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1
Foundation Concepts and
Foundation Alternatives
Amit Prashant
Indian Institute of Technology Gandhinagar
Short Course on
Geotechnical Investigations for Structural Engineering
12–14 October, 2017
IITGN Short Course on Geotechnical Investigations for Structural Engineering
Bearing Capacity
2
2
IITGN Short Course on Geotechnical Investigations for Structural Engineering
3
Terzaghi’s Bearing Capacity Theory
Assumption
L/B ratio is large plain strain problem
Df ≤ B
Shear resistance of soil for Df depth is neglected
General shear failure
Shear strength is governed by Mohr-Coulomb Criterion
B
Df
neglected Effective overburden
q = g’.Df
Strip Footing
f’ f’ 45−f’/2 45−f’/2
Shear
Planes
a b
d e f
g i
j k
qu
Rough Foundation
Surface
c’- f’ soil B
I
II II
III III
IITGN Short Course on Geotechnical Investigations for Structural Engineering
4
Terzaghi’s Bearing Capacity Theory
. . 0.5 .u c qq c N q N B Ngg Terzaghi’s bearing
capacity equation
Terzaghi’s bearing capacity factors
Local Shear Failure:
Modify the strength parameters such as: 2
3mc c 1 2
tan tan3
mf f
Square and circular footing:
1.3 . . 0.4 .u c qq c N q N B Ngg
1.3 . . 0.3 .u c qq c N q N B Ngg
For square
For circular
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
Total and Effective Stress Analysis
Total stress parameters
c and f
From UC or UU test
In bearing capacity equation, use total overburden and
bulk/saturated density.
Effective stress parameters
c' and f'
From direct shear test, CU or CD test
In bearing capacity equation, use effective overburden and
bulk/submerged density.
5
IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Terzaghi’s Bearing Capacity Theory
Effect of water table:
B
B
Dw
Df
Limit of influence
Case I: Dw ≤ Df
Surcharge, . w f wq D D Dg g
Case II: Df ≤ Dw ≤ (Df + B)
Surcharge, . Fq Dg
In bearing capacity equation
replace g by-
w fD D
Bg g g g
Case III: Dw > (Df + B)
No influence of water table.
4
IITGN Short Course on Geotechnical Investigations for Structural Engineering
7
IS:6403-1981 Recommendations
Shape
Factors
1 0.2 tan 452
f
c
Dd
L
f
1 0.1 tan 452
f
q
Dd d
Lg
f
Inclination
Factors
Depth
Factors
Net Ultimate Bearing capacity: . . . . . 1 . . . 0.5 . . . . .nu c c c c q q q qq c N s d i q N s d i B N s d ig g g gg
. . . .nu u c c c cq c N s d i 5.14cN For cohesive soils where,
, ,c qN N Ng as per Vesic(1973) recommendations
1 0.2c
Bs
L 1 0.2q
Bs
L 1 0.4
Bs
Lg For rectangle,
1.3cs 1.2qs
0.8 for square, 0.6 for circles sg g
For square and circle,
for 10of
1qd dg for 10of
2
190
o
c qi i
2
1ig
f
IITGN Short Course on Geotechnical Investigations for Structural Engineering
8
Bearing Capacity
Correlations with
SPT-value
Peck, Hansen, and
Thornburn (1974)
&
IS:6403-1981
Recommendation
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Bearing Capacity Correlations with SPT-value
Teng (1962):
2 213 . . 5 100 . .
6nu w f wq N B R N D R
For Strip Footing:
2 21. . 3 100 . .
