08-03-2025
Analysis & Design of Geotechnical Systems
(Bearing Capacity of Pile Foundation)
Paramita Bhattacharya
Department of Civil Engineering
IIT Kharagpur
What is Pile?
• Pile is a slender, structural member made of steel, concrete, timber, plastic, or
composites used to transmit structural loads deep within the soil mass.
• Skin friction stress or shaft friction stress or adhesive stress (fs) is the frictional or
adhesive stress on the shaft of a pile.
• End bearing stress or point resistance stress or tip resistance stress (fb) is the stress
at the base or tip of a pile.
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Importance
• Pile foundations are often used when
• The soil near the surface has insufficient bearing capacity to support the
structural loads.
• The estimated settlement of the soil exceeds tolerable limits (i.e., settlement
greater than the serviceability limit state).
• Excessive differential settlement due to soil variability or nonuniform
structural loads.
• The structural loads consist of lateral loads, moments, and uplift forces,
singly or in combination.
• Excavations to construct a shallow foundation on a firm soil layer are difficult
or expensive.
Practical situation
Figure 1 Pile construction in a waterfront project. 4
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Key terms
• Ultimate load bearing capacity (Qult) is the maximum vertical load that the pile can
transfer to the soil.
• Allowable load bearing capacity or safe load bearing capacity (Qa) is the working
load that would ensure a margin of safety against the collapse of the structure from
shearing. The allowable load bearing capacity is usually a fraction of the ultimate
load bearing capacity.
• Factor of safety or safety factor (FS) is the ratio of the ultimate load bearing
capacity to the allowable load bearing capacity. In geotechnical engineering, a factor
of safety between 2 and 3 is used to calculate the allowable load bearing capacity.
Types of piles
Figure 2 6
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Types of piles
Figure 3 7
Types of piles
Figure 4
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Pile Installation
• Piles have to be installed into the ground. The method of installation
affects the structural integrity of the piles and the strength and
deformation characteristics of the soil. Consequently, the load
capacity is strongly dependent on pile installation methods.
Pile Installation
•Impact Hammers
Pile driven by a ram that is
released from above the pile
suddenly strikes the pile head.
Figure 5
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Pile Installation
Steam
Drop hammers
hammer Figure 6
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Pile Installation
•Vibratory Driver / Extractors
Drop Hammers
Air or Steam Hammers
Diesel Hammers
Hydraulic Impact Hammers
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Key points
• Piles can be installed using simple drop hammers but, most often,
they are installed using steam or pneumatic hammers.
• Drilled shafts are installed in a pre-bored hole.
• Pile installation methods can cause structural damage, remold the
soil and reduce the pile load capacity.
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Figure 7
Bearing Capacity Assumption
• The ultimate load capacity, Qult, of a pile is conventionally
taken as consisting of two parts.
One part is due to friction, called skin friction or
shaft friction or side shear, Qf.
The other part is due to end bearing at the base or
tip of the pile or pile toe, Qb.
Wp
• If the skin friction is greater than about 80% of the end-
bearing load capacity, the pile is called a friction pile and,
if the reverse, an end-bearing pile. If the end bearing is
neglected, the pile is called a floating pile.
Ultimate load capacity (Qult ) = skin friction (Qf) + end bearing (Qb) – pile weight (Wp)
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Allowable bearing capacity
• The allowable or safe load bearing capacity is
𝑄
𝑄
FS
FS is the factor of safety ranging from 2 to 3
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Load Transfer
• For coarse-grained soils, the load
transfer is approximately linear with
depth higher loads at the top and
lower loads at the bottom.
• In fine-grained soils, the load transfer
is non-linear and decreases with
depth elastic compression of the
pile is not uniform; more
compression occurs on the top part
than on the bottom part of the pile.
Figure 8
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Load Transfer
• The full skin friction and full end bearing are not mobilized at the same
displacement.
For mobilizing the full skin friction a vertical displacement of 5 to 10 mm is required. The
actual vertical displacement depends on the strength and stiffness of the soil and is
independent of the pile length and pile diameter.
The full end-bearing resistance is mobilized in driven piles when the vertical displacement is
about 8-10% of the pile tip diameter. A vertical displacement of about 30% of the pile tip
diameter is required for drilled shafts and bored piles. The full end-bearing resistance is
mobilized when slip or failure zones similar to shallow foundations are formed.
• IMPORTANCE: Different safety factors can be applied to skin friction and to end
bearing.
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Methods of Determining Pile Load Capacity- driven piles
• Statics - and method + (other methods) semi-empirical
• Pile load test
• Pile driving formulae
• Wave analysis (pile driving analysis)
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Pile Load Capacity
Importance
To get an estimate of the pile load capacity, at least in the preliminary design
stages, recourse is made to Statics, experience and to correlations using lab and
field test results.
• STATICS
• method – TSA
• method – ESA
• Pile Load Test (ASTM D 1143)
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Pile Load Test
The purposes of a pile load test are:
• to determine the load capacity of a single pile
• to verify load capacity estimated from statics
• to determine the settlement of a single pile at working loads
• to obtain information on load transfer in skin friction and end bearing.
• to verify pile length.
