ce-632 geotechnical...

1

1

Foundation Concepts and Foundation Alternatives

Amit Prashant Indian Institute of Technology Gandhinagar

Short Course on

Geotechnical Investigations for Structural Engineering 15 – 17 October, 2015

IITGN Short Course on Geotechnical Investigations for Structural Engineering

Bearing Capacity

2

2


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


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 2tan tan

3mf 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

3


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


6

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


7

IS:6403-1981 Recommendations

Shape Factors

1 0.2 tan 452

fc

Dd

L

f

1 0.1 tan 452

fq

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


8

Bearing Capacity Correlations with SPT-value

Peck, Hansen, and Thornburn (1974)

&

IS:6403-1981 Recommendation

5


9

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 fw w

f

D DR R

D

Water Table Corrections:

B

B

Dw

Df

Limit of influence


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 cmc

q

qN Ng

6


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


12

Effective Area Method for Eccentric Loading

B

Df

ey ex

L’=L-2ey

B’=B-2ey

AF=B’L’

yx

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

7


Settlement

13


14

Pressure Bulb Square Footing Strip Footing

8


15

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


16

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


17

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


18

Elastic settlement of Foundation

0 0

1H H

e z z s x s ys

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 fs

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


19

Steinbrenner’s Influence Factors for Settlement of the Corners of

loaded Area LxB on Compressible Stratus of = 0.5, and Thickness Ht


20

Elastic settlement of Foundation Soil Strata with Semi-infinite depth

11


21


E in kPa

Several other sets of correlations available


22


12


23

Strain Influence Factor Method for Sandy Soil: Schmertmann and Hartman (1978)

2

1 20

zz

e fs

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


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

46av t m b

t

b

m

13


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)

Voi

d R

atio

, e

pe

1t 2teVoid 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


26

Total Settlement from SPT Data for Cohesionless soil

Multiply the settlement by factor W'

14


27

Total Settlement from CPT Data for Cohesionless soil

3

2c

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


28

Fox’s Depth Correction

Factor for Rectangular Footings of (L)x(B) at Depth (D)

Depth factorc 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

15


Shallow Foundation Design

29


30

Common Types of Footing

Strip footing Spread Footing

16


31

Common Types of Footing

Combined Footing Raft or Mat footing


32

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.

17


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


34

Plate Load Test – IS:1888-1982

18


35

Plate Load Test: Bearing Capacity

uf f

up p

q B

q B

For cohesioless soil

uf upq q

For cohesive soil


36

Modulus of Sub-grade Reaction

19


37


38

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

20


39

Total and Differential Settlement for Clays


40

Total and Differential Settlement for Sands

21


41

Design values of D/ Ratios


42

How Accurate are our

Settlement Predictions?

22


43

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


44


Teng’s (1962) Correlation:

depth correction factor 1 2fD

DC

B

Sa in mm and all other dimensions in meter.

.cor NN C N

1.75

for 0 1.050.7N o a

o a

C PP

3.5

for 1.05 2.80.7N 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


45


Meyerhof’s (1974) Correlation:

210.49 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

22

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

22

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


46


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


47

Net vs. Gross Allowable Bearing Pressure

2 2g 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

2c

n g f c c

Qq q D D

Bg g g

cg g is small, so it may be neglected

2c

n

Qq

B

Soil Soil

cD

fD

2c

a net

Qq

B

2c

g c c c

Qq D t

Bg g

t

2c

n g f c c f

Qq q D D t D

Bg g g

Usually Dc+t is much smaller than Df

2c

n f

Qq D

Bg

2c

a net f

Qq D

Bg

2c

a gross

Qq

B


48

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

Foundationg

Q Q Qq

A

n g fq q Dg

from

Bearing capacity calculationuq

nu u fq q Dg

25


49

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.

nuns

qq

FOS

gs ns fq q Dg

from settlement analysissq

Minimum of

bearing capacity and

settlement analysis

a netq


50

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.

26


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.


52

27


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.


54

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

28


55

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


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

29


57

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


Pile Foundation Design

58

30


63

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

.z

sz

Qq

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

upQ usQ

uQ

sQz

z


64

IS:2911 Pile Load Capacity in Cohesionless Soils

31


65



For Bored Piles

For Driven Piles

66

32


67



68


IS code recommends K-value to be chosen between 1 and 2 for driven piles and 1 and 1.5 for bored piles. However, it is advisable to estimate this value based on the type of construction and fair

estimation of the disturbance to soil around pile. Typical values of ratio between K and Ko are listed below.

33


69

IS:2911 Pile Load Capacity in Cohesive Soils

0.5

For 1 0.5 , but 1 v vu uc c

0.25

For 1 0.5 , but 0.5 and 1 v vu uc c


70

IS:2911 Pile Load Capacity in Cohesive Soils

34


71

Meyerhof’s Formula for Driven Piles based on SPT value

For L/D > 10

A limiting value of 1000 t/m2 for point bearing and 6 t/m2 is suggested

For Sand:

For Non-plastic silt and fine sand:

For Clays:


72

IS:2911 Pile Load Capacity in Non-Cohesive Soils Based on CPT data

The ultimate point bearing capacity:

35


73

IS:2911 Pile Load Capacity in Non-Cohesive Soils Based on CPT data

Correlation of SPT and CPT:

The ultimate skin friction resistance:


74

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

QQ

FS

36


75

Load Tests on Piles

Note: Piles used for initial testing are loaded to failure or at least twice the design load. Such piles are generally not used in the final construction.

Note: During this test pile should be loaded upto 1.5 times the working (design) load and the maximum settlement of the test should not exceed 12 mm. These piles may be used in the final construction.


76

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.

37


77

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


78

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

38


79

Vertical Load Test: Maintained Load Test Safe Load as per IS: 2911

Safe Load for Single Pile:

Safe Load for Pile Group:


80

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

39


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.

81

Normal Project Flow


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? 82

40


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

83

What is the allowable bearing pressure for

different foundations?


Alternative Foundation Systems for Buildings

Strip Footing Spread Footing? Combined Footing? Raft with Basement? Piled Assisted Raft System? Raft with Ground improvement?

84

41


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

85


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

86

42


Foundation Systems for Taller Buildings

87

Raft Foundation with more Basements

Pile Foundation With Cap

Pile Assisted Raft Foundation

Raft Foundation with Ground Improvement


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

88

43


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.

89



90

44



91



92

Poulos, H. G. (2001) "Piled raft foundations: design and

applications", Geotechnique, 51(2), 95-113

45


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

93


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.

94

46


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?

95


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

96

47


Thank You

ce-632 geotechnical...

Documents