3nu w f wq N B R N D R
For Square and
Circular Footing:
For Df > B, take Df = B
0.5 1 1ww w
f
DR R
D
0.5 1 1w f
w w
f
D DR R
D
Water Table Corrections:
B
B
Dw
Df
Limit of influence
IITGN Short Course on Geotechnical Investigations for Structural Engineering
10
Bearing Capacity Correlations with CPT-value
0 100 200 300 400
0
0.0625
0.1250
0.1675
0. 2500
nuq
qc
B (cm)
1fD
B
0.50
IS:6403-1981 Recommendation:
Cohesionless Soil
1.5B
to
2.0B
B
qc value is
taken as
average for
this zone
Schmertmann (1975):
2
kg in
0.8 cm
cq
qN Ng
6
IITGN Short Course on Geotechnical Investigations for Structural Engineering
11
Bearing Capacity Correlations with CPT-value
IS:6403-1981 Recommendation:
Cohesive Soil
Soil Type Point Resistance Values
( qc ) kgf/cm2
Range of Undrained
Cohesion (kgf/cm2)
Normally consolidated
clays qc < 20 qc/18 to qc/15
Over consolidated clays qc > 20 qc/26 to qc/22
. . . .nu u c c c cq c N s d i
IITGN Short Course on Geotechnical Investigations for Structural Engineering
12
Effective Area Method for Eccentric Loading
B
Df
ey ex
L’=L-2ey
B’=B-2ey
AF=B’L’
y
x
V
Me
F
xy
V
Me
F
In case of Moment loading
In case of Horizontal Force at
some height but the column is
centered on the foundation
.y Hx FHM F d
.x Hy FHM F d
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
Settlement
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Pressure Bulb Square Footing Strip Footing
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Newmark’s Chart
Determine the depth, z, where you
wish to calculate the stress increase
Adopt a scale as shown in the figure
Draw the footing to scale and place
the point of interest over the center
of the chart
Count the number of elements that
fall inside the footing, N
Calculate the stress increase as:
Point of
stress
calculation
Depth = z1
Depth = z2
IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Approximate
Methods
.
.z
B Lq
B z L z
2
2z
Bq
B z
z
Bq
B z
Rectangular Foundation:
Square/Circular Foundation:
Strip Foundation:
9
IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Settlement
Immediate Settlement: Occurs immediately after the construction. This is computed using elasticity theory (Important for Granular soil)
Primary Consolidation: Due to gradual dissipation of pore pressure induced by external loading and consequently expulsion of water from the soil mass, hence volume change. (Important for Inorganic clays)
Secondary Consolidation: Occurs at constant effective stress with volume change due to rearrangement of particles. (Important for Organic soils)
Settlement S = Se + Sc + Ss
Immediate Settlement
Se
Primary Consolidation
Sc
Secondary Consolidation
Ss
For any of the above mentioned settlement calculations, we first need vertical stress
increase in soil mass due to net load applied on the foundation
IITGN Short Course on Geotechnical Investigations for Structural Engineering
18
Elastic settlement of Foundation
0 0
1H H
e z z s x s y
s
S dz dzE
sE Modulus of elasticity
H Thickness of soil layer
s Poisson’s ratio of soil
Elastic settlement:
Elastic settlement for Flexible Foundation:
21e s f
s
qBS I
E
fI = influence factor: depends on the rigidity and shape of the foundation
sE = Avg elasticity modulus of the soil for (4B) depth below foundn level
10
IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Steinbrenner’s Influence Factors for Settlement of the Corners of
loaded Area LxB on Compressible Stratus of = 0.5, and Thickness Ht
IITGN Short Course on Geotechnical Investigations for Structural Engineering
20
Elastic settlement of Foundation Soil Strata with
Semi-infinite depth
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Elastic settlement of Foundation
E in kPa
Several other sets of
correlations available
IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Elastic settlement of Foundation
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
23
Strain Influence Factor Method for Sandy Soil: Schmertmann
and Hartman (1978)
2
1 2
0
z
ze f
s
IS C C q D z
Eg
1C Correction factor for foundation depth
1 0.5 f fD q Dg g
2C Correction factor for creep effects
q For square and circular foundation:
For foundation with L/B >10:
Interpolate the values for 1 < L/B < 10
1 0.2log time in years 0.1
IITGN Short Course on Geotechnical Investigations for Structural Engineering
24
Settlement due to Primary Consolidation
log log1 1
s c c c c o avc
o o o c
C H C HS
e e
log1
s c o avc
o o
C HS
e
log1
c c o avc
o o
C HS
e
For NC clay
For OC clay o av c
For OC clay o c o av
c o o av
o Average effective vertical stress before construction
av Average increase in effective vertical stress
c Effective pre-consolidation pressure
oe Initial void ratio of the clay layer
cC Compression Index
sC Swelling Index
cH Thickness of the clay layer
1
46
av t m b
t
b
m
13
IITGN Short Course on Geotechnical Investigations for Structural Engineering
25
Settlement Due to Secondary Consolidation
2
1
log1
cs
p
C H tS
e t
pe
C Secondary Compression Index
2 1log
e
t t
Time, t (Log scale)
Void
Ratio
, e
pe
1t 2t
eVoid ratio at the end of primary consolidation
cH Thickness of Clay Layer
Secondary consolidation settlement is more important in the case of
organic and highly-compressible inorganic clays
IITGN Short Course on Geotechnical Investigations for Structural Engineering
26
Total Settlement
from SPT Data
for Cohesionless
soil
Multiply the settlement
by factor W'
14
IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Total Settlement from CPT Data for Cohesionless soil
3
2
c
o
qC
lnt ot
o
HS
C
Depth profile of cone resistance can be divided in several segments of average cone resistance
Average cone resistance can be used to calculate constant of compressibility.
