundations
Now that you understand the strength limit state checks for deep
foundations, let's move on to the extreme limit state checks.
120
1.70 Deep Foundations – Extreme Limit State Checks
You’ve learned about the service and strength limit state checks for deep
foundation design, now let’s move on to the extreme limit state checks.
Deep foundation extreme limit state checks are similar to the strength and
service limit state checks but include the following considerations.
121
Lesson Review
Let's take a moment to review the concepts you learned during this lesson.
122
Structural resistance of deep foundations includes which of
the following?
a) Axial resistance
b) Shear resistance
c) Wind resistance
d) Flexure resistance
123
Extreme Event I limit state for shallow foundations evaluates
which of the following?
Extreme Event I limit state for shallow foundations evaluates which of the
following?
124
In order for a driven pile to develop its design geotechnical
resistance, the pile must be:
In order for a driven pile to develop its design geotechnical resistance, the
pile must be:
d) Driven with enough energy to force the pile tip into the intended
bearing stratum
The correct answer is d) Driven with enough energy to force the pile tip into
the intended bearing stratum.
125
Lesson Summary
You learned about the limit state checks for shallow and deep foundations.
For shallow foundations, service, strength, and extreme event limit states,
as well as eccentricity are to be checked. Service limit state includes
checks for vertical settlement, overall stability, and horizontal displacement.
Strength limit state includes checks for soil bearing resistance and sliding
resistance. The extreme event limit state includes checks for total and
differential settlement, overall stability, soil bearing resistance, and sliding
resistance.
For deep foundations, service, strength, and extreme event limit states are
to be checked. Service limit state includes checks for overall stability,
vertical displacement, and horizontal displacement. Strength limit state
includes checks for structural resistance, axial geotechnical resistance, and
drivability. Extreme limit state includes check flood for scour, vessel and
vehicle collision, seismic loading, and other site-specific situations the
design engineer determines should be included.
126
This concludes Lesson 2: Limit State Checks for Foundation Design.
127
Lesson Conclusion
If you would like to further review the material covered in this lesson,
please return to the beginning of this lesson.
If you are confident that you understand the learning outcome, please
continue on to the next lesson.
128
Lesson 3: Resistance for Foundations
Let’s move on to the third lesson, which explains the resistance for shallow
and deep foundations. At the end of this lesson, you will understand basic
soil resistance analysis for shallow foundations and understand basic
resistance of deep foundations.
129
Load and Resistance Factor Design Equation
Let’s begin Lesson 3 with a review of the Load and Resistance Factor
Design, or LRFD, equation introduced in Module 2. The equation states
that the sum of the factored dead loads plus the sum of the factored live
loads all multiplied by a load modifier is less than or equal to the nominal
resistance multiplied by a resistance factor.
If you recall, the LRFD golden rule states that the factored load is less than
or equal to the factored resistance. This lesson will focus on the resistance
side of the equation.
Equation Description: The variables used in this equation are load modifier
applied to all loads, referred to as eta, load factor applied to dead loads,
referred to as gamma sub DL, load factor applied to live loads, referred to
as gamma sub LL, deal loads, referred to as DL, live loads, referred to as
LL, resistance factor, referred to as phi, and nominal resistance or strength
of the element under consideration, referred to as R sub n. The equation
states that the sum of the factored dead loads plus the sum of the factored
130
live loads all multiplied by a load modifier is less than or equal to the
nominal resistance multiplied by a resistance factor.
131
Shallow Foundations – Soil Resistance
The strength needed by the soil to support the spread footing and avoid
failure is determined by the calculation of:
• Bearing resistance;
• Sliding;
• Settlement; and
This will be discussed in more detail, starting with the bearing resistance.
132
Shallow Foundations – Bearing Resistance of Soil
Let’s start by looking at the equation for bearing resistance. For strength
limit state design, the nominal bearing resistance of soil is to be estimated
using accepted soil mechanics theories and should be based on measured
soil properties. The nominal bearing resistance of a soil layer, in kips per
square foot, is calculated using the equation shown. This equation
represents the general shear failure of the soil. To better understand this
equation, it has been broken down into three components with each of
them covered separately.
