Foundation Engineering

Course Outcomes (CO)
CO1 Describe various subsurface exploration
techniques and Identify a suitable geotechnical
structure for a given situation.

CO2 Discuss and Analyse earth pressure distribution on
retaining structures and stability of slopes

CO3 Analyse and Design shallow and deep
foundations from the geotechnical aspect.

Syllabus

Text Books
1.B.M.Das, ‖Principles of Foundation Engineering‖, Cengage
Learning, 7th Edition
2.Gopal Ranjan and A.S.R. Rao (2016),―Basic and Applied
Soil Mechanics‖,New Age International Publishers, 3rd
Edition
3.Murthy, V. N. S.(2003), ―Geotechnical Engineering:
Principles and practices of Soil Mechanics and Foundation
Engineering ―, Marcel Dekker Inc., New York

Reference Books
1.IS 1888 : 1982,‖ Method of load test on soils (Second
Revision)‖, IS 1892 : 1979‖ Code of practice for subsurface
investigation for foundations (First Revision)‖
2.IS 1080 : 1985,‖ Code of practice for design and
construction of shallow foundations in soils (Other Than
Raft, Ring And Shell) (Second Revision)‖, IS 2911,‖ Design
and construction of pile foundations‖
3.Couduto, Donald P.(2017), ―Geotechnical Engineering –
Principles and Practices‖, Prentice-Hall.,2nd Edition
4.NPTEL notes

What we have learnt in last course?
Soil Mechanics
Basic scientific principles regarding Soil behaviour under loads
and environmental conditions

The “design of foundations” generally requires

The load that is coming on the foundation
The requirements of the local building code
The behavior of soil that will support the foundation system
The geological condition of the soil

Recap of Soil Mechanics

Plasticity

Mathematically…..

Choice of shear strength test



Major Reference: NPTEL course Material : Foundation
Engineering Course by Prof. Kaushik Deb

SOIL EXPLORATION

Field and lab investigation to provide information for design and
construction, project feasibility decision

PRIMARY OBJECTIVES OF SOIL
EXPLORATION

DEPTH OF
EXPLORATION
Governed by depth of influence zone

•Depth at which vertical stress is 20% of contact pressure

•Depth is normally 1.5 x width of square footing and raft, 3 x
width of strip footing, 1.5 x width of pile group, below the pile
tip.

LATERAL EXTENT OF
EXPLORATION
For small, less imp projects, one BH/pit at the
center
For area greater than 0.4 hectares, atleast 5
BH/pits at corners and center.
For large, multistoreyed structures, BH at
corners and at loaded areas
For highways, Shallow BH along C/L, at every
100 - 300 m
For bridges, one BH at every pier/abutmenet
For Dams, Deep BH at 40-80 m c/c, along dam
line.

EXPLORATION
METHODS
•Direct method : Trial pits, Trenches, Plate load Test
•Semi-Direct methods : Boring
Auger boring (Simple/Solid Flight Augers)
Wash boring
Rotary drilling
Percussion drilling
Rock drilling
•Indirect Methods
Sounding methods : SPT, DCPT, CPT etc.
Geophysical methods : GPR, Seismic surveys

DIRECT METHODS

AUGER BORING

•May be hand-operated (< 3 -
5m),or power-driven ( up to 60 to
70m in case of continuous-flight
augers);
•Partially saturated sand ,silts and
medium to stiff cohesive soils.
•Process:
•A rotating auger is pushed into
ground
•When it is full take it out
•Take out highly disturbed samples

WASH BORINGS

SOIL SAMPLING

UNDISTURBED THIN -WALLED
TUBE SAMPLING

PRESERVATION OF SAMPLES
•Undisturbed samples, which are tested after some time, should be
maintained in such a way that the natural water content is retained
and no evaporation is allowed.
•Usually, two coats of 12mm thick paraffin wax and petroleum
jelly are applied in molten state on either end of the sample is
preserved in a humidity controlled room.
•In the absence of such facilities, the sampling tubes should be
covered by Hessian bags and sprinkled with water from time to
time.
•Block samples may be coated with 6mm thick paraffin wax and kept
in air-tight box with saw dust filling the annular space between the
box and the sample.