Settlement of each layer is calculated separately due to foundation loading and then added together
IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Fox’s Depth Correction
Factor for Rectangular
Footings of (L)x(B) at
Depth (D)
Depth factor
c Embedded
c Surface
S
S
Rigidity Factor as per
IS:8009-1976 Total settlement of
rigid foundation
Total settlement at the center
of flexible foundation
Rigidity factor 0.8
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
Shallow
Foundation Design
29
IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Common Types of Footing
Strip footing
Spread Footing
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
31
Common Types of Footing
Combined Footing
Raft or Mat footing
IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Location and depth of Foundation
IS:1904-1986: Minimum depth of foundation = 0.50 m.
Foundation shall be placed below the zone of Excessive volume change due to moisture variation (usually
exists within 1.5 to 3.5 m depth)
Topsoil or organic material
Unconsolidated material such as waste dump
Foundations adjacent to flowing water (flood water,
rivers, etc.) shall be protected against scouring.
A raised water table may cause damage to the
foundation by Floating the structure
Reducing the effective stress beneath the foundation
Water logging around the building: proper drainage system
around the foundation may be required so that water does not
accumulate.
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
33
Location and depth of Foundation
Footings on surface rock or sloping rock faces
Shallow rock beds: foundation on the rock surface after chipping
Rock bed with slope: provide dowel bars of minimum 16 mm
diameter and 225 mm embedment into the rock at 1 m spacing.
Footings adjacent to existing structures
Minimum horizontal distance between the foundations shall not be
less than the width of larger footing. Otherwise, the principal of
2H:1V distribution be used to minimize influence to old structure
Proper care is needed during excavation phase of foundation
construction beyond merely depending on the 2H:1V criteria.
Excavation may cause settlement to old foundation due to lateral
bulging in the excavation and/or shear failure due to reduction in
overburden stress in the surrounding of old foundation
IITGN Short Course on Geotechnical Investigations for Structural Engineering
34
Plate Load Test – IS:1888-1982
18
IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Plate Load Test: Bearing Capacity
uf f
up p
q B
q B
For cohesioless soil
uf upq q
For cohesive soil
IITGN Short Course on Geotechnical Investigations for Structural Engineering
36
Modulus of
Sub-grade
Reaction
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Differential Settlement
Caused by variations in soil profile and structural loads
Consider construction tolerances
Consider best case/worst case scenarios
Use D/ ratios
D
L
= maximum settlement
D= differential settlement
D/ = angular distortion
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Total and Differential Settlement for Clays
IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Total and Differential Settlement for Sands
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Design values of D/ Ratios
IITGN Short Course on Geotechnical Investigations for Structural Engineering
42
How Accurate
are our
Settlement
Predictions?
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Allowable Bearing Pressure
Maximum bearing pressure that can be applied on the soil satisfying two fundamental requirements
Bearing capacity with adequate factor of safety – net safe bearing capacity
Settlement within permissible limits (critical in most cases) – net safe bearing pressure
IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Allowable Bearing Pressure
Teng’s (1962) Correlation:
depth correction factor 1 2f
D
DC
B
Sa in mm and all other
dimensions in meter.
