Design Working Manual Part1

Civil Design Handbook

Design Working ManualPart 1: Geotechnical Parameters and Foundations

Civil Design Department

May 2004

Civil Design Handbook Design Working Manual (May 2004)

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First published in May 2004

Amendments issued since this publicationAmd. No. Date Affected Clause(s)

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Civil Design DepartmentEngineering DivisionNo. 1 Hampshire Road, Singapore 219428Tel: (65) 1800 CALL LTA Fax: (65) 6396 1383

Civil Design Handbook Design Working Manual (May 2004)

PREFACE

This handbook is intended as a quick guide on the design of foundations and reinforcedconcrete structures based on the relevant standards and codes of practice. It is a collation ofthe requirements covered in the various documents for convenient and easy referenceduring design. Commonly used formulae, charts and tables are complied for quickreference. They are useful for conceptual design, preliminary sizing and detail designcheck. This handbook is suitable for designers familiar with the theoretical background ofthe relevant subjects. Worked examples that illustrate the full design process are included.

This design manual consists of two parts as follows:-Part 1 Geotechnical parameters and foundations,Part 2 Reinforced concrete structures.

1GEOTECHNICAL PARAMETERS AND FOUNDATIONS

Table of ContentContent Page

Geotechnical Parameters

1 Introduction and Background1.1 Soil Correlation

33

2 Basic Soil Characterisation2.1 Index Properties2.2 Soil Classification and Engineering Behaviour

447

3 Lateral Earth Pressure3.1 At Rest Condition (Ko)3.2 Active and Passive Conditions (Ka and Kp) in Cohesionless Soil

3.3 Lateral Earth Pressure in Soil with Cohesion

891012

4 Soil Strength4.1 Effective Stress Analysis4.2 Total Stress Analysis4.3 Relevance of Laboratory Strength Tests to Field Conditions4.4 Correlations with Index Parameters for Undisturbed Clays

1212141517

6 Elastic Deformability 5.1 Definition of Various Coefficient and Modulus

1818

7 Time Dependent Deformability6.1 Compression Index Cc and Modified Compression Index Cce6.2 Effective Preconsolidation Stress (sp) in Cohesive Soil6.3 Coefficient of Consolidation cv6.4 Coefficient of Secondary Compression

2222242526

8 Permeability 27

9 References 28

Deep Foundations

1 Pile Foundation1.1 Design of Piles

3030

2 Geotechnical Capacity2.1 Static Method2.2 Empirical Method

313237

3 Structural Capacity (Allowable Material Stress)3.1 Bored Pile3.2 Steel Pile3.3 Precast Pile3.4 Reinforced Concrete Pile

38

24 Dynamic Formulae4.1 Hileys formula (Trial and error method)4.2 ENF (modified) formula4.3 Janbus Formula

38393941

5 Pile group5.1 In Clay5.2 In Sand5.3 In Rock

42424444

6 Settlement6.1 Friction Pile6.2 End Bearing Pile6.3 Non Homogeneous Soil6.4 Soil Parameters6.5 Pile Group Analysis

444446484849

7 Negative Skin Friction7.1 Distribution of Negative Skin Friction on Single Pile7.2 Magnitude of Negative Skin Friction on Single Pile7.3 Safety Factor For Negative Skin Friction

54545455

8 Pile Load Test8.1 Static Load Test8.2 Dynamic Load Test

555556

9 Special Topics9.1 Micropiles9.2 Timber Piles9.3 Bakau Piles

59596061

10 Typical Sizes10.1 Bored Piles10.2 H-piles10.3 Precast Piles10.4 Timber Piles10.5 Micropiles

616161636363

11 Worked Examples 64

Shallow Foundations

1 Footing 70

2 Bearing Capacity of Footing 71

3 Contact Pressure 74

4 Estimation of Settlement of Footing 75

5 Plate Test 84

6 Influence of Ground Water 85

7 Examples 86

31. Introduction and Background

Soil is a complex engineering material, which has been formed by a combination ofvarious geological, environmental, and chemical processes. These processes, bothnatural or man made are ongoing. Thus soil properties are not unique or constant butvary with many environmental factors such as time, stress history, water tablefluctuation, etc.

Due to the complexity of soil behaviour, empirical correlations are used extensively inevaluating soil parameters.

1.1 Soil correlation

Before any geotechnical activity can proceed on site, it is necessary to perform ananalysis based on the soil engineering parameters for that site. The parameters areoften obtained through field/laboratory tests conducted as part of a borehole/CPT siteinvestigation programme. Sometimes, the engineer may be expected to perform apreliminary assessment/analysis at short notice and with limited soil information. Insuch a situation, correlations are very useful.

However, one must avoid using correlations as a black box to obtain the requiredsoil properties. The source, extent, and limitation of each correlation should beexamined carefully before use.

Items to look out for when using correlation include:

1) Number of data points, n

2) Standard deviation, s.d.

3) Coefficient of determination, r2 (r2 = 0 no correlation, r2 = 1 perfect correlation)

4) r is the statistic for testing the significance of a simple two variable linearrelationship, i.e. how well the data fit a linear relationship

Local calibrations, where available, should be preferred over any generalisedcorrelations.

Values for various geotechnical parameters are indicated in Table 1. It should benoted that the values are meant for preliminary design when site specific data are notavailable. For final design, site specific soil investigation data should always be usedto obtain the parameters required. Also, as the LTA Design Criteria has specified thevalues of various geotechnical parameters to be used, Table 1 and site specific datawill provide a verification or for knowing the design safety margin available.

Suggested correlations are available in the later sections.

Civil Design Department Design Working Manual Geotechnical Parameters & Foundations

4

2. Basic Soil Characterisation

2.1 Index Properties

Cohesive Soils

Cohesive soils are represented by simple index parameters which are expressed as watercontents at particular soil states known as Atterberg limits; they also represent boundariesbetween different engineering behaviours.

The commonly used index parameters for geotechnical engineering are:

wn = In situ natural water content

wL or LL = Liquid limit

wp or PL = Plastic limit

PI or Ip = Plasticity index

LI or IL = Liquidity index

For most soils: 0 < LL < 100 and PL < 40

Liquid limit is the minimum water content of the soil to make it behave like a viscousliquid. Plastic limit is the minimum water content of the soil to make it assume a plasticstate.

Plasticity Index (PI)Plasticity Index, PI (or Ip) is an index to describe the range of water content over which asoil was plastic. Therefore, PI = LL PL (or WL WP)

High PI, i.e. > (25 30) may mean troublesome soils with low strength and highcompressibility.

Liquidity Index (LI)The Liquidity Index, LI (or IL) tells us the likelihood for the sample to behave as a plastic, abrittle solid or even possibly a liquid. It is defined as:

LI = (wn PL) / PI or (wn PL) / (LL PL)

The LI is also an excellent indicator of geologic history and relative soil properties, asshown in Table 2, Fig. 2 and Fig. 3.


5

Table 2 General behaviour of soil in relation to Liquidity Index, LI.

LI < 0 i.e. wn < PL soil will have a brittle fracture if sheared

0 < LI < 1 i.e. PL < wn < LL soil will behave like a plastic if sheared

(rule of the thumb:- LI > 0.5: likely to be NC;LI < 0.5: likely to be OC)

LI > 1 i.e. wn > LL soil will be a very viscous liquid when sheared

NC = normally consolidated OCR = overconsolidation ratio (sp/svo)

LOC = lightly overconsolidated Ko = in-situ coefficient of horizontal soil stress (sho/svo)

HOC = highly overconsolidated

>1 < 0

wnPLp

LLp

NC LOC HOCSensitiveDecreasing water content

Increasing OCR, Ko

Increasing strength, modulus

Decreasing compressibility

Decreasing LI1 0

Figure 2 Liquidity Index Variations. (adapted from Kulhawy, 1990)


6

Cohesionless Soil

Cohesionless soil are represented by simple index parameters expressed in terms of eitherunit weight or density.

The relative density or density index of sand is defined as:

densityratiovoidewhereee

eeD

ddd

dddr ==-

-=

--

= rrrrrrr

,)()(

minmax

minmax

minmax

max

Note: The above definition is only limited to cohesionless soils having less than 15% fines

The relative density of sand may be described as below (note that the range definition mayvary slightly from source to source):

State:

Water content:

Liquidityindex:

Brittlesolid

Semi-solid: Plastic solid Liquid

0 SL(Solid Limit)

PL LLwn (%)

0 < LI < 1LI < 0 LI = 0 LI = 1 LI > 1

t

e

w < PLStress

xt

e

t

e

w LL

w PL

w > LL

Figure 3 Water content continuum showing the various states of a soil as well asthe generalised stress-strain response. (adapted from Holtz and Kovacs, 1981)

Strain


7

Table 3 Categorisation of relative density.