Standard Penetration Test (SPT)
IS: 2131-1981

VARIOUS SOUNDING METHODS

SPT

SPT
•Simple and inexpensive.
•In-situ dynamic penetration .
•Crude form of testing but results are widely accepted globally.
•SPT ‘N’ – a widely used parameter for estimating soil
resistance and well developed correlations are available
relating all most all soil strength properties., viz., relative
density and friction angle, undrained cohesion, shear wave
velocity and also estimation of settlement.
•The test uses a thick – walled sample tube (split spoon
sampler),– Area ratio more than 100%, hence, produces
disturbed samples.
•Collection of disturbed samples for Atterberg limits test,
Triaxial tests, etc.

•Very useful in ground conditions where it may not
be possible to obtain borehole samples of adequate
quality like gravels, sands, silts, clay containing sand
or gravel and weak rock.

•Most suitable for fine-grained sands and reasonably
well for coarser sands and silty sands.

•Clays and gravelly soils yield results which may be
very poorly representative of the true soil
conditions

•empirical determination of a sand layer's
susceptibility to earthquake liquefaction.

Dilatancy Correction

Precautions in SPT

LOGO
Earth Pressure
STABILITY OF EARTH-RETAINING STRUCTURES

LOGO
Syllabus

LOGO GATE :Section 3:
Geotechnical Engineering (8 %)
Soil Mechanics:
Origin of soils, soil structure and fabric; Three-phase system and phase relationships, index
properties; Unified and Indian standard soil classification system ;
Permeability - one dimensional flow, Darcy’s law; Seepage through soils - two-dimensional flow,
flow nets, uplift pressure, piping; Principle of effective stress, capillarity, seepage force and
quicksand condition;
Compaction in laboratory and field conditions;
One-dimensional consolidation, time rate of consolidation;
Mohr’s circle, stress paths, effective and total shear strength parameters, characteristics of clays
and sand.

Foundation Engineering:
Sub-surface investigations - scope, drilling bore holes, sampling, plate load test, standard
penetration and cone penetration tests;
Earth pressure theories -Rankine and Coulomb ;
Stability of slopes - finite and infinite slopes, method of slices and Bishop’s method;
Stress distribution in soils - Boussinesq’s and Westergaard’s theories, pressure bulbs;
Shallow foundations - Terzaghi’s and Meyerhoff’s bearing capacity theories, effect of water table;
Combined footing and raft foundation; Contact pressure;
Settlement analysis in sands and clays;
Deep foundations - types of piles, dynamic and static formulae, load capacity of piles in sands and
clays, pile load test, negative skin friction.

LOGO

LOGO
Retaining Structures

LOGO

LOGO TYPES OF RETAINING WALLS AND
MODES OF FAILURE

•Rigid : concrete walls relying on gravity for stability
These are called cast-in-place (CIP) gravity and
semi-gravity walls

•Flexible: consists of long, slender members of
either steel or concrete or wood or plastic and
relies on passive soil resistance and anchors for
stability

LOGO
Rigid Retaining Walls

LOGO

LOGO
Flexible retaining walls

LOGO
Modes of failure for Rigid Retaining Walls

LOGO
Failure modes for flexible retaining walls

LOGO Seepage-induced failure
•Rigid retaining walls : adequate drainage systems,
–Flownets. discussed are used in designing drainage systems
•Flexible retaining walls are often used in waterfront structures
and as temporary supports for excavations. Seepage forces
are generally present and must be considered

LOGO
Importance
The first to be analyzed using mechanics
•(Coulomb’s analysis of the lateral earth pressure on the fortresses
protected by soil

Categories
•mass gravity,
•flexible,
•mechanically stabilized earth walls.