.cor NN C N
1.75
for 0 1.050.7
N o a
o a
C PP
3.5
for 1.05 2.80.7
N o a
o a
C PP
o Effective Overburden stress
2
0.31.4 3
2n cor w D a
Bq N R C S
B
Net safe bearing pressure
2kN m
23
IITGN Short Course on Geotechnical Investigations for Structural Engineering
45
Allowable Bearing Pressure
Meyerhof’s (1974) Correlation:
2
10.49 for 1.2 mn D aq N R S kN m B
1 depth correction factor
1 0.2 1.2
D
f
R
D
B
2
2
2
0.30.32 for 1.2 mn D a
Bq N R S kN m B
B
Net safe bearing pressure
2 depth correction factor
1 0.33 1.33
D
f
R
D
B
Bowel’s (1982) Correlation:
2
10.73 for 1.2 mn D aq N R S kN m B
2
2
2
0.30.48 for 1.2 mn D a
Bq N R S kN m B
B
N-value corrected for overburden using bazaraa’s equation, but
the N-value must not exceed field value
IITGN Short Course on Geotechnical Investigations for Structural Engineering
46
Allowable Bearing Pressure
IS Code recommendation: Use total settlement correlations with SPT
data to determine safe bearing pressure.
Correlations for raft foundations:
Rafts are mostly safe in bearing capacity and they do not show much
differential settlements as compared to isolated foundations.
20.7 3 n w D aq N R C S kN m Teng’s Correlation:
Peck, Hanson, and Thornburn (1974): 20.88 a net w aq C N S kN m
Correlations using CPT data:
Meyerhof’s correlations may be used by substituting qc/2 for N,
where qc is in kg/cm2.
24
IITGN Short Course on Geotechnical Investigations for Structural Engineering
47
Net vs. Gross Allowable Bearing Pressure
2 2
g c c c f cQ Q B D B D Dg g
Gross load
2 2
g cg f c c
Q Qq D D
B Bg g g
2
cn g f c c
Qq q D D
Bg g g
cg g is small, so it may be neglected
2
cn
B
Soil Soil
cD
fD
2
ca net
B
2
cg c c c
Qq D t
Bg g
t
2
cn g f c c f
Qq q D D t D
Bg g g
Usually Dc+t is much smaller than Df
2
cn f
Qq D
Bg
2
ca net f
Qq D
Bg
2
ca gross
B
IITGN Short Course on Geotechnical Investigations for Structural Engineering
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SUMMARY of Terminology
Net Loading Intensity Pressure at the level of foundation causing actual
settlement due to stress increase. This includes
the weight of superstructure and foundation only.
Ultimate Bearing capacity:
Maximum gross intensity of loading that the
soil can support against shear failure is
called ultimate bearing capacity.
Net Ultimate Bearing Capacity:
Maximum net intensity of loading that the
soil can support at the level of foundation.
Gross Loading Intensity
Total pressure at the level of foundation
including the weight of superstructure,
foundation, and the soil above foundation.
superstructure Foundation soil
Foundation
g
Q Q Qq
A
n g fq q Dg
from
Bearing capacity calculation
uq
nu u fq q Dg
25
IITGN Short Course on Geotechnical Investigations for Structural Engineering
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SUMMARY of Terminology
Gross Safe Bearing capacity:
Maximum gross intensity of loading that the soil
can safely support without the risk of shear failure.
Safe Bearing Pressure:
Maximum net intensity of loading that can be
allowed on the soil without settlement
exceeding the permissible limit.
Allowable Bearing Pressure:
Maximum net intensity of loading that can
be allowed on the soil with no possibility of
shear failure or settlement exceeding the
permissible limit.
Net Safe Bearing capacity:
Maximum net intensity of loading that the soil can
safely support without the risk of shear failure. nu
ns
FOS
gs ns fq q Dg
from settlement analysissq
Minimum of
bearing capacity and
settlement analysis
a netq
IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Loads on Foundation
Permanent Load: This is actual service load/sustained loads of a structure which give rise stresses and deformations in the soil below the foundation causing its settlement.
Transient Load: This momentary or sudden load imparted to a structure due to wind or seismic vibrations. Due to its transitory nature, the stresses in the soil below the foundation carried by such loads are allowed certain percentage increase over the allowable safe values.
Dead Load: It includes the weight of the column/wall, footings, foundations, the overlaying fill but excludes the weight of the displaced soil
Live Load: This is taken as per the specifications of IS:875 (pt-2) – 1987.
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
51
Loads for Proportioning and Design of Foundation
IS:1904 - 1986
Following combinations shall be used
Dead load + Live load
Dead Load + Live load + Wind/Seismic load
For cohesive soils only 50% of actual live load is considered for design (Due to settlement being time dependent)
For wind/seismic load < 25% of Dead + Live load
Wind/seismic load is neglected and first combination is used to compare with safe bearing load to satisfy allowable bearing pressure
For wind/seismic load ≥ 25% of Dead + Live load
It becomes necessary to ensure that pressure due to second combination of load does not exceed the safe bearing capacity by more than 25%. When seismic forces are considered, the safe bearing capacity shall be increased as specified in IS: 1893 (Part-1)-2002 (see next slide). In non-cohesive soils, analysis for liquefaction and settlement under earthquake shall also be made.