Sand Dr (%)

Very loose 0 to 20

Loose 20 to 40

Medium 40 to 60

Dense 60 to 80

Very dense 80 to 100

2.2 Soil Classification and Engineering Behaviour

Soil classification provides a systematic method of categorising soils according to theirprobable engineering behaviour.

Soil Classification System used in LTA

The current soil classification used by LTA is the British Standard Soil Classification forEngineering Purposes, which has been adopted since 1999. Previously, the Unified SoilClassification System (USCS) was used.

Plasticity Chart for Soil Classification and Engineering Behaviour

For plastic soils, the plasticity chart is also used (only 1 plasticity chart exists Casagrandes Plasticity Chart), as shown in Fig. 4.

Fig. 4 Casagrandes plasticity chart, showing several representative soil types.


8

Note for Plasticity Chart:

1) Any soils plot above the U line in the plasticity chart, the data should be questioned andverified.

2) The A line generally separates the more claylike materials from those that are silty and also theorganics from the inorganics. The exception is organic clays (OL and OH) which are below theA-line. However, these soils do behave similarly to soils of lower plasticity.

3) The dividing line between low and high liquid limits was set arbitrarily at 50. Several differentsoil types tend to be plotted in approximately the same area on the LL-PI chart, which meansthat these soils tend to have about the same engineering behaviour.

4) The chart should be used as a reference and not as an absolute measure of the behaviour of soil.

From the plasticity chart, we can observe the behaviour of soil as it moves on the chart:

Table 4 Behaviour of soil in relation to plasticity.

Characteristic Soils at Equal LL

with Increasing PI (LL PL)

Soils at Equal PI

with Increasing LL

Dry Strength Increases Decreases

Permeability Decreases Increases

Compressibility About the same Increases

Rate of volume change Decreases -

3. Lateral Earth Pressures

The coefficient of lateral earth pressure K is:

v

hK''

ss

=

Where sh = horizontal effective stress and sv = vertical effective stress.K is an indicator of the lateral earth pressures acting on a retaining wall.

Three important soil conditions are defined: the at rest condition (Ko), the active condition(Ka), and the passive condition (Kp), where Ka < Ko < Kp.


9

3.1 At Rest Condition (Ko)

Ko is called the coefficient of lateral earth pressure at rest (i.e. no lateral strain).

The general formula for Ko is:

( ) ( )2sin tan5.01sin1 bf f +-= OCRKo or

Ko (overconsolidated) Ko (nc) OCR

where:f = friction angle of soilOCR = overconsolidation ratio of soilb = inclination of ground surface from horizontal

Ko can be estimated using the Plastic Index (PI) where:

Ko = 0.44 + 0.42(PI/100) Massarsch, 1979

Ko = 0.4 + 0.007(PI) for 0 < PI < 40 } Brooker & Ireland (1965)Ko = 0.64 + 0.001(PI) for 40 < PI < 80 }

Table 4 Typical ranges of Ko.Type Ko

Sedimentary soils 0.4 0.5

Normally consolidated clay 0.5 0.9

Over-consolidated clay > 1

Loose sand 0.45 0.6

Dense sand 0.3 0.5(extracted from various sources, see reference list)

Note for Ko:

1) Within a homogenous soil, Ko is a constant, independent of the depth and the location of groundwater table.

2) For sand, Ko = 1 sin f provides reasonable estimates.

3) For clay, Ko tends to increase with PI and OCR.

4) The magnitude of Ko may be measured directly either in the laboratory using special testingequipment, or in the field using devices such as the pressuremeter or total stress cells. However,these direct methods may be subject to unavoidable disturbance effects during sampling and in-situ testing.


10

3.2 The Active and Passive Conditions (Ka and Kp) in Cohesionless Soil

A small movement in soil would alter the lateral earth pressure. Fig. 5 Shows the relativemagnitude and the relative movement required to mobilise Ka and Kp in sands of differingdensity.

The active and passive earth pressure coefficient developed by Rankine and Coulomb arewidely used. The equations are given in Table 4.

Fig. 5 Effect of wall movement on lateral earth pressure in sand (extract from Coduto, 1994).


11

Table 5 Rankine and Coulomb lateral earth pressure equation.

Active PassiveRankine

(Fig. 6) fbfbb

fbb

-+

--=

22

22

coscoscos

coscoscosaK

if b = 0, the above equation reduces to:

-= 245tan

2 faK

fbfbb

fbb

--

-+=

ap K

orK1

coscoscos

coscoscos22

22

if b = 0, the above equation reduces to:

+= 245tan

2 fpK

( )

( ) ( ) ( )( ) ( )

2

2

2

coscossinsin

1coscos

cos

-+-+

++

-=

baafbfaf

afa

af

w

ww

aK( )

( ) ( ) ( )( ) ( )

2

2

2

coscossinsin

1coscos

cos

--++

--

+=

baafbfaf

afa

af

w

ww

pK

Coulomb

(Fig. 7)

a) Formula valid only for bfb) When designing concrete walls it is common practice to use fw = 0.67f. Steel

walls have less sliding friction, perhaps on the order of fw = 0.33f

Note for Ka and Kp:

1) The above coefficients of earth pressure are the value of K derived for a cohesionless soil.

2) For Ka, use either Rankine or Coulomb. For Kp, use Rankine as Coulombs theory produceserroneously high values of Kp.

Figure 7 Parameters for Coulombs equations.

H

b

a

Pa/b

Va/b

fw

H

b

Pa/bVa/

W/

T/b

N/bb

45 +

H

b

Pp/Vp/

W/

T/b

N/bb

45 -

(a) (b)

Figure 6 Free body diagram of soil behind a retaining wall usingRankines solution: (a) active case; and (b) passive case.


12

3.3 Lateral Earth Pressure in Soil with Cohesion

Creep in clayey soil was not considered in Rankine or Coulombs equation. If the soilbehind the wall is clayey, the value of K would be higher. Earth pressure distributions incohesionless soil and cohesive soil are different as illustrated in Fig 8.

Suggest: Use K between active and at-rest values and not to rely on full passive pressure.

Note on cohesive soil wall:Theoretically, if H < Hc, the earth will stand vertically without a wall. In practice, we need to applysome factor of safety to Hc before deciding not to build a wall. The potential of surface erosion andother modes of failure e.g. slope failure needs to be considered too.

4. Soil Strength

The soil strength is not uniquely defined but varies with many parameters. The strength ofsoils is commonly expressed by the Coulomb-Mohr failure criterion:

t = c + stanf

The criterion is usually used in two alternative forms, based on effective stress or totalstress analysis.

4.1 Effective Stress Analysis

The shear strength (i.e. shear stress at failure) is expressed as:

t = c + stanf

where c is the effective cohesionf is the effective friction angle

H

Hc

(b)(a)

Figure 8 Distributions of earth pressures for (a) cohesionless soil in active and passivecondition, (b) cohesive soil in active condition (c 0, f 0) ,and (c) cohesive soil in passivecondition (c 0, f 0).

H

(c)


13

sn is the effective normal stress

Effective stress laboratory test data are often interpreted incorrectly to show a moderatelyhigh c and an unrealistically low f because the true failure envelope curvature is not

being addressed.

Fig. 9 shows the correct interpretation of f where c = 0 for a wide range of soil type.

Linear interpolation of any of these data over a limited stress range would suggest a c andf, but these values would not be the true soil strength parameters.

For a given soil at a constant normal effective stress (s), the friction angle will varies withdensity state and strain, as shown in Fig. 10.

Fig. 9 Strength Envelopes for a range of soil types (fm Kulhawy, 1990).

Fig. 10 Friction angle definitions.


14

Table 6 Typical strain at peak strength.

Type Typical Strain

at Peak Strength

Very dense sand or clay with high OCR < 5%

Structured clay 4 7%

Very loose sand or weak NC clay 10 to 20%

Note on c and f:

1) c = 0 (except in truly cemented soils, partially saturated soils, and heavily overconsolidatedclays). For the stability of some numercial analysis, a small value of c is assumed, e.g. c = 1.

2) At very large strain (excess of 100%), fcv is reduced to the residual state. This residual state isonly considered for very large strain problems, such as in soil containing pre-existing shearfailures.

4.2 Total Stress Analysis

In total stress analysis,

t = c + stanf ;

f = 0 and t = c =cu = su*

The total stress analysis is normally adopted for simplicity. In reality, the failure of all soils(sand, silts, and clay) occurs on the effective stress envelope. In low permeability soilssuch as clays, loading generates changes in pore water stresses (Du). These pore waterstresses change the effective stress envelope. Since the total stress loading path and themagnitude of the changes in pore water stresses may not be known with confidence, a totalstress analysis provides a simple analysis alternative.