133
referred to as B, factored unit weight (footing width) term (drained loading)
bearing capacity factor, referred to as N sub gamma m, and correction
factors to account for location of groundwater table, referred to as C sub w
gamma. The equation states that the nominal bearing resistance is equal to
cohesion times the cohesion term (undrained loading) bearing capacity
factor, plus the total moist unit weight of soil above the base of the footing
times the footing embedment depth times the factored surcharge
(embedment) term (drained or undrained loading) bearing capacity factor
times the correction factor for the groundwater table, plus one half times
the total moist unit weight of soil below the footing times the footing width
times the unit weight (footing width) term (drained loading) bearing capacity
factor times the correction factor for the groundwater table.
In which: The variables used in this equation are cohesion term (undrained
loading) bearing capacity factor, referred to as N sub c, footing shape
correction factor, referred to as s sub c, and load inclination factor, referred
to as i sub c. The equation states that the factored cohesion term
(undrained loading) bearing capacity is equal to the cohesion term
(undrained loading) bearing capacity factor times the footing shape
correction factor times the load inclination factor.
134
capacity factor is equal to the surcharge (embedment) term (drained or
undrained loading) bearing capacity factor times the footing shape
correction factor times the correction factor for shearing resistance times
the load inclination factor.
In which: The variables used in this equation are unit weight (footing width)
term (drained loading) bearing capacity factor, referred to as N sub gamma,
footing shape correction factor, referred to as s sub gamma, and load
inclination factor, referred to as i sub gamma. The equation states that the
factored unit weight (footing width) term (drained loading) bearing capacity
factor is equal to the unit weight (footing width) term (drained loading)
bearing capacity factor times the footing shape correction factor times the
load inclination factor.
135
Soil Bearing Resistance – Cohesion Term
136
Bearing Capacity Factors
137
Shape Correction Factors
138
Load Inclination Factor
The load inclination factors account for the reduction in bearing resistance
from an inclined load. AASHTO mentions that most geotechnical engineers
nationwide have not used the load inclination factors. This may be due in
part to the lack of knowledge of the vertical and horizontal loads at the time
of the geotechnical exploration and lack of preparation of bearing
resistance recommendations. Also, the load inclination factors were derived
for footings without embedment and may be overly conservative for
footings with an embedment depth to footing width ratio greater than or
equal to one. Therefore, for footings with modest embedment,
consideration may be given for omission of the load inclination factor.
Even though these factors are rarely used, the equations are shown for
informational purposes.
139
Soil Bearing Resistance – Surcharge (Embedment) Term
Gamma is the total moist unit weight of the soil above or the bearing depth
of the footing. C sub wq is the correction factor to account for the location
of the groundwater table. D sub q is the correction factor to account for the
shearing resistance along the failure surface. Each of these are determined
from tables in AASHTO.
140
Bearing Capacity Factors
141
Coefficient for Various Groundwater Depths
When the position of the groundwater is at a depth less than 1.5 times the
footing width below the footing base, the bearing resistance is affected. The
highest anticipated groundwater level should be used in design.
142
Friction Angle
The table shown on the screen, AASHTO Table 10.4.6.2.4-1, can be used
to correlate the friction angle from the Standard Penetration Test, or SPT,
data in cohesionless soil, or can be measured directly by laboratory tests or
in-situ testing. After determining the friction angle from this table, it can then
be used with AASHTO Table 10.6.3.1.2a-1 to determine the bearing
capacity factor. For cohesive soils, the friction angle is determined by
laboratory or in-situ testing.
Table Description:
143
Load Inclination Factor
The load inclination factors account for the reduction in bearing resistance
from an inclined load. AASHTO mentions most geotechnical engineers
nationwide have not used the load inclination factors. This may be due in
part to the lack of knowledge of the vertical and horizontal loads at the time
of the geotechnical exploration and lack of preparation of bearing
resistance recommendations. Also, the load inclination factors were derived
for footings without embedment and may be overly conservative for
footings with an embedment depth to footing width ratio greater than or
equal to one. Therefore, for footings with modest embedment,
consideration may be given for omission of the load inclination factor.
Even though these factors are rarely used, the equations are shown for
informational purposes.
144
Depth Correction Factors
The depth correction factors are only to be used when the soils above the
footing bearing elevation are as competent as the soils beneath the footing
level. Otherwise, the depth correction factor should be taken as 1.0.
145
Shape Correction Factors
146
Soil Bearing Resistance – Unit Weight Term
The third component of the bearing resistance equation, called the unit
weight term, uses the unit weight (footing width) term of the bearing
capacity factor and considers drained loading. This component is equal to
one half times the total moist unit weight of soil below the footing times the
footing width times the combined bearing capacity factor, shape factor, and
inclination factor for unit weight, times the correction factor for the
groundwater table.