Loads on Retaining strucutres:
•static and dynamic loads,
•fluid pressure,
•seepage forces).
Stability is synonymous with ultimate limit state, but
serviceability limit state is also important.

Some Examples

A flexible retaining wall under construction

LOGO
Sheet Pile
During installation Sheet pile wall

LOGO
Lateral Support
Crib walls have been used in Queensland.
Interlocking
stretchers
and headers
filled with
soil
Good drainage & allow plant growth.
Looks good.

LOGO
Bank Requiring a Retention System

LOGO
Retaining Walls - Applications
Road
Train

LOGO
Retaining Walls - Applications
basement wall
High-rise building

LOGO

LOGO
DEFINITIONS OF KEY TERMS
Backfill
•soil retained by the wall.
Active earth pressure coefficient (K
a)
•ratio between the lateral and vertical principal
effective stresses at the limiting stress state when an
earth-retaining structure moves away (by a small
amount) from the backfill (retained soil).
Passive earth pressure coefficient (K
p)
•ratio between the lateral and vertical principal
effective stresses at the limiting stress state when an
earth-retaining structure is forced against a soil
mass.

LOGO
Static Pressures on Retaining Walls
strongly influenced by wall and soil movements.

LOGO Illustration of active and passive pressures with usual
range of values

LOGO
Static Pressures on Retaining Walls
strongly influenced by wall and soil movements.

Active earth pressures develop as a retaining wall moves
away from the soil
behind it, inducing extensional lateral strain in the soil.
When the wall movement is sufficient to fully mobilize the
strength of the soil behind the wall, minimum active earth
pressures act on the wall.

Because very little wall movement is required to develop
minimum active earth pressures (for the usual case of
cohesionless backfill materials), free-standing retaining walls are
usually designed on the basis of minimum active earth
pressures.

LOGO
Earth Pressure at Rest
Where lateral wall movements are restrained, such as;
•tieback walls,
•anchored bulkheads,
•basement walls,
•bridge abutments,
•static earth pressures may be greater than minimum active.

LOGO
Passive earth pressures develop as a
retaining wall moves toward the soil,
thereby producing compressive lateral
strain in the soil.
When the strength of the soil is fully
mobilized, maximum passive earth pressures
act on the wall.
The stability of many free standing retaining
walls depends on the balance between active
pressures acting predominantly on one side of
the wall and passive pressures acting on the
other.

for attaining the active and passive states
in various soils,

LOGO

LOGO
Note
prediction of actual retaining walls forces and
deformations is a complicated soil—structure
interaction problem.
Deformations are rarely considered explicitly in
design—the typical approach is to estimate the
forces acting on a wall and then to design the
wall to resist those forces with a factor of safety
high enough to produce acceptably small
deformations.
 A number of simplified approaches are available
to evaluate static loads on retaining walls.

LOGO
Rankine Earth Pressure Theory
Rankine (1857) developed the simplest
procedure for computing minimum active
and maximum passive earth pressures. By
making assumptions

Rendered the lateral earth pressure
problem determinate

LOGO
Rankine (1820-1872)
William J.M. Rankine (1820-1872), the
famous Scot engineer and physicist is
best known as one of the founders of
the science of thermodynamics.

He held the Queen Victoria Chair of
civil engineering at the University of
Glasgow.

In soil mechanics, he simplified
Coulomb’s theory for cases when
the surface of the backfill is
horizontal, the friction between
the wall and the backfill is
negligible and the retaining wall is
vertical.

LOGO Rankine Active and Passive States of Plastic
Equilibrium
Plastic equilibrium
•if every point of soil is on the verge of failure.

Assumptions made in the originally proposed Rankine’s
theory
1. The soil mass is homogenous and semi-infinite.
2. The soil mass is cohesionless and dry.
3. The surface of soil is a plane which may be horizontal or inclined.
4. The back of the wall is vertical.
5. The back of the wall is smooth, so that there will be no shearing
stresses between the wall and soil.
Because of this assumption the stress relationship for any element adjacent
to the wall is the same as that for any other element far away from the wall.
6. The wall yields about the base and thus satisfies the deformation
condition for plastic equilibrium.