IITGN Short Course on Geotechnical Investigations for Structural Engineering
52
27
IITGN Short Course on Geotechnical Investigations for Structural Engineering
53
Other considerations for Shallow Foundation Design
For economical design, it is preferred to have square footing for
vertical loads and rectangular footing for the columns carrying
moment
Allowable bearing pressure should not be very high in comparison
to the net loading intensity leading to an uneconomical design.
It is preferred to use SPT or Plate load test for cohesionless soils
and undrained shear strength test for cohesive soils.
In case of lateral loads or moments, the foundation should also be
checked to be safe against sliding and overturning. The FOS shall
not be less than 1.75 against sliding and 2.0 against overturning.
When wind/seismic loads are considered the FOS is taken as 1.5
for both the cases.
IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Combined Footings
Combined footing is preferred when The columns are spaced too closely that if isolated footing is
provided the soil beneath may have a part of common influence zone.
The bearing capacity of soil is such that isolated footing design will require extent of the column foundation to go beyond the property line.
Types of combined footings Rectangular combined footing
Trapezoidal combined footing
Strap beam combined footing
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Rectangular Combined Footing
If two or more columns are carrying almost equal loads, rectangular combined footing is provided
Proportioning of foundation will involve the following steps
Area of foundation
Location of the resultant force
For uniform distribution of pressure under the foundation, the resultant load should pass through the center of foundation base.
Length of foundation,
Offset on the other side,
The width of foundation,
1 2
a net
Q QA
q
2
1 2
Q Sx
Q Q
12L L S
2 1 0L L S L
B A L
Q1 Q2
Q1+Q2
x
L1 S L2
IITGN Short Course on Geotechnical Investigations for Structural Engineering
56
Trapezoidal Combined Footing
If one of the columns is carrying much larger load than the other one, trapezoidal combined footing is provided
Proportioning of foundation will involve the following steps if L, and L1 are known
Area of foundation
Location of the resultant force
For uniform distribution of pressure under the foundation, the
resultant load should pass through the center of foundation base. This gives the relationship,
Area of the footing,
1 2
a net
Q QA
q
2
1 2
Q Sx
Q Q
1 21
1 2
2
3
B B Lx L
B B
1 2
2
B BL A
Solution of these
two equations
gives B1 and B2
Q1 Q2
Q1+Q2
x
L1 S L2
B1 B2
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Strap Combined Footing
Strap footing is used to connect an eccentrically loaded column footing to an interior column so that the moment can be transferred through the beam and have uniform stress distribution beneath both the foundations.
This type of footing is preferred over the rectangular or trapezoidal footing if distance between the columns is relatively large.
Some design considerations:
Strap must be rigid: Istrap/Ifooting > 2.
Footings should be proportioned to have approximately equal soil
pressure in order to avoid differential settlement
Strap beam should not have contact with soil to avoid soil reaction to it.
Q1 Q2
M2
IITGN Short Course on Geotechnical Investigations for Structural Engineering
Pile Foundation
Design
58
30
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Load Transfer Mechanism of Piles
The frictional resistance per unit area at any depth
Ultimate skin friction resistance of pile
Ultimate point load
Ultimate load capacity in compression
Ultimate load capacity in tension
.