Gamma, the total moist unit weight of the soil, is taken below the bearing
depth of the footing.
147
Friction Angle
The table shown on the screen, AASHTO Table 10.4.6.2.4-1, can be used
to correlate the friction angle from Standard Penetration Test, or SPT, data
in cohesionless soil, or can be measured directly by laboratory tests or in-
situ testing for cohesive and cohesionless soils. After determining the
friction angle from this table, it can then be used with AASHTO Table
10.6.3.1.2a-1 to determine the bearing capacity factor.
148
Coefficient for Various Groundwater Depths
When the position of the groundwater is at a depth less than 1.5 times the
footing width below the footing base, the bearing resistance is affected. The
highest anticipated groundwater level should be used in design.
149
Bearing Capacity Factors
150
Shape Correction Factors
151
Load Inclination Factor
The load inclination factors account for the reduction in bearing resistance
from an inclined load. AASHTO mentions that most geotechnical engineers
nationwide have not used the load inclination factors. This may be due in
part to the lack of knowledge of the vertical and horizontal loads at the time
of the geotechnical exploration and lack of preparation of bearing
resistance recommendations. Also, the load inclination factors were derived
for footings without embedment and may be overly conservative for
footings with an embedment depth to footing width ratio greater than or
equal to one. Therefore, for footings with modest embedment,
consideration may be given for omission of the load inclination factor.
Even though these factors are rarely used, the equations are shown for
informational purposes.
152
Which of the following are true for the bearing resistance
equation?
Which of the following are true for the bearing resistance equation?
153
Presumptive Values – AASHTO Table 10.6.2.6.1-1
Now that you understand the bearing resistance equation, let’s look at
some alternatives. This table may look familiar to you, it was introduced in
Lesson 2 of this module.
Table Description:
154
Bearing Resistance (ksf) (column title)
155
Consistency in Place: Medium hard rock
156
Consistency in Place: Loose
Type of Bearing Material: Coarse to medium sand, and with little gravel
(SW, SP)
157
Bearing Resistance Recommended Value of Use: 5 ksf
Type of Bearing Material: Fine sand, silty or clayey medium to find sand
(SP, SM, SC)
158
Bearing Resistance Ordinary Range: 2 to 6 ksf
Type of Bearing Material: Inorganic silt, sandy or clayey silt, varved silt-
clay-fine sand (ML, MH)
159
Shallow Foundations – Bearing Resistance on Rock
Now, let’s look at information regarding the bearing resistance on rock for
shallow foundations.
For conditions of intact, sound rock that is stronger and less compressible
than concrete, footings on rock with small applied loads are generally
stable and do not require extensive study of the strength and
compressibility characteristics of rock behavior. However, site investigation
is still required to confirm the consistency and extent of rock formations
beneath a shallow foundation.
160
classification is beyond the scope of this course but is presented in
AASHTO 10.4.6.4 Rock Mass Strength.
161
Rock Mass Rating System - RMR
The Rock Mass Rating System, known as RMR, was developed for tunnel
design and includes life-safety considerations which can be thought of as a
built-in factor of safety. This procedure may yield conservative results.
162
AASHTO Table 10.4.6.4-1
Table Description:
163
compressive strength. Ranges of values: Greater than 4320 ksf, 2160 to
4320 ksf, 1080 to 2160 ksf, 520 to 1080 ksf, 215 to 520 ksf, 70 to 215 ksf,
20 to 70 ksf. Relative rating: 15, 12, 7, 4, 2, 1, 0. Section 2: Parameter: Drill
core quality RQD. Range of Values: 90 percent to 100 percent, 75 percent
to 90 percent, 50 percent to 75 percent, 25 percent to 50 percent, less than
25 percent. Relative Rating: 20, 17, 13, 8, 3. Section 3: Parameter: Spacing
of joints. Range of values: less than 10 feet, 3 to 10 feet, 1 to 3 feet, 2
inches to 1 foot, less than 2 inches. Relative Rating: 30, 25, 20, 10, 50.
Section 4: Parameter: Condition of joints. Ranges of values: Very rough
surfaces, Not continuous, No separation, Hard joint wall rock; Slightly rough
surfaces, separation of less than 0.05 inches, Hard joint wall rock; Slightly
rough surfaces, Separation of less than 0.05 inches, Soft joint wall rock;
Slicken sided surfaces, or Gouge of less than 0.02 inches thick, or Joints
open 0.05 to 0.2 inches, Continuous joints; Soft gouge of more than 0.02
inches thick, or joints open greater than 0.2 inches, continuous joints.