LOGO Diagrams for Lateral Earth-Pressure
Distribution Against Retaining Walls
Backfill—Cohesionless
Soil with Horizontal
Ground Surface

LOGO
RANKINE’S LATERAL EARTH PRESSURE
For cohesion less soils,
coefficient of Rankine’s active earth pressure

LOGO
Rankine’s Passive Pressure

For cohesion less soils,

LOGO
c-phi soil: Min. Active Pressure case

LOGO
c-phi soil: Max. Passive Resistance case

LOGO

LOGO Backfill—Partially Submerged Cohensionless Soil Supporting a
Surcharge

Problem : What is the total active force/unit width of wall and
what is the location of the resultant forces

12
11
2 11
1sin32 1sin30
0.307 0.333;
1sin32 1sin30
At 0 from top,
100*0.30730.7kPa
At just above 3.5 m interface,
10016.5*3.5*0.30748.4kPa
At just below 3.5 m interface
aa
a
a
KK
z
pqK
z
pqzK
z
2' 12
3
,
10016.5*3.5*0.33352.5kPa
10016.5*3.519.25*3.59.82*3.5*0.33363.5kP a
a
pqzK
p

By using triangles and rectangles as shown, the total wall force is
the sum from the several areas and the forces act through the
centroids of the areas as shown so that we can easily sum
moments about the base to obtain

LOGO Backfill—Cohesive Soil with Horizontal Backfill
Active Case

LOGO
Passive Case

Passive Earth Pressure with Water

LOGO
Coulomb (1736-1806)
Charles-Augustine de Coulomb
(1736-1806) was a military
engineer and a famous French
physicist that discovered the force
between two electrical charges.
Less known was his development
of the first thoroughly analytical
study of lateral earth pressures
which he published in 1776.
That theory remains the standard
choice of analysis for lateral forces
upon structures in soils.

LOGO
STABILITY OF RIGID RETAINING WALLS
Cantilever walls (CIP semigravity walls) utilize the backfill
to help mobilize stability and are generally more economical
than CIP gravity retaining walls.

 A rigid retaining wall must have an adequate factor of
safety to prevent excessive
1.translation,
2.rotation,
3.bearing capacity failure,
4.deep-seated failure, and
5.seepage-induced instability.

LOGO
Coulomb’s Active Pressure

BC is a trial failure surface
F—the resultant of the shear and normal forces on the surface of
failure, BC.

LOGO
Coulomb’s Passive Pressure

LOGO

LOGO
Summury Coments
the lateral active pressure on a retaining wall can be
calculated using Rankine’s theory only when the wall moves
sufficiently outward by rotation about the toe of the footing
or by deflection of the wall.
If sufficient wall movement cannot occur (or is not allowed
to occur), then the lateral earth pressure will be greater
than the Rankine active pressure and sometimes may be
closer to the at-rest earth pressure.
It is a general practice to assume a value for the soil
friction angle (f) of the backfill in order to calculate the
Rankine active pressure distribution, ignoring the
contribution of the cohesion (c).

LOGO
lateral earth pressure on a retaining wall is increased
greatly in the presence of a water table above the base of
the wall.
 Most retaining walls are not designed to withstand full
hydrostatic pressure; hence, it is important that adequate
drainage facilities are provided to ensure that the backfill
soil does not become fully saturated.
This can be achieved by providing weepholes at regular
intervals along the length of the wall

Problems

LOGO
Solution

Problem 2:
Compute
Active Earth
pressure
thrust and
its location.

LOGO

For
Practical
batch

LOGO

At z=0,
To calculate depth of tension crack,
substitute p=0,
At base, active earth pressure is,

LOGO

Shallow Foundations

Syllabus

A foundation is that part of structure which transfers
the load of the structure to the sub soil.