*In many older references, the term cohesion was used to designate su. In recent references, su isreferred to as the undrained shear strength or undrained shearing resistance. The older definitionhas led to much confusion and misinterpretation with the effective stress cohesion intercept (c)

Note on total stress analysis:

1) all the four terms can be used interchangeably to represent the undrained shear strength of thesoil.

2) Detailed studies conducted have shown that the UU and UCT tests often in gross error because ofsampling disturbance effects and omission of a reconsolidation phase. Strictly speaking, thesetests should only be considered as general indicators or relative behaviour and not to be useddirectly in design.

3) Simple hand held devices are intended primarily for field inspection purposes; not to obtainparameters for design.


15

4.3 Relevance of Laboratory Strength Tests to Field Conditions

The strength of soils can be measured by a number of different laboratory and field strengthtests (see Fig 11 and Fig 12). Each of these tests will give different results (both c and f)because each subjects the soil to different boundary conditions and loading stress path.

Fig. 11 Common laboratory strength tests and field tests.


16

To adopt various tests pertinent for a particular field condition is likely to be an excessiverequirement for common and routine design cases. It is recommended that the isotropicallyconsolidated, triaxial compression test for undrained/drained loading be carried out. Theresults of this test can then be used as the standard reference to compare the results of allother tests. For example, the value of f(tc), as shown in Table 7.

Fig. 12 Relevance of laboratory strength tests to field conditions.


17

Table 7 Comparison of f from different laboratory tests (from Kulhawy, 1990).

Test Type Friction Angle (degrees)

Triaxial compression1 (TC) 1.0ftcTriaxial extension (TE) 1.22ftcPlane strain compression (PSC) 1.10ftcPlane strain extension (PSE) 1.34ftcDirect shear2 (DS) tan-1[tanfpsccosfcv]

1 isotropically consolidated, triaxial compression test for undrained/drained loading2 Speculative, based on results from sand

4.4 Correlations with Index Parameters for Undisturbed Clays

Correlation with PI

For NC, non-fisssured, organic, sensitive, or unusual clays, the correlation by Skempton(153)may be used.

su (vst) / svo = 0.11 + 0.0037PI

Correlation with preconsolidation stress, sp

For low OCR clays and low to moderate PI, the approximation (to DSS) by Jamiolkowski(1988) is useful:

su /sp = 0.23 0.04

Correlation with CPT qc value

The theoretical relationship for the cone tip resistance in clay is given by:

qc = Nk su + svo

Different theoretical models adopted general different range of values for Nk. Thus, Nk isusually determined empirically by calibrating CPT data with a know measured value of su.To get a correct Nk, consistent reference su, cone type and correction for qc must be applied.


18

Correlation with PMT resultsThe applied stress (normalised with atmospheric pressure) is plotted against the volumetricstrain as shown in Fig. 13. The slope is su/Pa. This su should be close to the value obtainedfrom plane strain compression (PSC) tests.

5. Elastic Deformability

The elastic behaviour of soils governs the initial, time-independent, movement offoundations under static loads. These deformation properties vary with many parametersand therefore are not defined uniquely.

The deformation properties of elastic materials are described most often by Youngsmodulus (E) and Poissons ratio (v). Although these parameters are strictly defined onlyfor elastic materials under uniaxial loading, they are used commonly in a generic sensewith inelastic material such as soils.

It should be noted that the following properties are non-linear and stress dependent.

5.1 Definition of Various Coefficient and Modulus

Youngs Modulus, E

E = stress/strain is often obtained from the results of triaxial compression tests, i.e. theslope of the curve.

E can be defined as the initial tangent modulus (Ei), the tangent modulus (Et) at a specifiedstress level, or the secant modulus (Es) at a specified stress level as shown in Fig. 14.

Fig. 13 Pressuremeter results in Bartoon Clay (fm Kulhawy, 1990).


19

Table 8 General ranges of E (from Bowles, 1997).

Soil Undrained Modulus,

Eu (MPa)

SPT(different table; same ref)

Clay

Very soft 2 to 5 0 - 2

Soft 5 to 15 3 - 5

Medium 15 to 50 6 - 9

Hard 50 to 100 10 - 30

Sandy 25 to 200

Sand

Silty 5 to 20

Loose 10 to 25 4 10

Dense 50 to 80 30 - 50

Sand and Gravel

Loose 50 to 150

Dense 100 to 200

The pressuremeter test provides a measurement of the horizontal modulus (EPMT) in soils.In clays, it is commonly assumed that EPMT = Eu.

Note on E:

1) In sophisticated numerical models, the actual stress path can be followed, and the modulus canbe evaluated for each stress strain state along the stress path. In simpler closed-form solutions,an effort must be made to estimate the overall average modulus from the initial to the final stressstates.

Fig. 14 Modulus definitions.


20

2) Factors affecting su will also affect Eu. Therefore, the value of Eu will be dependent on test typeand test specifics.

Poissons Ratio, v

Poissons ratio is defined in an analogous form for triaxial tests in which both axial andvolumetric strains are measured.

Poissons ratio = radial strain/axial strain

For drained loading, volume change occurs, and the drained Poissons ratio (vd) varies withsoil type and consistency. Typical values are give below, which are representative ofsecant values at common design stress levels:

Table 9 Typical range of vd (from Kulhawy, 1990).

Soil Drained Poissons Ratio, vd

Clay 0.2 to 0.4

Dense sand 0.3 to 0.4

Loose sand 0.1 to 0.3

Note for v:

1) The range of v is relatively small compared with the range of E

2) For isotropic elastic materials, the entire range of v is from 0 to 0.5.

3) For undrained (f = 0) loading of saturated cohesive soil, no volume change occurs. Therefore,the undrained Poissons ratio (vu) is equal to 0.5 by definition.

4) General, vd is higher for soil with higher PI and OCR.

Shear Modulus, G

For undrained loading, the modulus of cohesive soils can be described by either theundrained Youngs modulus (Eu) or the shear modulus (G). The shear modulus actuallydescribes the soil skeleton response, so it is independent of drainage conditions, all otherfactors being equal. The shear modulus is the slope of the shear stress-strain curve fromtests such as the Direct shear or Direct simple shear results. As with E and v, G is nonlinearand stress-dependent.

For undrained loading,

Eu = 3G (vu = 0.5).

For elastic materials, Youngs modulus and Poissons ratio are interrelated uniquely withthe shear modulus (G) as follows:


21

G = E/2(1 + v)

Constrained Modulus, M

This modulus is defined for one-dimensional compression, where the lateral strains arezero. From elastic theory, M is related to E and v as follows:

( )( )( )uu

u211

1-+

-=

EM

Coefficient of Compressibility, av

The slope of the compression curve (void ratio versus effective stress), when the results areplotted arithmetically, is called the coefficient of compressibility, av. Since the curve is notlinear, av is approximately constant over a small pressure range, s1 to s2; or

12

21

'' ss --

=ee

av

Coefficient of Volume Change, mv

When the results are plotted in terms of the percent consolidation or strain, then the slope ofthe compression curve is the coefficient of volume change, mv

Mea

vm

o

vvv

11'

=+

=DD

=se

ev is the vertical compression or strainM or Eoed is the contrained or oedometric modulusIn one dimensional compression, ev is equal to De/(1+eo)

Subgrade Reaction, ks

The concept of subgrade reaction is often used for evaluation the behaviour of footings,mat/raft foundations, and laterally loaded deep foundations. In subgrade reaction models,there is a basic parameter which is analogous to a spring constant. This parameter isdefined as the modulus of subgrade reaction (ks), given by:

ks = p/d unit is force per length cubed

p = applied stress,d = displacement under p

As with Youngs modulus, ks varies with stress level. However, unlike Youngs modulus,ks also varies with foundation width.

The most logical procedure to evaluate ks is to present it in terms of Youngs modulus (E)and Poissons ration (v) of the soil as given by Vesic below:


22

-

= 2

121

4

165.0

vE

IEEB

Bk

ffs

Ef = foundation Youngs modulusIf = foundation moment of inertiaEfIf = foundation stiffness

6. Time Dependent Deformability

The parameters that define the time-dependent deformability of soils are important forevaluating the settlement of foundations.