Relative rating: 25, 20, 12, 6, 0. Section 5: Parameter: Groundwater
conditions (use one of the three evaluative criteria as appropriate to the
method of exploration). Parameter: Inflow per 30 foot tunnel length. Ranges
of values: None, less than 400 gallons per hour, 400 to 2000 gallons per
hour, greater than 2000 gallons per hour. Parameter: Ratio equals joint
water pressure/major principal stress. Ranges of values: 0, 0.0 to 0.2, 0.2
to 0.5, less than 0.5. Parameter: General conditions. Ranges of values:
Completely Dry, Moist only (interstitial water), Water under moderate
pressure, Severe water problems. Relative rating: 10, 7, 4, 0.
164
Shallow Foundations – Footings on Rock
FHWA-NHI GEC 6 and FHWA NHI Course 132037 Spread Footing: LRFD
165
Shallow Foundations – Soil Resistance
Now that you understand bearing resistance for shallow foundations, let's
move on to sliding.
166
Shallow Foundations – Nominal Sliding Resistance
The resistance factor for shear resistance between the soil and foundation,
and the resistance factor for passive resistance of shallow foundations can
be found in AASHTO Table 10.5.5.2.2-1.
167
resistance between soil and foundation, referred to as R sub tau,
resistance factor for passive resistance, referred to as phi sub ep, and
nominal passive resistance of soil throughout design life, referred to as R
sub ep. The equation states that nominal resistance against failure by
sliding is equal to a resistance factor times the nominal sliding resistance
which is equal to the resistance factor for shear resistance times the
nominal sliding resistance between the soil and foundation plus the passive
resistance factor times the nominal passive soil resistance of the soil.
168
Shallow Foundations – Cohesionless Soil Nominal Sliding
Resistance
The calculation for cohesionless soil nominal sliding resistance takes the
factored vertical forces at the internal friction angle into account.
In practice, the second equation term of the passive earth pressure is often
ignored. This is done for the following three reasons.
2. The soil providing the passive resistance is often in the zone subject
to freezing and thawing cycles. During the thaw cycle, the strength of
169
this soil may be reduced to near zero resulting in no passive
resistance; and
170
When are presumptive values of bearing resistance used?
171
Shallow Foundations – Soil Resistance
Now that you understand nominal sliding resistance for spread footings,
let's move on to settlement.
172
Shallow Foundations – Settlement
Soil deforms under loading. If the footing movements from the loads
exceed the tolerable settlement, the service limit state is reached. In
general, the total settlement of a foundation is the summation of elastic
settlement, primary consolidation settlement, and secondary consolidation
settlement.
173
Shallow Foundations – Settlement
174
Shallow Foundations – Settlement
175
Shallow Foundations – Settlement
176
Se – Elastic Settlement of Shallow Foundations on
Cohesionless Soils
Now let’s take a look at the equations used to calculate the elastic
settlement on cohesionless soils.
The first equation states that the elastic settlement is equal to the
summation of the elastic settlement of all layers of a soil. The second
equation states that the elastic settlement of a specific layer is equal to the
177
initial height of the layer times one divided by the bearing capacity index, all
multiplied by the log of the initial vertical effective stress plus the increase
in vertical stress divided by the initial vertical effective stress.
Remember from the previous screens, the effective stresses are taken at
the midpoint of the soil layer.
178
Bearing Capacity Index, C’
179
Sc - Primary Consolidation Settlement of Shallow
Foundations on Cohesive Soils
180
• Normally consolidated soils are soils that have never consolidated
under loads other than the current load; and
181
Consolidation Settlement (Sc) for Overconsolidated Soils
When the maximum past vertical effective stress is greater than the initial
vertical effective stress, the soil is said to be overconsolidated and
AASHTO equation 10.6.2.4.3-1 is used to calculate the consolidation
settlement when the void ratio is known. Also, note that the vertical
effective stress measurements are taken at the midpoint of the soil layer
under consideration.
where:
182
• Hc = initial height of compressible soil
• Cc = compression index
• σ’f = final vertical effective stress in soil at midpoint of soil layer under
consideration (ksf)
Equation Description 1: The variables used in this equation are initial height
of compressible soil, referred to as H sub c, initial void ratio, referred to as
e sub o, recompression index, referred to as C sub r, maximum past
vertical effective stress, referred to as sigma prime sub p, initial vertical
effective stress, referred to as sigma prime sub o, compression index,
referred to as C sub c, and final vertical effective stress, referred to as
sigma prime sub f. The equation states that the consolidation settlement is
equal to the initial height of compressible soil divided by one plus the initial
void ratio, all multiplied by the recompression index times the log of the
maximum past vertical effective stress over the initial vertical effective
183
stress plus the compression index times the log of the final vertical effective
stress over the maximum past vertical effective stress.