Allowable Bearing Pressure (q
a-net)

Net Safe Bearing Capacity (q
ns)

Shear Failure Criteria
Net Safe Bearing Pressure

Settlement Criteria

Suppose, q
u= 100 Tonne/ m2
q
nu= 100-(18*2) = 64 Tonne/ m2
q
ns= 64/3= 21.33 Tonne/ m2

q
s= 21.33 + (18*2) = 57 .33Tonne/ m2

Suppose, Safe bearing Pressure = 15 Tonne/ m2
Allowabale Bearing Pressure =
{min. of q
ns= 21.33 Tonne/ m2
and safe bearing pressure =15 Tonne/ m2}

Meyerhof’s
Analysis
β increases with
an increase in
depth Df and is
equal to 90° for
deep foundation

IS code method (6403-1981)

Shape Factors

Depth Factors

Inclination Factors

Bearing capacity of granular soils
based on SPT
•Teng (1962)

•D
w = depth of water table below the ground surface limited
to the depth equal to D
f

•D’
w = depth of water table measured from base level of the
footing with a limiting value equal to the width of footing B

Bearing capacity of footings on
layered soils:
•Weighted Average of c and phi

Factors influencing bearing capacity
•i) For c
u = 0

•ii) For ϕ = 0

•Using IS Code Method

Ex.2: A rectangular footing of size 3m X 6m is founded at a depth
of 1m in a homogeneous sandy soil. The water table is at a great
depth. The unit wt of soil 18 kN/m3 . Determine net ultimate
bearing capacity. c= 0 and ϕ = 22° . Assume local shear failure

1. Terzaghi’s theory

IS :8009 (Part I) -1976

Summury…
•Method c and d etc are not in syllabus

•V N S Murthy… Adv. Foundaion..

•Compute Consolidation Settlement based
upon odometer data and site details as given
in the figure

•At depth =2m,
•P= 100kN/m2 * A= 100*(8m*12m) = 9600kN
•At depth =4m,
•Intensity = P/A= 9600/(10m*14m) =
68.57kN/m2

Change in load due to Raft 100 kn/M2
P0 z B L Delta p Load Cc e0 Hi Sc
kN/m2 m m m kN/m2 kN m m
at
Foundatio
n Level 2 8 12 100 9600
48.38 Layer 1 4 10 14 68.57143 9600 0.16 0.93 4 0.127117817
78.14 Layer 2 8 14 18 38.09524 9600 0.14 0.84 4 0.052489166
105.805 Layer 3 11.5 17.5 21.5 25.51495 9600 0.11 0.76 3 0.017592097
139.815 Layer 4 15.5 21.5 25.5 17.51026 9600 0.09 0.73 5 0.013329544
Tatal Settlement 0.210528624 m
210.528624 mm

Combined footings
classified as:
•a. Rectangular combined footing
•b. Trapezoidal combined footing
•c. Strap footing

Rectangular Combined Footing

Need:
•In several instances, the load to be carried by a
column and the soil bearing capacity are such
that the standard spread footing design will
require extension of the column foundation
beyond the property line.
•In such a case, two or more columns can be
supported on a single rectangular foundation,
•If the net allowable soil pressure is known, the
size of the foundation can be determined

Design Steps

•For a uniform distribution of soil pressure under the
foundation, the resultant of the column loads should pass
through the centroid of the foundation.
Note that the magnitude of L2 is will be known
and depends on the location of the
property line.
The width of the foundation is then B = A/L

Trapezoidal Combined Footing
•sometimes used as an isolated spread
foundation of columns carrying large loads
where space is tight
Determine the location of the resultant for the column loads

•From the property of a trapezoid,
Obtain B1 and B2
Note that, for a trapezoid

Cantilever Footing
•uses a strap beam to connect an eccentrically loaded column foundation
to the foundation of an interior column.
•They may be used in place of trapezoidal or rectangular combined footings
when the allowable soil bearing capacity is high and the distances
between the columns are large

Module 3 :
Stress Distribution in Soils

Syllabus

2:1 Approximate method

Vertical Stress Increase in a Soil Mass
Caused by Foundation Load
Stress Due to a
Concentrated
Load
In 1885,
Boussinesq
developed the
mathematical
relationships

•relationships for determining the normal and
shear stresses at any point inside
homogeneous, elastic, and isotropic mediums
due to a concentrated point load located at
the surface,
Nore: Eqn is s not a function of Poisson’s ratio of the soil.