6.1 Compression Index Cc and Modified Compression Index, Cce

When the results are plotted in terms of the void ratio versus the logarithm of effectivestress, then the slope of the virgin compression curve is called the compression index Cc, or

1

2

21

''

logssee

Cc-

=

The degree of compressibility of clay, expressed in terms of the compression index is asbelow:

Table 10 Degree of compressibility

Compressibility Cc

Slight or low < 0.2

Moderate or Intermediate 0.2 to 0.4

High > 0.4

Correlation for Cc

Based on modified Cam Clay model, Wroth and Wood showed that:

Cc 0.5Gs(PI/100)

for Gs = 2.7; Cc PI/74 and;

Cr PI/370 , which is about 20% of Cc.

The table below shows some compilation of estimates of Cc using eo and wn.

Table 11 Various correlation for Cc and Cce.


23

Equation Regions of Applicability

Cc = 1.15(eo 0.35) All clays

Cc = 0.30(eo 0.27) Inorganic, cohesive soil; silt,silty clay

Cc = 1.15(10-2wn) Organic clay

Cc = 0.75(eo 0.50) Soils of very low plasticity

Cce = 0.156 eo + 0.0107 All clays

Modified Compression Index, Cce

The slope of the virgin compression curve when the tests results are plotted as percentconsolidation or vertical strain versus logarithm of effective stress is called the modifiedcompression index, Cce:

o

cvc e

CC

+=

D=

1''

log1

2

sse

e

Note on consolidation graphs:

There are a few advantages in using the percent consolidation or vertical strain versus logarithm ofeffective stress curve to compute settlements:

1) Estimating field settlement is simple. The percent compression can be read directly from thegraph once the in situ vertical overburden stress is known.

2) The graph can be plotted during the test itself. This enables early evaluation compared to thevoid ratio versus log effective stress curve which requires the determination of the dry mass ofsolid to compute the initial and final void ratio. By looking at development of the curve duringthe test, the load increment near the preconsolidation pressure can be reduced to obtain a betterdefinition of the transition between the reloading curve and the virgin compresson curve.

3) Two samples may show very different e versus log svc plots but may have similar verticalstrain versus log effective stress curves because of difference in void ratio.


24

6.2 Effective Preconsolidation Stress (sp) in Cohesive Soils

sp can be estimated from the index parameter using the following by Stas and Kulhawy(1984):

sp/Pa = 10(1.11 1.62LI)

where Pa is the atmospheric pressure = 1 bar = 100 kPa

Fig. 15 Factors affecting the laboratory determination of sp: (a) effect of sample disturbance; and (b) effect of load.


25

Where possible, sp should be obtained from 1-D consolidation tests on undisturbedsamples.

Factors affecting determination of sp, Cc and Cr

Sample disturbance will cause the transition part of the consolidation curve to be less sharp.As a result, lower sp and Cc are obtained compare to actual in-situ values while a higher Cris obtained.

For soft, sensitive clays, small stress change or even vibration may drastically alter the soilstructure. For such soils, the load increment ratio (Ds/sinitial) should be smaller than 1.

6.3 Coefficient of Consolidation cv

The field value of the coefficient of consolidation (cv) is a difficult parameter to estimatebecause common field situations include seams, lenses and boulders, etc., which laboratorypredicted values of cv different from in-situ values.

A first order estimate for cv of clays can be obtained using the liquid limit (LL) as in Fig.16.

Fig. 16 Coefficient of consolidation vs Liquid limit (fm Kulhawy, 1990).


26

Field estimates using CPTU

The piezocone tests have been used to give field estimates of horizontal permeability (kh)and horizontal coefficient of consolidation (cvh) in clays. The basic equation for thehorizontal coefficient of consolidation is:

cvh = TR2/t

T = time factorR = equivalent cavity (piezocone) radiust = time to achieve desired degree of excess pore water stress dissipation.

The approach is based on cavity expansion theory, and therefore it depends on the rigidityindex of the soil.

Fig. 17 gives the piezocone time factors. Most commonly, the dissipation test is conductedfor a period of time (t) which will allow 50% dissipation of the original insertion excesspore water stress (Du). The time factor corresponding to this dissipation time is thenintroduced to the equation above to compute the coefficient of consolidation. Cylindericaltheory would be used for a pore water sensor behind the tip, while spherical theory wouldbe used for a sensor at the tip.

6.4 Coefficient of Secondary Compression

The coefficient of secondary compression (Ca) defines the rate of settlement with time afterprimary consolidation is complete. This coefficient may be expressed either in units ofstrain (Cae) or void ratio (Cae) per log cycle of time.

For a wide variety of clays, Cae has been correlated to the natural water content.For NC clay: Cae 0.0001wn

For most OC clay: 0.0005 < Cae < 0.001

Fig. 17 Pore water stress decay vesus Piezocone time factor (fm Kulhawy, 1990).


27

For NC clays, the ratio of the coefficient of secondary compression to the compressionindex is relatively constant for a given soil. Table 16 lists Cae/Cc for a variety of soils.

Table 12 Ratio of Cae/Cc for various types of soil (as cited in Kulhawy, 1990)

Soil Type Cae/Cc

Soft Clay 0.025

Other Clay 0.025 to 0.06

Silty Clay 0.03

Silt 0.03 to 0.06

Organic Clay and Silt 0.04 to 0.06

Note on time rate of consolidation:

In inorganic soil, primary consolidation is usually the largest component of total settlement,whereas secondary compression constitutes a major part of the total settlement of peats and otherhighly organic soils.

In engineering practice, only estimates of the time rate of settlement can be made because of thegreat dependence that the rate of settlement has on the drainage path. Another factor is our inabilityto accurately predict cv. If possible, estimates should be field checked, especially for importantjobs.

7. Permeability

The coefficient of permeability (k) of soil, also known as the hydraulic conductivity,describes the rate of water flow through soil. This soil property is often difficult to evaluatewith certainty, because it varies over many orders of magnitude and in-situ soil conditionsare highly variable. In addition to controlling the amount and rate of ground water inflowinto foundation excavations, the coefficient of permeability also governs the rate of primaryconsolidation and equalisation of pore water stresses.

The value of the coefficient of permeability can vary over a wide range, as shown in Table13. It is also clear that k is highly dependent upon the soil particle size.


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Table 13 Range of coefficient of permeability for different soil.

Soil Coefficient ofPermeability, k (m/sec)

Relative Permeability

Gravel > 10-3 High

Sandy gravel, clean sand, finesand

10-3 to 10-5 Medium

Sand, dirty sand, silty sand 10-5 to 10-7 Low

Silt, silty clay 10-7 to 10-9 Very Low

Clay < 10-9 Practicallyimpermeable

In geotechnical problems, drainage can occur horizontally as well as vertically. The ratioof horizontal to vertical permeability (kh/kv) is generally less than 1.5 for marine clays andother massive deposits (Kulhawy, 1990). However, in varved clays and stratified fluvialdeposits, kh/kv can easily exceed 10.

k can also be obtained indirectly from the consolidation test:

o

vwv

egac

k+

=1r

whererw = density of waterg = gravitational force (10ms-2)cv = coefficient of consolidationav = coefficient of compressibilityeo = initial void ratio

The value of eo is the void ratio at the start of the time rate readings for a given loadincrement.

8. References

1. Bowles, J.E. (1997) Foundation Analysis and Design, McGraw-Hill International,Singapore, 5th Edition.

2. Coduto, D.P. (1990) Foundation Design Principles and Practices, Prentice-Hall,Inc., Englewood Cliffs, New Jersey 07632.

3. Holtz, R.D. and Kovacs, W.D. (1981) An Introduction to GeotechnicalEngineering, Prentice-Hall, Inc., Englewood Cliffs, New Jersey 07632.

4. Kulhawy, F.H. (1990) Manual on Estimating Soil Properties for Foundation,Research Project 1493-6, Geotechnical Engineering Group, Cornell University,Ithaca, New York.


29

5. LTA Design Criteria, Chapter 5 Geotechical Parameters.

6. Guidance notes on weathering and classification, Technical Sharing Material byChiam, S.L., LTA-CDE.

7. Selection of Geotechnical Design Parameters, Technical Sharing Material by Wen,D.Z., LTA-CDE.


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1. Pile Foundation

Piled foundation is selected when large settlement is likely for shallow foundation or whereno stratum of sufficient bearing capacity exists close to the surface.

The main function of bearing piles is to transfer the load from the structure to the lowerlevels of the grounds where are capable of sustaining the load with an adequate factor ofsafety and without settling at the working loads by an amount detrimental to the structurethat they support. Piles derive their carrying capacity from a combination of friction alongtheir sides and end bearing at the pile toes. The former is likely to predominate for piles inclays and silts and the latter for piles terminating in a stratum such as compact gravel, hardclay or rock.