184
Consolidation Settlement (Sc) for Normally Consolidated
When the maximum past vertical effective stress is equal to the initial
vertical effective stress, the soil is said to be normally consolidated and
AASHTO equation 10.6.2.4.3-2 is used to calculate the consolidation
settlement when the void ratio is known. Also, note that the vertical
effective stress measurements are taken at the midpoint of the soil layer
under consideration.
where:
185
• Cc = compression index
• σ’f = final vertical effective stress in soil at midpoint of soil layer under
consideration (ksf)
Equation Description 1: The variables used in this equation are initial height
of compressible soil, referred to as H sub c, initial void ratio, referred to as
e sub o, compression index, referred to as C sub c, final vertical effective
stress, referred to as sigma prime sub f, and maximum past vertical
effective stress, referred to as sigma prime sub p. The equation states that
the consolidation settlement is equal to the initial height of compressible
soil divided by one plus the initial void ratio, multiplied by the compression
index times the log of the final vertical effective stress over the maximum
past vertical effective stress.
186
Consolidation Settlement (Sc) for Underconsolidated
When the maximum past vertical effective stress is less than the initial
vertical effective stress, the soil is said to be underconsolidated and
AASHTO equation 10.6.2.4.3-3 is used to calculate the consolidation
settlement when the void ratio is known. Also, note that the vertical
effective stress measurements are taken at the midpoint of the soil layer
under consideration.
where:
187
• eo = initial void ratio
• Cc = compression index
• σ’f = final vertical effective stress in soil at midpoint of soil layer under
consideration (ksf)
Equation Description 1: The variables used in this equation are initial height
of compressible soil, referred to as H sub c, initial void ratio, referred to as
e sub o, compression index, referred to as C sub c, final vertical effective
stress, referred to as sigma prime sub f, and the current effective stress,
referred to as sigma prime sub pc. The equation states that the
consolidation settlement is equal to the initial height of compressible soil
divided by one plus the initial void ratio, multiplied by the compression
index times the log of the final vertical effective stress over the current
effective stress that does not include the stress due to the footing loads.
188
states that the maximum past vertical effective stress is less than the initial
vertical effective stress.
189
Consolidation Settlement (Sc) for Overconsolidated
When the maximum past vertical effective stress is greater than the initial
vertical effective stress, the soil is said to be overconsolidated and
AASHTO equation 10.6.2.4.3-4 is used to calculate the consolidation
settlement when the vertical strain is known. Also, note that the vertical
effective stress measurements are taken at the midpoint of the soil layer
under consideration.
where:
190
• Crε = recompression ratio
• σ’f = final vertical effective stress in soil at midpoint of soil layer under
consideration (ksf)
Equation Description 1: The variables used in this equation are initial height
of compressible soil, referred to as H sub c, recompression ratio, referred
to as C sub r epsilon, maximum past vertical effective stress, referred to as
sigma prime sub p, initial vertical effective stress, referred to as sigma
prime sub o, compression ratio, referred to as C sub c epsilon, and final
vertical effective stress, referred to as sigma prime sub f. The equation
states that the consolidation settlement is equal to the initial height of
compressible soil multiplied by the recompression ratio times the log of the
maximum past vertical effective stress over the initial vertical effective
stress plus the compression ratio times the log of the final vertical effective
stress over the maximum past vertical effective stress.
191
Consolidation Settlement (Sc) for Normally Consolidated
When the maximum past vertical effective stress is equal to the initial
vertical effective stress, the soil is said to be normally consolidated and
AASHTO equation 10.6.2.4.3-5 is used to calculate the consolidation
settlement when the vertical strain is known. Also, note that the vertical
effective stress measurements are taken at the midpoint of the soil layer
under consideration.