• To check the structural integrity of the pile, especially the effects of the
proposed installation method.
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Set Up of Pile Load Test
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Figure 9
Pile Load Test
http://www.panynj.gov/airtrainnewark/images/construction_history/08-98/08-98_5.jpg
Figure 10
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PILE LOAD TEST IN ACTION -1
Figure 11
2320
Pile Load Test Results
load
Failure D
load
reloading
B
Shaft friction
A
End bearing
unloading
O C
settlement settlement
Figure 12
• OA ELASTIC; SHAFT FRICTION ONLY
• AB ELASTO-PLASTIC ; MOSTLY SHAFT FRICTION
• BC UNLOADING; OC = PERMANENT SET (PLASTIC)
• CD – RELOADING – END BEARING INCREASES BEYOND A, MAXIMUM SHAFT FRICTION MOBILIZED; END BEARING ONLY TOWARDS D.
VERY NEAR TO D, MAXIMUM END BEARING REACHED PILE PLUNGES DOWN
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Interpretation of Failure Load
Figure 13 25
Key points
• A pile load test provides the load capacity and the settlement of a pile
at the working load at a particular location in a job site.
• Various criteria and techniques are used to determine the allowable
load capacity from pile load tests.
• The tests require accurate measurements of loads and displacements
and careful installation.
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EXAMPLE
The results of a load test on a 0.45m diameter pile are shown in the
table Determine (a) the ultimate pile load capacity (b) the allowable
load for a factor of safety of 2 (c) the allowable load capacity at 10%
pile displacement.
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Solution
• Step1: Plot displacement-load graph.
Figure 14
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Solution
• Step 2: Determine ultimate pile load capacity.
The failure load is ill-defined. Locate the intersection of the
tangents at the beginning and the end of the curve. The
ordinate of this intersection is the ultimate pile load capacity.
Qult = 1780kN
Solution
• Step 3: Determine allowable pile load capacity.
FS = 2
𝑄 1780
𝑄 890 kN
𝐹𝑆 2
• Step 4: Determine pile load capacity at 10% pile
diameter.
Displacement = 450 x 0.01 = 4.5mm
From the figure, Qa = 510kN
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Figure 15
Bearing Capacity Assumption
• The ultimate load capacity, Qult, of a pile is conventionally
taken as consisting of two parts.
One part is due to friction, called skin friction or
shaft friction or side shear, Qf.
The other part is due to end bearing at the base or
tip of the pile or pile toe, Qb.
Wp
• If the skin friction is greater than about 80% of the end-
bearing load capacity, the pile is called a friction pile and,
if the reverse, an end-bearing pile. If the end bearing is
neglected, the pile is called a floating pile.
Ultimate load capacity (Qult ) = skin friction (Qf) + end bearing (Qb) – pile weight (Wp)
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STATICS METHOD
• Skin Friction
ESA -- method
𝑓 𝛽𝜎
𝑄 𝑓 perimeter length
Fine-grained soils (Burland, 1973) :
𝛽 1 sin 𝜙 OCR 0.5 tan 𝜙
j is the number of soil layers within the embedded
length of the pile Coarse-grained soils (Poulos, 1988):
TSA -- method 𝛽 1 sin 𝜙 tan 𝜙
fs is lower of the following expressions
(Randolph and Murray, 1985)
𝑓 0.5 𝑠 𝜎 where i is the soil-pile interface
friction angle
𝑓 0.5 𝑠 . 𝜎 .
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DRIVEN PILES - SKIN FRICTION FACTORS - TSA
𝛼 (Tomlinson, 1987)
Figure 16
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Skin Friction
Typical range of interfacial friction angle
Material Steel Concrete Timber
i ′ to 0.8′ 0.9′ to 1.0′ 0.8′ to 1.0′
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End Bearing ( method)
• The end-bearing capacity is estimated from:
TSA: 𝑄 𝑓𝐴 𝑁 𝑠 𝐴
where fb is the base resistance stress, Nc is a bearing capacity
coefficient, (su)b is the undrained shear strength of the soil at the base
of the pile, Ab is the cross-sectional area of the base of the pile.
•𝑁 9 for 3 and 𝑠 25 kPa
•𝑁 6 for 𝑠 25 kPa.
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End Bearing ( method)
• ESA: 𝑄 𝑓𝐴 𝑁 𝜎 𝐴
where,
𝑓 𝑁 𝜎 is the base resistance stress,
Nq is a bearing capacity coefficient that is a function of ’
𝜎 is the vertical effective stress at the base, and
Ab is the cross-sectional area of the base.
Janbu (1976)
𝑁 tan 𝜙 1 tan 𝜙 exp 2 tan 𝜙 ; We will use this
equation
𝑁 0.6 exp 0.126𝜙 ; 𝜙 is in degrees Budhu
p is called angle of pastification 36
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Numerical Problem-1
• A cylindrical timber pile of diameter 400 mm is driven to a depth of
10 m into a clay with 𝑠 40 kPa, ′ 28°, OCR 2, and 𝛾 =18
kN/m . Groundwater level is at the surface. Estimate the allowable
load capacity for a factor of safety of 2. Is the pile a friction pile?
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