zsz
S z
perimeter of pileS
suQ
.pu pu pQ q A
u pu suQ Q Q
u suQ Q
bearing capacity of soilpuq
bearing area of pilepA
upQusQ
uQ
sQz
z
IITGN Short Course on Geotechnical Investigations for Structural Engineering
64
IS:2911 Pile Load Capacity in Cohesionless Soils
31
IITGN Short Course on Geotechnical Investigations for Structural Engineering
65
IS:2911 Pile Load Capacity in Cohesionless Soils
IITGN Short Course on Geotechnical Investigations for Structural Engineering
66
IS:2911 Pile Load Capacity in Cohesionless Soils
(1 to 1.5 for
bored piles)
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
For
Bored
Piles
For
Driven
Piles
67
IITGN Short Course on Geotechnical Investigations for Structural Engineering
68
IS:2911 Pile Load Capacity in Cohesive Soils
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
69
Meyerhof’s Formula for Piles based on SPT value
A limiting value of 400 NAp for point bearing
For Sand:
For Non-plastic silt and fine sand:
Driven Piles:
A limiting value of 130 NAp for point bearing
For Sand:
For Non-plastic silt and fine sand:
Bored Piles:
IITGN Short Course on Geotechnical Investigations for Structural Engineering
70
IS:2911 Pile Load Capacity in Non-Cohesive
Soils Based on CPT data
The ultimate point
bearing capacity:
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71
IS:2911 Pile Load Capacity in Non-Cohesive
Soils Based on CPT data
Correlation of SPT and CPT:
The ultimate skin friction resistance:
IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Allowable Pile Capacity
Factor of Safety shall be used by giving due consideration to the following points
Reliability of soil parameters used for calculation
Mode of transfer of load to soil
Importance of structure
Allowable total and differential settlement tolerated by structure
Factor of Safety as per IS 2911:
uall
FS
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
Load Tests on Piles: Initial Test
73
Purpose:
Numbers:
Note: Piles used for initial testing are loaded to failure or at least 2.5 times the
design load with maximum settlement not exceeding 10% of pile diameter. Such
piles are not used in the final construction.
IITGN Short Course on Geotechnical Investigations for Structural Engineering
Load Tests on Piles: Routine Test
74
Purpose:
Numbers:
Note: During this test pile should be loaded up to 1.5 times the working (design)
load and the maximum settlement of the test should not exceed 12 mm for pile
diameter upto 600 mm and and 18 mm or 2% of pile diameter whichever is less
for piles of diameter more than 600 mm. These piles may be used in the final
construction.
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75
Vertical Load Test: Maintained Load Test
The test can be initial or routine test
The load is applied in increments of 20% of the estimated safe load. Hence the failure load is reached in 8-10 increments.
Settlement is recorded for each increment until the rate of settlement is less than 0.1 mm/hr.
The ultimate load is said to have reached when the final settlement is more than 10% of the diameter of pile or the settlement keeps on increasing at constant load.
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76
Vertical Load Test: Maintained Load Test
After reaching ultimate load, the load is released in decrements of 1/6th of the total load and recovery is measured until full rebound is established and next unload is done.
After final unload the settlement is measured for 24 hrs to estimate full elastic recovery.
Load settlement curve depends on the type of pile
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Vertical Load Test: Maintained Load Test
Ultimate Load
De Beer (1968):
Load settlement curve is plotted in a log-
log plot and it is assumed to be a bilinear
relationship with its intersection as failure
load
Chin Fung Kee (1977):
Assumes hyperbolic curve.
Relationship between settlement
and its division with load is taken
as to be bilinear with its
intersection as failure load
IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Vertical Load Test: Safe Load as per IS: 2911
Safe Load for Single Pile:
Safe Load for Pile Group:
for pile diameter upto 600 mm and and 18 mm
or 2% of pile diameter whichever is less for piles
of diameter more than 600 mm
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
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Dynamic Pile Formula for Driven Piles:
Modified Hiley Formula
. . .
/ 2u
W HQ
S C
W
H
S uQ
Weight of hammer
Height of fall
Pile resistance or Pile capacity
Pile penetration for the last blow
Hammer fall efficiency
Efficiency of blow
Sum of temporary elastic compression
of pile, dolly, packing, and ground C
Note: Dynamic pile formula are not used for soft clays due to pore pressure evolution
IITGN Short Course on Geotechnical Investigations for Structural Engineering
Step 1: The client/owner defines the project scope and specific project requirements.
Step 2: The architect in consultation with the owner and sometime with structural consultants decides the overall initial layout and the type of structures to suit the owners requirements.
Step 3: Soil Investigations and arriving at optimum foundation solutions, through continuous interaction between Foundation and Structural Consultants.
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Normal Project Flow
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
Factors to consider
How deep is good strata? Sand is Good!
How is water table situation?
What is the season when geotechnical investigation was done?
(Moisture content effect, when clayey strata)
Are there any special soil conditions?
Expansive, dispersive or soft clay?
Poorly graded soils? Gravel mix deposits!
What is the architectural plan and what are the expected
loads on different columns?
Clubbing the columns into category A, B, C … based on the load
ranges, before making decisions
Column spacing at different locations?