Sc=HcCcε(σ’f / σ’pc)
where:
192
• σ’f = final vertical effective stress in soil at midpoint of soil layer under
consideration (ksf)
Equation Description 1: The variables used in this equation are initial height
of compressible soil, referred to as H sub c, compression ratio, referred to
as C sub c epsilon, final vertical effective stress, referred to as sigma prime
sub f, and maximum past vertical effective stress, referred to as sigma
prime sub p. The equation states that the consolidation settlement is equal
to the initial height of compressible soil times the compression ratio times
the log of the final vertical effective stress over the maximum past vertical
effective stress.
193
Consolidation Settlement (Sc) for Underconsolidated
When the maximum past vertical effective stress is less than the initial
vertical effective stress, the soil is said to be underconsolidated and
AASHTO equation 10.6.2.4.3-6 is used to calculate the consolidation
settlement when the vertical strain is known. Also, note that the vertical
effective stress measurements are taken at the midpoint of the soil layer
under consideration.
Sc=HcCcε(σ’f / σ’pc)
where:
194
• σ’f = final vertical effective stress in soil at midpoint of soil layer under
consideration (ksf)
Equation Description 1: The variables used in this equation are initial height
of compressible soil, referred to as H sub c, compression ratio, referred to
as C sub c epsilon, final vertical effective stress, referred to as sigma prime
sub f, and current effective stress, referred to as sigma prime sub pc. The
equation states that the consolidation settlement is equal to the initial
height of compressible soil times the compression ratio times the log of the
final vertical effective stress over the current effective stress that does not
include the stress due to the footing loads.
195
Ss – Secondary Settlement for Shallow Foundations
Now let’s look at the final type of settlement, secondary settlement. Two
equations are used to calculate secondary settlement depending on which
laboratory tests have been conducted.
The equation used if the consolidation parameters are based on void ratio
states that the secondary settlement is equal to the secondary compression
index divided by one plus the void ratio times the initial height of
compressible soil layer times the log of the arbitrary time that could
represent the service life of the structure, divided by the time when
secondary settlement begins.
196
Settlement of Shallow Foundations on Rock
Now that shallow foundation of soil settlement has been explained, let’s
look at settlement of shallow foundations on rock. For shallow foundations
bearing on fair to very good rock, using the RMR procedure, elastic
settlements are generally assumed to be less than a half inch. In most
cases, it is sufficient to determine settlement using the average bearing
stress under the footing. Where the foundations are subjected to a very
large load or where settlement tolerance may be small, settlements of
footings on rock may be estimated using elastic theory.
197
Shallow Foundations – Soil Resistance
Now that you understand settlement, let's discuss overall stability of the
soil.
198
Overall Stability – Shallow Foundations
199
True or False. When the movement of a shallow foundation is
greater than the tolerable movement of the shallow
foundation, it can be said that the extreme limit state has
been reached.
a) True
b) False
200
Deep Foundations
The first portion of this lesson covered resistance for shallow foundations.
Now, let’s take a look at the resistance of deep foundations.
201
Deep Foundations – Methods for Determining Structural
Resistance
202
Deep Foundations – Structural Resistance Factors
203
Deep Foundations – Methods to Determine Geotechnical
Resistance
204
Deep Foundations – Computation of Static Geotechnical
Resistance
• The first equation states that the factored resistance of the pile is
equal to the pile resistance factor times the resistance.
• The third equation states that the pile tip resistance is equal to the
area of the pile tip times the unit tip resistance of the pile.
• And the last equation states that the side resistance of the pile is
equal to the surface area of the pile side in the bearing strata times
the unit side resistance of the pile.
205
Determining Geotechnical Resistance of Piles – Field Tests
The field methods used for determining the geotechnical resistance of piles
are:
• Driving formulae.
206
Static Load Test 1
Image Description: Photo of a pile static load test in which test piles support
a loaded beam.
207
Static Load Test 2
For this test, the pile is loaded by increasing the load on the pile in various
load increments, typically until the pile fails. The settlement is recorded at
the various load increments. The structural strength of the foundation
element or the capacity of the loading system is reached at the point of
geotechnical failure, and the subsequent settlement is recorded.
208
Driving Formulae
For driving formulas, the penetration resistance observed in the field and
the driving energy are used to compute ultimate resistance using dynamics.
209
Dynamic Testing
Each hammer impact during driving is essentially a short term load test that
results in geotechnical failure of the pile, which we see as penetration of
the pile. Dynamic testing equipment measures the applied load using a
strain gauge on the pile, and the displacement using an accelerometer.