Stress below a Rectangular Area
•The integration technique of Boussinesq’s
equation also allows the vertical stress at any
•point A below the corner of a flexible
rectangular loaded area to be evaluated.
•At point A ;

•In most cases, the vertical stress below the center of a
rectangular area is of importance. This can be given by
the relationship

•B.M. Das Book

Problem
•A flexible rectangular area measures 2.5 m X
5 m in plan. It supports a load of 150 kN/ m2.
•Determine the vertical stress increase due to
the load at a depth of 6.25 m below the
center of the rectangular area.

Stress Increase under an Embankment

simplified
form of the
equation is

Problem
•Stress increase at Points A1 and A2 ?

Hence, I’= 0.445

MODULE 5:
DEEP
FOUNDATIONS

Syllabus

Stress and settlement distribution
below a foundation

Assumed Distribution

Pile Foundations

Uses of piles
1. To carry vertical load
•End Bearing Pile : If all the (majority amount) loads are
transferred to the pile tips
•Friction Piles : If all the (majority amount) loads are
transferred to the soil along the length of pile
•Compaction pile: Short piles used for compacting loose
sand.

2. To resist uplift load

•Tension pile or
Uplift:
•Below some
structures such
as transmission
tower, offshore
platform which
are subjected to
tension.

Types of Piles

•Timber pile: suitable for light loads varies from 100 to
250 kN per pile. Suitable for soft cohesive soil.
• Concrete Pile: all load condition. Most frequently used
piles. Strong, durable.
•Steel pile: Used to carry heavy load

•a) circular, b) square, C) rectangular, d) hexagonal,
e) H- section, f) pipe

•Note:
•Rock or very dense sand - H pile and open ended pipe
pile (least driving effort)
•Under the vertical load, the type of pile cross section does
not play a important role. However, under horizontal load,
square and H section pile perform well as compared to
circular pile

•Cohesive soil
under laid by a
granular soil –
Cylindrical pile

•Loose to medium dense
granular soil –Tapered
pile
•(for efficient transfer of
load along the length of
pile.
•efficient distribution of
pile materials)

Under-reamed pile

•Expansive soil –Under-reamed Pile:•
•150-200 mm shaft diameter
•3 to 4m long
•Under-reamed portion is 2 to 3 times the
shaft dia
•Used for expansive soil such as B.C. Soil

Mode of Load Transfer
End- bearing pile
• Act as column
• Transmit the load through a weak soil to a hard stratum
• The ultimate load carried by pile= load carried by the
bottom end

Friction pile
• Do not reach hard stratum
. Transfer the load through skin friction between embedded
soil and pile
• The ultimate load carried by pile= load transferred by skin
friction

Combined end-bearing and friction pile
••The ultimate load carried by pile = load transferred by
skin friction + load carried by the bottom end of pile

Bored Piles

•Driven Pile: loose granular soil (compact the soil, thus
increase its shear resistance)

•Bored pile: best suited to clay soil

•Jetted pile: used if granular soil are in a very compact
state

Method of forming
•Precast concrete piles:
•Formed in a central casting yard to the specified length,
cured and shipped to the construction sites.or
•If space is available, casting yard may be provided at the
site-Length upto 20m and precast hollow pipe piles can
go up to 60m
•- Shorter piles can carry load up to 600kN, and capacity of
longer pile can be as large as2000K N (in some cases)