When friction piles are driven into a deep deposit of fairly uniform consistency in order totransfer the foundation pressure to the lower levels, they should be long enough to ensure asubstantial advantage over a shallow foundation. In these circumstances, it should be bornein mind that for the same superficial area of pile surface, a few long piles forming a pilefoundation are more effective and will support the load with smaller settlement than manyshort piles.

The load should be applied concentrically and the axis of the pile is at the centre of gravityof the pile group. Allowance should be made in the design for inaccuracies in positioningthe piles, particularly for isolated piles or pairs of piles. Such piles should be designed toaccommodate the resulting moments or should be restrained by an adequately designed pilecap to resist lateral and rotational movements.

The types of piled foundation system adopted by LTA for structures are

a. Bored piles

b. Driven H-piles

c. Precast piles

d. Micropiles

The types of piles that are not commonly used like

e. Timber piles

f. Bakau piles

1.1 Design of piles

The design of piles requires specialised knowledge of the ground and the environmentalconditions, the properties of the various types of piles and the effects produced by theapplied load on the piles and the supporting soils. The type of piles to be chosen should becarefully considered to ensure its suitability in relation to the ground and environmentalconditions. The conditions must be properly defined by means of adequate siteinvestigation works to permit appropriate selection of piles type and the design of the pile.


31

Where piles are installed in groups, the effects of the placing a number of piles in closeproximity to each other will need to be taken into account.

The design of pile should satisfy the following requirements

a) An adequate factor of safety against failure of the pile element or its surroundingsoil shall be adopted.

b) The settlement of the piles foundation as a whole and the differential settlements ofthe piles shall be kept within permissible limits.

The design of the structural strength (commonly referred to as nominal working load) of apile shall be based on its required material strength with an adequate factor of safety toensure that the pile has necessary strength when installed to transmit the loads imposed onit to the soil. For a driven pile, it shall be capable of withstanding without damaging thestresses arising during handling and installation.

In designing the pile, allowance should be made for the additional weight of the pile capand pile. These additional weights of the pile cap and the pile can contribute up to 15% ofthe column load. In addition, pile group action is taken into account. Thus, these factorsshould be taken into consideration.

2. Geotechnical capacity

Qu = Qb + Qs= qbAb + fsAs

Where Qb = Total base resistance Qs = Total shaft resistance qb = Unit base resistance Ab = Cross Sectional area of pile fs = Unit shaft resistance As = Surface area of pile

A factor of safety (FOS) is used to evaluate the allowable load

= QuQaFOS

The minimum geotechnical length required as practised in LTA is based on the factor ofsafety imposed for skin friction and end bearing as show in the table below:


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Loading Condition FOS for Qs FOS for Qb

Compression 2.5 2.5

Compression 1.5 -

Tension (With load test) 2.5 -

Tension (No load test) 3.5 -

2.1 Static Method

i) Cohesionless Soil (b Method)

(1) Shaft resistance

Qs = S fsAs

Wherefs = Ks po tand flKs = Coefficient of lateral earth pressured = Angle of friction between pile and soilpo = Vertical earth pressurefl = Limiting friction = 100 kN/m2

Meyerhofs relationship of Ks tand versus f (Angle of friction of sand) is given in Figure1. f can be estimated from N values obtained from standard penetration test of conepenetration test (See Figure 2 ). Alternatively, f may be taken as (28+15RD) where RD isthe relative density of sand.

(Note fl can be increased up to 200kN/m2 provided the value can be verified by pile loadtests numerous in the case of local soils)


33

Figure 1 Values of Kstand versus f

Figure 2 Determination of angle of shearing resistance of granular soil from in-situtests. (a) Relationship between SPT (N values) and angle of shearing resistance of

granular soil (Peck, Hanson & Thorburn). (b) Meyerhofs correlation between staticcone penetration resistance and angle of shearing resistance of sand.


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(2) End bearing

Qb = qbAb

Whereqb = poNq qlpo = Effective overburden pressure at the pile tipNq = Bearing capacity factor (See Figure 3)ql = Limiting value (See Figure 4) for driven pile = 200 to 1100 t/m2 for bored piles

Figure 3 Relationship between Nq and f


35

Figure 4 Limiting static cone resistance versus ffor sand (Meyerhof, 1976)

Broms et al. suggested

Nq = 9 if the diameter of pile is 1.0m

ii) Cohesive Soil (a Method)

(1) Shaft resistance

qs = aCu

Where a = Adhesion/reduction factor to take into account the loss in shear strengthdue to pile installation

Typical values for bored piles,

Type of Soil a

Over consolidated clay 0.3-0.6

Normally consolidated clay 0.8-1.0

Adverse ground 0.3


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If no previous data or experience available, use a = 0.45 provided

a. There is adequate load testing

b. aCu >/ 100 KN/m2.

For driven piles,See Figure 5 for different values in different grounds.

Figure 5 Adhesion factors for driven piles in clay(Tomlinson, 1969)

(2) End Bearing

qb = qCu

The end bearing is unlikely to be in clay, more likely to be on silt or sand. This can beevaluated from the equation given for cohesionless soil. Alternatively, this can also beestimated by Meyerhorfs method based on SPT values.


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2.2 Empirical Methods

iii) Using Standard Penetration Test (Based on Meyerhorfs method)

Qu = S C1 Ns As + C2 Nb Ab

Where C1 = 2 KN/m2 for driven pile = 1 KN/m2 for bored pileC2 = 40 (Db/B) for driven pile with a limiting value of 400kPa = (40/3) (Db/B) for bored pile with a limiting value of 400/3 kPaDb= Depth of pile in bearing stratumB = Pile width or diameterNs, Nb = SPT values along pile shaft (average) and pile base (corrected)

respectively in blow/300mm.As, Ab = Area of pile shaft and pile base respectively in m2

Note :

C2 Nb qbl

Where qbl = Figure 2 (for driven pile)= 2000 11000 KN/m2 (for bored pile)

Broms et al. suggested fs = 2N for residual soil in Singapore (max 120 KN/m2).

iv) Using Cone Penetration Test

Qu = qcb Ab + 2 fs As

Where qcb = CPT value at base of pile fs = Average shaft friction as measured on friction jacket

or fs = 0.005 qcb (for displacement piles)= 0.0025 qcb (for steel H-piles)

BearingStratum Db

B


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3. Structural Capacity (Allowable Material Stress)

3.1 Bored pile

Qa = 0.25fcuAb

Wherefcu = Concrete cube strength at 28 daysAb = Cross sectional area of pile

3.2 Steel pile

Qa = 0.3fyAb (For driven pile)= 0.5fyAb (For jacked pile)

Wherefy = Yield stress depending on grade of steel

= 275 N/mm2 for Grade 43A

3.3 Precast pile

Qa = 0.25 (fcu - prestress after loss) Ab

The loss of prestress should be calculated in accordance with SS CP65.

3.4 Reinforced Concrete pile

Qa = 0.25fcuAb

4. Dynamic Formulae

The formulae are limited to cohesionless soil and driven pile. In its simplest form,

Energy of hammer = Work done in overcoming resistance.W h = Ru s

Where W = Work Doneh = Height of hammerRu = Soil resistances = Set

This formula has been modified to take into account the energy losses in pile, cap and soil.


39

Commonly used dynamic formulae are

a. Hileys formula

b. ENF (Modified) formula

c. Janbus formula

4.1 Hileys formula (Trial and error method)

Energy = Work + Impact loss + Losses in cap, pile, soil

Qu = (ef W H h ) / (s + c/2)

Wherec = c1(Pile) + c2(Cap) + c3(Soil)h = (W + n2Wp) / (W + Wp)

Values can be obtained from Tables 1-3.

Assume an initial driving resistance Q. Iterate until Q Qu

4.2 ENF (modified) formula

Qu = [(ef W H h ) / 0.025(s + 0.1)] / [(W + n2Wp) / (W + Wp)]

ef and n can be obtained from Table 1-2.

Whereef = Hammer coefficientW = Weight of hammerWp = Weight of pileH = Height of hammer droph = Efficiency of driving systems = Pile penetration for last blow or setc = Sum of temporary elastic compressionn = Coefficient of restitution


40

Table 1 Values of hammer efficiency, ef

Table 2 Values of coefficient of restitution, h


41

Table 3 Values of c1, c2, c3 for Hiley formula.

4.3 Janbus formula

Qu = (1/Ku) (WH/s)

Where Ku = Cd [1+ (1+ le/Cd)]Cd = 0.75 + 0.15 Wp/Wle = (WHL) / (ApEps2)L = Pile lengthAp = Pile cross sectional areaEp = Modulus of elasticity of pile

There are a number of limitations to the driving formulae. They are

1. Assumptions made in the formulae pay little attention to the actual forces andmotions occurring during driving a real pile or to the nature of the soil and itsbehaviour.