210
Deep Foundations – Static Analysis Methods
• For cohesive soils, the alpha method in which the unit side resistance
and the unit tip resistance of the pile are a function of the undrained
shear strength;
• For cohesionless soils, the beta method in which the unit side
resistance and the unit tip resistance of the pile are a function of
effective overburden pressure;
• The lambda method in which the unit side resistance of the pile is a
function of the passive effective lateral earth pressure;
• SPT and CPT which are empirical methods based on in-situ tests.
211
Drilled shafts methods include the same alpha and beta methods used for
driven piles, but also include side friction and tip resistance in rock
methods.
212
Geotechnical Resistance Factors for Piles
The last three resistance factors are based on methods used for pile driving
acceptance by visual observation of pile set and hammer performance. The
resistance factors associated with these methods are lower than for
methods based on testing because they are less reliable. The resistance
factor for the ENR equation seems unusually low but is correct. The original
ENR equation incorporated a factor of safety of six, since it was written to
give an “allowable capacity” for ASD. The factor of safety was removed for
LRFD, thus the resistance factor is low to start with. It has been further
downgraded based on the poor reliability of the method.
213
signal matching analysis. Condition/Resistance Determination Method
(column header). Resistance Factor (column header). Nominal Bearing
Resistance of Single Pile - Dynamic Analysis and Static Load Test
Methods, phi subscript dyn (Row Title). Method: Driving criteria established
by successful static load test of at least one pile per site condition and
dynamic testing* of at least two piles per site condition, but no less than 2%
of the production piles. Resistance Factor: 0.80. Method: Driving criteria
established by successful static load test of at least one pile per site
condition without dynamic testing. Resistance Factor: 0.75. Method: Driving
criteria established by dynamic testing,* conducted on 100% of production
piles. Resistance Factor: 0.75. Method: Driving criteria established by
dynamic testing,* quality control by dynamic testing* of at least two piles
per site condition, but no less than 2% of the production piles. Resistance
Factor: 0.65. Method: Wave equation analysis, without pile dynamic
measurements or load test but with field confirmation of hammer
performance. Resistance Factor: 0.50. Method: FHWA-modified Gates
dynamic pile formula (End of Drive condition only). Resistance Factor: 0.40.
Method: Engineering News (as defined in Article 10.7.3.8.5) dynamic pile
formula (End of drive condition only). Resistance Factor: 0.10. * Dynamic
testing requires signal matching, and best estimates of nominal resistace
are made from a restrike. Dynamic tests are calibrated to the state load
test, when available.
214
Geotechnical Resistance Factors for Piles (con.)
There are additional resistance factors for driven piles given by AASHTO
Table 10.5.5.2.3-1 and include:
• Uplift resistance;
• Group uplift;
• Drivability analysis.
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Pile-Static Analysis Methods, phi subscript stat (row title). Method: Side
resistance and End Bearing: Clay and Mixed Soils: Alpha method
(Tomlinson, 1987; Skempton, 1951), Resistance Factor: 0.35. Beta Method
(Esrig & Kirby, 1979; Skempton, 1951), Resistance Factor: 0.25. Lambda
Method (Vijayvergiya & Focht, 1972; Skempton, 1951), Resistance Factor:
0.40. Method: Side Resistance and End Bearing: Sand: Nordlund/Thurman
Method (Hannigan et al. , 2005), Resistance Factor: 0.45. SPT-method
(Meyerhof), Resistance Factor: 0.30. Method: CPT-Method
(Schmertmann), Resistance Factor: 0.50. Method: End bearing in rock
(Canadian Geotech. Society, 1985), Resistance Factor: 0.45. Block Failure,
phi subscript b1 (row title). Method: Clay, Resistance Factor: 0.60. Uplift
resistance of Single Piles, phi subscript up (row title). Method: Nordlund
Method, Resistance Factor: 0.35. Method: Alpha method, Resistance
Factor: 0.25. Method: Beta Method, Resistance Factor: 0.20. Method:
Lambda Method, Resistance Factor: 0.30. Method: SPT Method,
Resistance Factor: 0.25. Method: CPT Method, Resistance Factor: 0.40.