•Prestressed concrete piles:
•Formed by tensioning high-strength steel (f1 =1700 to 1860
MPa) prestress cables and casting the concrete pile about the
cable
•The prestress cables are cut, when the concrete hardens

•Cast in situ pile
•Formed by making a hole in the ground and filling it with
concrete
•If the hole is formed by drilling, then it is called bored cast in
situ.
• If it is formed by driving a metallic shell or a casing into the
ground, then it is called driven cast in situ.
•If during concreting the casing is left in position, then it is
termed as cased pile.
• If the casing is gradually withdrawn, then it is termed as
uncased pile.

Precast concrete pile
Advantages
•Piles are cast in controlled environment
•The required number of piles can be cast in advance
•Loose granular soil is compacted
•The reinforcements remain in proper position.

Disadvantages
•Addition reinforcements are required due handling and
transportation
•Special equipment's are required for handling and driving
•Piles can be damaged during handling and transportation
•lf the soil is saturated, then pore water pressure is developed
which reduces the shear strength of the soil.
•Length adjustment is difficult

Advantages of cast-in-situ concrete pile:

•The length of the shell or pile can be increased or decreased
•No additional reinforcement is required
•Additional pile can be installed quickly
•Little chance of damage due to handling and transportation

Disadvantages of cast-in-situ concrete
pile:
•Proper quality control
•Loose granular soil is not compacted significantly
•A lot of storage space is required for materials

Bored cast-in-situ piles:
•Large diameter pile can be made.
•Installation can be made without appreciable noise or vibration.
•Boring may be loosen the granular soil.
• In uncased pile, concreting is difficult due to the presence of drilling
mud.
•Bored piles are commonly cheaper.
•Length of the pile can be changed or varied depending the ground
condition.
Driven cast-in-situ piles:
•Diameter of the pile can not be made too large.
•More noise and vibration.
•Granular soil is compacted.
•Drilling mud is not required.
•It is costlier (especially the cased one). Length adjustment is difficult.

Based on displacement of soil:

•Displacement Piles: All driven piles are displacement
piles as the soil is displaced laterally when the piles is
installed.
•Non-Displacement Piles: Bored piles are non-
displacement piles

Typical length and capacities of various
piles (Ranjan and Rao, 1991)

Static pile load formulae
•The ultimate load capacity of the pile (Q
u)

•The ultimate point load
•A
b= sectional area of the pile at its base
•The ultimate skin friction
Therefore, the ultimate load capacity (Q
u
)

IS:2911 (Part i): 2010 Piles in granular soil
For driven piles in loose to dense sand (φ=30°to40°), K
i =1 to 2
For bored piles in loose to dense sand(φ=30°to40°), K
i =1 to 1.5

•Note: Because the width D of a pile is relatively small, the
related term may be dropped without introducing a
serious error;

The allowable load Q
a:

“It is the depth of the pile up to which the effective stress will increase linearly.
After the critical depth, the effective stress becomes constant”
Note: As per IS Code [IS:291 1
(Partl/Sec 1 ):2010],
for piles longer than 15 to 20
times the pile diameter,
maximum effective overburden
stress at pile tip should
correspond to the pile length
equal to 15 (if phi <= 30°) to 20
(if Phi>= 40°) times of the
diameter.

Piles in clay :
•The ultimate load capacity of pile (Q
u):

Values of reduction
factor α
Murthy (2001)…….