2. Formulae are unreliable for long piles.


42

3. Formulae are applicable to granular soil (sand, gravel) where changes with time arevery small.

4. Formulae neglect influence of pile grouping may have to modify predictions toallow for this.

5. Pile group

The ultimate load capacity of a pile group is not necessarily equal to the sum of the ultimateload capacities of the individual piles in the group. The ratio of the two loads is defined asthe efficiency of the pile group. In general, the pressure bulbs of neighbouring piles tend tooverlap, creating a greater stress concentration on the surrounding soil. Such phenomenonleads to greater settlement of the pile group and is termed as group action. With sufficientstress overlap, either the soil will fail in shear (local failure) or pile group will settleexcessively (block failure).

Other important factors for design consideration include the influence of pile spacing andpile cap. BS8004 recommended that

1. For friction piles, the spacing centre to centre should be not less than the perimeterof the pile, or circular piles, three times the diameter.

2. For end bearing piles, the distance between the surfaces of the shafts of adjacentpiles should not be less than the least width of the pile.

5.1 In clay

Group action is important in the case of friction piles in clay. The ultimate load capacity ofa pile group (QG) in clay is the lesser of the two following relationships :

i) Local failure

QG = h m n Q

Whereh = Pile group efficiency (Figure 6)

= 1 q/90 [{(n-1)m + (m-1)n}/m n]m = Number of rows of pilesn = Number of piles in a rowd = Diameter of piless = Centre to centre spacing of pilesq = tan-1 (d/s)Q = Ultimate capacity of single pile


43

Figure 6 Pile group efficiency in cohesive soils

ii) Block failure

The ultimate bearing capacity of the whole block

QG = cb Nc Bg Lg + a Cu [2 D (Bg + Lg)]

WhereD = Depth of pile in bearing stratumBg = Width of pile groupLg = Length of pile group


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5.2 In sand

Action of driving piles is to compact the sand around the pile to a radius of at least 3 timesthe width of the pile. In loose sand, the pile group efficiency may be greater than 1 becauseof the effects of densification of the sand between the piles. In dense sand, pile drivingcauses loosening and efficiency less than 1 may result.

In general, block failure is a consideration only if the pile centre to centre spacing is lessthan 7 diameters. The ultimate load capacity of a pile group in sand is the lesser of (a) sumof load capacity of individual piles and (b) load capacity of the pile group block.

5.3 In rock

The ultimate capacity of a pile group installed to rock is the sum of capacity of individualpiles in the group. Block failure is a consideration only if foundations are on a sloping rockformation, and sliding may occur along favourable weakness planes. The possibility ofsuch an occurrence must be evaluated from the site geology and field exploration.

6. Settlement

One of the most widely used approach to compute settlement of piles is the elastic theory.Poulos provided dimensionless parametric solutions from which estimates of pilesettlement behaviour can be rapidly obtained based on given pile and soil properties. Bothfriction piles and end bearing piles are considered in the analysis.

6.1 Friction pile

The pile is considered to be a cylinder of length L, shaft diameter d, and base diameter db,and loaded with an axial force P at the ground surface. In a homogeneous soil mass havingconstant Youngs modulus Es and Poissons ratio ns, the settlement of the pile head r isgiven by

r = P I / (Es d)

Where I = Io Rk Rh RnIo = Settlement influence factor for incompressible pile in semi-infinite mass

for ns = 0.5 (Figure 7)Rk = Correction factor for pile compressibility [Where the pile stiffness factor

k = Ep RA / Es where RA = area ratio = Ap / (pd4/4) = 1 for a solid pile] SeeFigure 8

Rh = Correction factor for finite depth of layer on a rigid base. See Figure 9Rn = Correction factor for ns. See Figure 10


45

Figure 7 Settlement influence factor, Io

Figure 8 Correction factor for pilecompressibility, Rk


46

Figure 9 Correction factor for depth for settlement, Rh

Figure 10 Correction factor for Poissons ratio for settlement, Rv

6.2 End bearing pile

The settlement of pile head r is given by

r = P I / (Es d)

Where I = Io Rk Rb RnRb = Correction factor for stiffness of bearing stratum. See Figure 11


47

Figure 11 Base modulus correction factor for settlement, Rb


48

6.3 Non homogenous soil

If the modulus variation between successive layers along the length of the pile is not large,the settlement may be calculated from the expressions for a pile in uniform soil using anaverage soil modulus Eav as follows

Eav = (1/L) S (Ei hi)

WhereEi = Modulus of layer ihi = Thickness of layer i

In cases where the pile passes through distinct layers of soil, having large difference in soilmodulus, the uniform soil solutions may be utilized in an approximate manner. Forexample, for a simple case of a pile penetrating one layer and founded in a second layer, thesettlement may be estimated by treating the portion of the pile in the first layer as an endbearing pile and determining the settlement of this and the amount of load in the pile at theinterface of the two layers. The settlement is added to the previously calculated settlementof the upper portion to obtain the overall settlement of the pile head.

6.4 Soil parameters

iii) Clay

The total settlement of pile head rTF = ri + rCF where ri is the immediate settlement andrCF is the final consolidation settlement. Drained parameters such as Es and n should beused to calculate rTF . On the other hand, undrained soil parameters such Eu and nu shouldbe used to determine ri.

Eu = 1.5 Es / (1+n? )

Suggested n? values are

Soil n?

Stiff overconsolidated clay 0.1 0.2

Medium stiff clay 0.2 0.35

Soft normally consolidated clay 0.35 0.45

iv) Sand

For piles in sand, the final settlement may be considered to occur immediately onapplication of the load, so that drained soil parameters such as Es and n? should be used incalculating the settlement of the pile. In general, the soil modulus at the pile base Eb isconsiderably greater than the average modulus along the shaft.


49

Suggested average values of Es along the shaft of the pile

Sand density Range ofrelativedensity

Range of Es(MPa)

Loose < 0.4 27.5 - 55

Medium 0.4 0.6 55 70

Dense > 0.6 70 - 110

Poulos suggested that as an upper limit Eb = 10Es may be used for driven pile in dense sandand as lower limit Eb = 5Es may be used for bored piles in loose sand. An average value ofn? = 0.3 is reasonable when no test data are available.

v) Rock

The modulus for rock mass Em is highly affected by its joint spacing,

Em = j Mr quc

Wherej = A mass factor related to the joint spacing in the rock massMr = Modulus ratio

Values of Poissons ratio lie between 0.1 0.4 depending on the type of rock

6.5 Analysis of pile group

To analyse the settlement behaviour of a general pile group, superposition of two pileinteraction factors may be employed. Thus for a group of n identical piles, the settlementrk of any pile k in the group is given by superposition as

nrk = r1 S ( Pj akj ) + r1 Pk

j=1 jk

Where r1 = Settlement of single pile under unit loadPj = Load in pile jakj = Interaction factor for spacing between piles k and j.

For groups containing piles of different size and geometry, rk may be expressed as

nrk = r1 S ( rij Pj akj ) + r1k Pk

j=1 jk


50

Where rij = Settlement of single pile j under unit loadakj = Interaction factor for spacing between piles k and j and for the

geometrical parameters of pile j.

For vertical load equilibrium, the total pile group load PG is given as

nPG = S Pj

j=i

For a pile group of n piles, there will be n settlement and one load equations and these canbe solved for two simple conditions

vi) Flexible pile cap

Equal load or known load on all piles i.e. all the Pj are given to solve for all the rj andhence the differential settlement between piles.

For groups with equally loaded piles, the maximum settlement occurs at the centre pile,while the minimum settlement occurs at the corner pile. The ratio of the maximumdifferential settlement to the maximum settlement is shown in Figure 12 for some typicalgroups of incompressible friction piles in a semi infinite mass. The ratio increases withincreasing spacing but decreases if the layer depth is decreased or L/D is increased. Thevalue of K has relatively little influence.

For typical end bearing pile groups, the corresponding values are shown in Figure 13 for K= 100. For such compressible piles, relatively large differential settlement may occur,especially for large groups and slender piles. However, the relative differential settlementdecreases rapidly with increasing K and is zero for piles that can be consideredcompressible.


51

Figure 12 Differential settlement in floating pilegroups with equally loaded piles

Figure 13 Differential settlement in end-bearing groups with equallyloaded piles

vii) Rigid pile cap

Equal settlement for all the piles. All the rj are equal and only PG is given: Pj and hencedistribution of loads in the pile group as well as group settlement are to be computed.