Special walls, enclosures or columns? 81
IITGN Short Course on Geotechnical Investigations for Structural Engineering
Factors to consider
Is there any variation in the
strata across the building plan
Change in soil type
Inclines strata (Clays?)
Partial fill ground
…..?
In case bearing capacity criteria prevails Is it total
stress or effective stress analysis possible in calculation?
What are the neighboring structures and where is the
line of property?
What are the construction requirements and constraints?
Despite all that, there is uncertainty at each level
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What is the allowable
bearing pressure for
different foundations?
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
Alternative Foundation Systems for
Buildings
Strip Footing
Spread Footing?
Combined Footing?
Raft with Basement?
Piled Assisted Raft System?
Raft with Ground improvement?
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
Strip Footing or Spread Footing?
Loads are relatively small (3 to 4 storey buildings)
Ground is firm from shallow depth
Columns loads are reasonable and well spaced
Most preferred due to ease of construction
Consider uniform depth as much as possible vary the
dimensions; but depth can change if needed
Plan of utilities and finished level minimum depth of
foundation
Comparison of field test data and laboratory test data
major discrepancy means lack of confidence in design
parameters find the reasons or conduct more tests
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
Strip Footing or Spread Footing?
Allowable bearing pressure varies significantly based on
the size and shape of foundation
2x2 footing may have almost 2-times bearing pressure than 6x6
footing in some cases Savings?
One bearing pressure for the whole building is not a good option,
but one depth can be good for ease of construction
Consider differential settlement between different
columns: threshold limits to be checked
Close spacing of footings is alarming to go for alternative
foundation systems Combined footing or Raft
foundations
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
Foundation Systems for Taller Buildings
86
Raft Foundation with more Basements
Pile Foundation With Cap
Pile Assisted Raft Foundation
Raft Foundation with Ground Improvement
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
Raft Foundation with more Basements
Advantage of Gross Bearing Pressure!
Thumb Rules:
On an average per storey load is usually 1.5 tonnes and bulk
density is slightly more than that
One storey per meter of basement depth per basement three
storey building with no bearing pressure requirement
Allowable bearing pressure increases with depth
Some bearing pressure means more storeys.
Adjacent structures can put constraints
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
Piled-Raft Foundation
Use of piles to reduce raft settlements and differential
settlements
Leads to considerable economy without compromising
the safety and performance of the foundation.
It makes use of both the raft and the piles, and is
referred to here as a pile enhanced raft or a piled raft.
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
Piled-Raft Foundation
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
Piled-Raft Foundation
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
Piled-Raft Foundation
91
Poulos, H. G. (2001) "Piled raft foundations: design and
applications", Geotechnique, 51(2), 95-113
IITGN Short Course on Geotechnical Investigations for Structural Engineering
Design of Piled-Raft
Design methods are available for vertical loads
Lateral load response for piled raft is yet to be fully
explored to develop design methods
Moments and static lateral forces are no problem, but seismic
forces are of concern
There are some studies available on construction arrangements
for reducing damage due to seismic forces
Demands accuracy of pile load capacity
The savings from it is worth making the effort for more of initial
and routine pile load tests at different locations on the site
Strategy of pile locations to reduce differential
settlements
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
Design Process for Piled-Raft
Preliminary stage
Assess the feasibility of using a piled raft
number of piles to satisfy design requirements
Second stage
Assess where piles are required
general characteristics of the piles
Final detailed design stage
Optimum number, location and configuration of the piles
Distributions of settlement, bending moment and shear in the raft
Pile loads and moments.
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
Pile Foundation with Cap
Is it really required? The answer is?
Is there a rock strata at shallow depth? Can it consider
shallow foundation or ground improvement in that case?
Is group action required to consider this option?
Is estimation of pile capacity a concern? Is there major
uncertainty about the strata?
Is project too small?
The raft is too small to worry about it?
Loads too heavy?
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
Raft with Ground improvement
No extra basement and no piles!
Can be highly cost effective in most cases
Clayey strata below can be a problem, but solutions are:
Stone columns, Sand pile
Soil mixing, injection systems
Pre-loading, vacuum loading
Sandy or Silty strata is easier to handle
Vibro-compaction
Dynamic compaction, vibro-replacement, vibro-concrete
Check availability and economy
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IITGN Short Course on Geotechnical Investigations for Structural Engineering
Thank You