Method: Static load test, Resistance Factor: 0.60. Method: Dynamic test
with signal matching, Resistance factor: 0.50. Group Uplift Resistance, Phi
subscript ug (row title). Method: All soils, Resistance Factor: 0.50. Lateral
Geotechnical Resistance of Single Pile or Pile Group (row title). Method: All
soils and rock, Resistance Factor: 1.0. Structural Limit State (row title).
Method: Steel piles - See the provisions of Article 6.5.4.2. Method:
Concrete piles - See the provisions of Article 5.5.4.2.1. Method: Timber
piles - See the provisions of Article 8.5.2.2 and 8.5.2.3. Pile Drivability
Analysis, Phi subscript da (row title). Method: Steel piles - See the
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provisions of Article 6.5.4.2. Method: Concrete piles - See the provisions of
Article 5.5.4.2.1. Method: Timber piles - See the provisions of Article
8.5.2.2. In all three articles identified above, use phi identified as
“resistance during pile driving”.
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Geotechnical Resistance Factors – Drilled Shafts
Now let’s look at the resistance factors for drilled shafts provided in
AASHTO Table 10.5.5.2.4-1. Side friction and tip resistance are included
for rock methods. This table is continued on the following screen.
The resistance factors are provided for each method in AASHTO Section
10.5. There are no resistance factors provided for tip resistance in tension
because tip resistance does not work in tension. Resistance factors are
lower for tip resistance than for side friction because tip resistance is
dependent on the cleanliness and firmness at the bottom of excavation and
they can have higher variability.
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sand: Beta method (Brown et al., 2010), Resistance Factor: 0.55. Method:
Tip resistance in sand (Brown et al., 2010), Resistance Factor: 0.50.
Method: Side resistance in cohesive IGMs (Brown et al., 2010), Resistance
Factor: 0.60. Method: Tip resistance in cohesive IGMs (Brown et al., 2010),
Resistance Factor: 0.55. Method: Side resistance in rock (Kulhawy et al.,
2005; Brown et al., 2010), Resistance Factor: 0.55. Method: Side
resistance in Rock (Carter and Kulhawy, 1988), Resistance Factor: 0.50.
Method: Tip resistance in Rock: Canadian Geotechnical Society (1985),
Pressuremeter Method (Canadian Geotechnical society, 1985), Brown et
al. (2010). Resistance Factor: 0.50. Block Failure, Phi subscript b1 (row
title). Method: Clay, Resistance Factor: 0.55. Uplift Resistance Single-
Drilled Shafts, Phi subscript up (row title). Method: Clay: Alpha Method
(Brown et al., 2010), Resistance Factor: 0.35. Method: Sand: Beta Method
(Brown et al., 2010), Resistance Factor: 0.45. Method: Rock (Kulhawy et
al., 2005; Brown et al., 2010), Resistance Factor: 0.40. Ground Uplift
Resistance, Phi subscript ug (row title). Method: Sand and Clay,
Resistance Factor: 0.45.
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Geotechnical Resistance Factors – Drilled Shafts (con.)
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(row title). Method: All materials, Resistance Factor: 0.70. Static load test
(uplift), phi subscript upload (row title). Method: All materials, Resistance
Factor: 0.60.
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Deep Foundations – Overall Stability
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Lesson Review
Let's take a moment to review the concepts you learned during this lesson.
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Which of the following is true for the settlement of footings
on fair to very good rock:
Which of the following is true for the settlement of footings on fair to very
good rock:
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Field tests for determining the geotechnical resistance of
piles includes which of the following:
c) Dynamic testing
d) Driving formulae
The correct answers are b) Static load test; c) Dynamic testing; and d)
Driving formulae.
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When soil deforms under load, elastic settlement refers to
which of the following?
When soil deforms under load, elastic settlement refers to which of the
following?
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Lesson Summary
Now that you’ve completed the third lesson of this module, you are able to
explain resistance for shallow and deep foundations. Equations used to
calculate resistance for spread footings were presented. You also learned
about the three types of settlement, elastic, primary consolidation, and
secondary, and how it differs for cohesive and cohesionless soils. Then you
moved on to learn about resistance for deep foundations. Field methods
and static analysis methods for driven piles and drilled shafts were
covered.
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Lesson Conclusion
If you would like to further review the material covered in this lesson,
please return to the beginning of this lesson.
If you are confident that you understand the learning outcome, please
continue on to the Module Conclusion.
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Module Summary
This module described the basic foundation analysis techniques, the limit
state checks for foundation design, and resistance for foundations.
• Explain limit state checks for shallow and deep foundations; and
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