52

However for most practical purpose, the average settlement of a group with equally loadedpiles is found to be equal to that of a group with a rigid cap. Thus the assumption of equalloading should be adequate in most cases, and the group settlement may be approximatelycalculated from a representative pile that is neither at the centre nor at the corner of thegroup.

The group settlement rg can be expressed in terms of the settlement ratio Rs where

Rs = rg / Settlement of single pile at same average load as a pile in a group

Theoretical values of Rs are shown in Table 4 for friction pile group in a deep layer ofuniform soil and in Table 5 for pile groups bearing in a rigid stratum.

The exact configuration of the piles in a group does not significantly influence Rs so thatvalues for other numbers of piles may be interpolated from the Tables 4 & 5. For groupscontaining more than 16 piles, it has been found that

Table 4 Theoretical values of settlement ratio Rs. Friction pile group, with rigid cap,on deep uniform soil mass


53

Table 5 Theoretical values of settlement ratio Rs. End-bearing pile group, with rigidcap, on deep uniform soil mass

Rs varies approximately linearly with the square root of the number of piles in the group.

Thus for a given value of pile spacing, K and L/d, Rs may be extrapolated from the valuesfor a 16 pile group and 25 pile group as follows

Rs = (R25 R16) ( n - 5) + R25

WhereR25 = Value of Rs for 25 pile groupn = Number of piles in group

Once Rs is determined, rg can be evaluated as follows,

rg = Rs Pav r1

WherePav = Average load on a pile in a groupr1 = Settlement of a single pile under unit load


54

7. Negative Skin Friction

Deep foundation elements installed through compressible materials can experiencedowndrag forces or negative skin friction along the shaft which results from downwardmovement of adjacent soil relative to the pile. Negative skin friction results primarily fromconsolidation of a soft deposit caused be dewatering or permanent placement of fill.

7.1 Distribution of negative skin friction or single pile

The distribution and magnitude of negative skin friction along the shaft depends on

a) Relative movement between compressible soil and pile shaft.

b) Relative movement between upper fill and pile shaft.

c) Elastic compression of pile under working load

d) Rate of consolidation of compressible soils

Negative skin friction develops along the portion of the pile shaft where settlement of theadjacent soil exceeds the downdrag displacement of the shaft. The neutral point is thatpoint of no relative movement between the pile and adjacent soil. Below this point, skinfriction acts to support the pile loads. The ratio of the depth of the neutral point to thelength of the pile in compressible strata may be roughly between 0.67 0.75. The positionof the neutral point may be estimated by trial and error procedure that compares thesettlement of the soil to the displacement of adjacent sections of the pile.

7.2 Magnitude of negative skin friction on single pile

The peak negative skin friction in granular soils and cohesive soils is determined as forpositive skin friction.

The peak unit negative skin friction can be estimated from

fn = bPo

Wherefn = Unit negative skin friction (to be multiplied by area of shaft in zone of subsiding soil)Po = Effective vertical stressb = Empirical factor from full scale tests

Table 6 - b values from full scale testsSoil b

Clay 0.20 0.35

Silt 0.25 0.35

Sand 0.35 0.50


55

7.3 Safety factor for negative skin friction

Since negative skin friction is usually estimated on the safe side, the factor of safetyassociated with the load is usually unity.

Qu - PnQa =Fs

WhereQa = Allowable pile loadQu = Ultimate Pile loadFs = Factor of safetyPn = Ultimate negative skin friction load

8. Pile Load Test

Pile tests are conducted on site to determine whether the foundation design is adequate.Namely they are:

a) Static Load Test

b) Dynamic Load Test

8.1 Static Load Test

A load test is carried out for the following reasons

1. To determine the load settlement relationship, particularly in the region of theanticipated working load

2. To serve as a proof test to ensure that failure does not occur before a load is reachedwhich is a selected multiple of the chosen working load. The value of the multipleis then treated as a factor of safety.

3. To determine the real ultimate bearing capacity as a check on the value calculatedfrom the dynamic or static formulae, or to obtain information that will enable otherpiles to be designed by empirical methods.

The load test may be applied to the pile by either

a. Direct load placed on a platform bearing on the pile.

b. Kentledge heavier than the required test load of a platform supported clear of thepile under test and brought to bear on the pile by the reaction of a jack.


56

Kentledge Pile Load Test Setup

Load cell and dial gauge

Survey equipment to measure thesettlement of pile

8.2 Dynamic load test

Dynamic load test is normally carried out on piles to evaluate the pile load capacity andpile integrity.


57

During the hammer impact on the pile, the Pile Driving Analyzer (PDA) process therecords and calculate the values for the maximum hammer transferred energy, maximumcompressive force and an evaluation of the piles mobilised static bearing capacity iscalculated by Case Method.

The field records from the test are further analysed using the computer CAPWAPC (CasePile Wave Analysis Program Continuous Version). This method combines the wave equalpile and soil model with the Case Method measurements. Thus the solution includes notonly the total and static bearing capacity values but also the skin friction, end bearing,damping factors and soil stiffness. A simulated static load test using the established soilcharacteristics is then performed yielding the load versus settlement curve.

A dynamic load test is deemed to have failed if the maximum resistance of pile (RMX) atany time during blow, using a Case Damping Coefficient (J) as approved by the Engineer,is less than 2 times the nominal working load of a working pile under test.

Test Setup

Instruments used to take measurements


58

Test in progress with reading being taken

Test completed

Checking of pile


59

viii) Case Method

The Case Method is a closed form solution based on a few simplifying assumptions such asideal plastic soil behaviour in an ideally elastic and uniform pile. Given the measured piletop force F(t) and pile top velocity v(t), the total soil resistance R(t) is

R(t) = 0.5[ F(t) + F(t + 2L/c)] + 0.5Z[v(t) v(t+2L/c)]

WhereZ = Mc/L is the pile impedance (for uniform piles equal to EA/c)L = Pile length below gaugesc = (E/rho)0.5 is the speed of the speed waveE = Elastic modulus of the pilerho = Pile mass densityA = Pile cross sectional areaM = Pile mass below gauges

The total resistance consists of a dynamic (Rd) and a static (Rs) component. Thus

Rs(t) = R (t) Rd(t)

The static resistance component is of course the desired pile bearing capacity. Thedynamic component may be computed from a soil damping factor, J and a pile velocity, vt(t) that is conveniently calculated for the pile toe. Using wave consideration, this approachleads immediately to the dynamic resistance

Rd(t) = J [ F(t) + Zv(t) R(t) ]

and finally to the static resistance by subtracting from the total soil resistance. Thissolution is simple enough to evaluate in real time i.e. between hammer blows, using thePDA.

9. Special Topics

9.1 Micropiles

Micropiles can be defined as bored piles with small diameter which derived their strengthcapacity from the structural steel core.

The conditions on which micropiles could be adopted are as follows

a) Micropiles are used as an alternative piling system to overcome boulders or to formshort piles in shallow granite outcrops which are too deep for footing.


60

b) They are also used as an alternative piling system to overcome site constraints e.g.low headroom, restricted access or piling close to existing structures.

c) Micropiles are used for carrying high compressive loads

d) Where minimal noise and ground vibration are critical considerations, micropilescan be an effective alternative

Allowable design load based on pile material

Qa = 0.25fcuAc + 0.35fyAst

Wherefy = Yield stress depending on grade of steelAst = Cross sectional area of reinforcementfcu = Design concrete strengthAc = Cross sectional area of pile

9.2 Timber piles

Timber piles are made of tree trunk with branches carefully trimmed off, usually treatedwith a preservative and driven with small end as a point. They are generally used asfoundation piles or in dolphins or fender systems to protect waterfront structures. Thetimbers Keruing and Kempas are recommended for use as foundation piles because of thefollowing properties :

a) High strength in compression parallel to grain

b) Ease of treatment good permeability enabling preservative to penetrate deeply intothe pile

Allowable design load based on pile material

Qa = Ap fa

WhereAp = Average pile cross sectional area at pile capfa = Allowable design stress value for the type of timber.