Ranjan and Rao 1991…

Types of failure of piles;
•indicate how strength of soil determines the type of
failure:
(a) Buckling in very weak surrounding soil,
•(b) general shear failure in the strong lower soil,
(c) soil of uniform strength,
•(ci) low strength soil in the lower layer, skin friction
predominant,
(e) skin friction in tension (Kézdi, 1975)

•N
q = 16.5
•Ny= 22.4

Consider only one layer,

Note that contribution of (o.5*D*Y*Ny) term is quite less (in this
case only 1.2%) as dia is quite small. So can be omitted D Depth Y phi K delta Fs
0.45 15 17.50.5235991 0.3926992.5 Ap Nq Ny Pd
0.15904316.5 22.4262.5 Q
utip P
di A
si Q
ufrictionQu
702.8832131.2521.205751152.8621855.745

kN
Qa
742.298

Problem 2
•Consider a 15-m long concrete pile with a cross section of
0.45 m X 0.45 m fully embedded in sand. Unit weight of
sand = 17kN/m
3
: and soil friction angle = 35°. Estimate
the ultimate pile capacity from tip resistance
Consider a 1 5-m long concrete pile with a cross section of 0.45 ni X 0.45 ni fully ciii
bedded in sand. For (lie sand. given: unit weight, y = 1 7kN/m3: and soil friction angle,
= 35°. Estimate the ultimate point Q with each of the following: 0.45 15 170.610865
D Depth Y phi 0.2025 55 48.3 255 2877.474
Ap Nq Ny Pd 0 Qutip

Problem 3

Total Pile capacity = 116.5+ 1581 = 1698 kN
Cu Dia
Thk. Of
layer Y A
p
N
c
Q
u tip
Adhe
sion
factor A
si

Q
adhesi
on

Layer
1 30 0.406 5 18 0.129 m2 9
116.5
kN 0.8 6.377 153.
Layer
2 30 5 18 0.8 6.377 153.
Layer
3 100 20 19.6 0.5 25.50 1275.
1581
kN

Problem 4

P = perimeter
Note : No need of cohesion etc. as directly skin resistance is
measured.
•Total frictional Capacity = 931 kN

Load carrying capacity of under-
reamed pile in Clay

•……..(As the pile settles, there is possibility of formation of a small gap between
the top of bulb)

•Whats wrong in this formulae? Multiply second term by
…..?:

Pile Load test
•only direct method for determining the allowable load
••in-situ test and the most reliable
••very useful for cohesion less soil.
••However, for cohesive soil, data from pile load test
should be used with caution because of pile driving
disturbance, pore water pressure development, and
inadequate time allowed for the consolidation settlement.

•Ultimate bearing capacity of pile group is not equal to
sum of all individual piles present in the group.

Negative skin friction:

SLIDE OF ROCK
BLOCK
ALONG STRESS
RELIEF JOINT

OVERHANGING SLOPE
IN HARD AND SLIGHTLY
JOINTED
CEMENTED SANDSTONE

FUNDAMENTALS
•failures of natural and artificial slopes
•an exposed inclined ground surface that is
unrestrained may be prone to mass movement due to
gravitational forces.
•a potential or known failure surface, could exceed the
shear strength of the soil and cause slope failure.
•The ratio of available shear strength to induced shear
stress on a potential failure surface most commonly is
referred to as the factor of safety.

MOST COMMONLY USED METHOD FOR
STABILITY ANALYSIS
limit equilibrium
the shear stress along a failure surface is
expressed as a quotient of the shear
strength over an unknown factor of
safety, and then the factor of safety is
solved by using the equilibrium
equations from statics.

FLOW

TILTING HOUSE

TILTING FENCE

FACTORS AFFECTING THE STABILITY OF SLOPES
•Either external and internal geometric factors,
•intrinsic properties of the materials,
•discontinuities and other structural weaknesses
(geological causes)
•The presence of fluids (air and water)
•weathering or ageing
•external loads from foundations and anchorages, and
stresses induced dynamically, Ex. seismic shaking.

•External geometric factors include ( height and slope angle,
and its shape, both in section and plan).
•Internal geometric factors include the disposition of different material types
inside the slope.
extremely common : weak layer , Natural deposits, particularly of sedimentary origin,
often contain strata which are weaker than the surrounding soils,

ANALYSIS: FELLENIUS METHOD

Note: it is assumed that the forces on the two sides of a slice are
zero, so they have no effect on the force normal to the failure
surface, Hence,

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