61

9.3 Bakau Piles

Baukau piles can only be used as friction piles. The shaft friction can be calculated usingthe empirical formula

W H N0.5F =100 A

WhereF = Average frictional value (t/m2)W = Weight of hammer (Tonnes)H = Drop height of hammer (m)N = Average number of blows per metreA = Cross sectional area of pile (m2)

10. Typical sizes

10.1 Bored Piles

The bored pile sizes adopted by LTA vary from 500mm diameter to 1500mm diameter.Details of the sizes and nominal working loads of the piles are given in the Tables below:

Diameter(mm)

Area ofPile (mm2)

Structural Working Load(kN)

Bar size(mm)

No ofBars

Area ofSteel (mm2)

Links

fcu = 35 MPa fcu = 40 MPa500 196350 1718 1964 20 7 2199 T 10 - 250600 282743 2474 2827 20 9 2827 T 10 - 250700 384845 3367 3848 25 8 3927 T 10 - 225800 502655 4398 5027 25 11 5400 T 10 - 200900 636173 5567 6362 25 13 6381 T 10 - 175

1000 785398 6872 7854 32 10 8042 T 13 - 2751100 950332 8315 9503 32 12 9651 T 13 - 2501200 1130973 9896 11310 32 14 11259 T 13 - 2251300 1327322 11614 13273 32 18 14476 T 13 - 2001400 1539380 13470 15394 32 20 16085 T 13 - 1751500 1767146 15463 17671 32 22 17693 T 13 - 175

Table 1 Typical sizes of circular piles


62

10.2 H-piles (Grade 43A, fy = 265 MPa)

Pile Type Sectionalarea (mm2)

Zx(cm3)

Zy(cm3)

Structural WorkingLoad (kN)

100x50x9.3 kg/m 1185 38 6 94100x100x17.2 kg/m 2190 77 27 174

125x60x13.2 kg/m 1684 56 10 134125x125x23.8 kg/m 3031 136 47 241

150x75x14.0 kg/m 1785 87 13 142150x100x21.1 kg/m 2684 138 30 213150x150x31.5 kg/m 4014 219 75 319

175x90x18.1 kg/m 2304 139 22 183175x175x40.2 kg/m 5121 330 112 407

200x100x18.2 kg/m 2318 160 23 184200x100x21.3 kg/m 2716 164 27 216200x150x30.5 kg/m 3901 277 68 310200x200x49.9 kg/m 6353 472 160 505200x200x56.2 kg/m 7153 498 167 569200x200x65.7 kg/m 8369 628 218 665

250x125x25.7 kg/m 3268 285 41 260250x125x29.6 kg/m 3766 324 47 299250x175x44.1 kg/m 5624 502 113 447250x250x64.4 kg/m 8206 720 233 652250x250x66.5 kg/m 8470 801 269 673250x250x72.4 kg/m 9218 857 292 733250x250x82.2 kg/m 10470 919 304 832

300x150x32.0 kg/m 4080 424 59 324300x150x36.7 kg/m 4638 481 68 369300x200x56.8 kg/m 7238 771 160 575300x200x65.0 kg/m 8238 890 189 655300x300x84.5 kg/m 10700 1150 365 851300x300x87.0 kg/m 11080 1270 417 881300x300x94.0 kg/m 11980 1360 450 952

300x300x106.0 kg/m 13480 1440 466 1072

Table 2 Typical sizes of H- piles


63

10.3 Precast Piles (fcu = 50 N/mm2)

Pile Size (mm) 155 180 205 230 255 280 305 330 355 380 405Allowable Pile Capacity

(Tonnes)28.67 39.92 50.97 67.41 79.64 102.05 114.68 138.76 159.68 184.00 139.81

Main Reinforcement (mm)12m - - - - - 4T16 4T20 4T20 4T20 4T22 4T2210m - - - 4T16 4T16 4T16 4T20 4T20 4T20 4T22 4T228m - - 4T13 4T16 4T16 4T16 4T20 4T20 4T20 4T22 4T226m 4T110 4T10 4T13 4T13 4T16 4T16 4T20 4T20 4T20 4T22 4T224m 4T110 4T10 4T13 4T13 4T16 4T16 4T20 4T20 4T20 4T22 4T22

Cover to main reinforcement(mm)

30 30 30 30 40 40 40 40 40 40 40

Percentage of reinforcement(%)

1.40 1.00 1.33 1.00 1.29 1.00 1.40 1.15 1.00 1.05 1.00

Distance for end links (mm) 450 531 600 690 750 849 900 990 1062 1140 1170Links Spacing (mm) 7R6 9R6 10R6 12R6 16R6 19R6 21R6 26R6 29R6 33R6 36R6Joint Plate Size (mm) 150 172 198 225 250 278 298 325 350 378 400

Joint Plate Thickness (mm) 6 6 6 6 8 8 8 8 9 9 9Collar Band Size (mm) 40 40 40 50 50 50 50 50 75 75 75

Collar Band Thickness (mm) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5Anchor Bar Size, 32f (mm) 4T10 4T10 4T13 4T13 4T16 4T16 4T20 4T20 4T20 4T22 4T22Anchor Bar Length (mm) 320 320 416 416 512 512 640 640 640 704 704

Table 3 Typical sizes of RC piles

10.4 Timber Piles (Compression parallel to grain, fa = 10.69MPa)

Size Area of pile(mm2)

Structural workingload (kN)

100 x 100 10000 80125 x 125 15625 140150 x 150 22500 200175 x 175 30625 270

Table 4 Typical sizes of timber piles

10.5 Micropiles (fy = 460 MPa, fcu = 30 MPa)

Pile diameter(mm)

Area ofpile (mm2)

Structuralworking load (kN)

Reinforcement Area of steel(mm2)

150 17671 430 3 T 28 1847175 24053 569 3 T 32 2413200 31416 843 3 T 40 3770225 39761 1107 4 T 40 5027250 49087 1177 4 T 40 5027300 70686 1339 4 T 40 5027

Table 5 Typical sizes of micropiles


64

11. Worked Examples

Example 1

Calculation of Negative Skin Friction

NEGATIVE SKIN FRICTION

NSF = bPoAs b = EMPRIRICAL FACTORPo= EFFECTIVE VERTICAL STRESS

As= PILE SHAFT AREA

SOIL b CLAY 0.20 - 0.25SILT 0.25 - 0.35

SAND 0.35 - 0.5

BOREHOLE UTR 001

DESCRIPTION THICKNESS EFFECTIVE EMPIRICAL FROM TO UNIT TOP BOTTOM MEAN FACTOR STRESS

h WEIGHTb 125x125x23.8 300x300x94

(m) (m) (m) (kN/m3) (kN/m2) (kN/m2) (kN/m2) (kN/m2) 500 1200

0 1 Top of pilecap 1 19 0 19 9.5 0 0 0 01 1.8 Pile COL :Fill 0.8 19 19 34.2 26.6 0 0 0 0

1.8 6 Fill 4.2 19 34.2 114 74.1 0.35 25.9 13.0 31.16 8 Sandy Clay 2 18 114 150 132 0.25 33 16.5 39.6

29 71

perimeter (mm)

DEPTHBy H-Pile Surface perimeter

Pile Type

NEGATIVE SKIN FRICTION (kN)EFFECTIVE STRESS


65

Example 2

Calculation of structural and geotechnical capacity of H-pile

1. PILE DATAPILE SECTION kg/m H 300 x 300 x 94Steel Grade 43A

characteristic strength of reinforcement, fy kN/mm2 265

Properties : sectional area, Ag mm2 10770flange width, B mm 302section depth, D mm 294flange thickness, T mm 12web thickness, t mm 12

2. PILE STRUCTURAL CAPACITYPile Working load :

Compression Load, N = 800 kNTension Load, T = 410 kN

Pile Structural Capacity, Qs = 0.3*fy Ag kN= 856 kN =>OK

3. PILE GEOTECHNICAL CAPACITY

Ultimate Frictional Resistance, Qs = 2* N* As kNUltimate Capacity of Base, Qb = 200* N* Ab kNUlitmate bearing Capacity, Qult = Qs + Qb kN

Case (i) Pile Working Load, Pw = (Qs + Qb) / 2.5Case (ii) Pile Working Load, Pw = Qs/1.5 Qb=0Case (iii) Pile Working Load, Pw = Qs/3.5 Qb=0

BOREHOLE REFERENCE UTR 001GROUND LEVEL OF SOIL INVESTIGATION (m) 114.4

Sectional area , Ab = mm2

Perimeter , P = mm

H-PILE DESIGN

887881192

B

D

Assume mode of failure

Depth Effective SPT qs qb Qs SQs QbFrom To Depth N

(m) (kN/m2) (kN/m2) (kN) (kN) (kN)

1 G.L. to C.O.L 0 0.5 0.5 0 0 02 FILL 0.5 6 5.5 5 66 65.563 Sandy CLAY 6 8 2 4 19 84.634 Sandy SILT 8 16 8 11 210 294.425 Very Stiff Sandy SILT 16 24 8 26 496 790.306 Hard Sandy SILT 24 30 6 49 701 1491.19 870.127 0 1491.19 0.008 0 1491.19 0.009 0 1491.19 0.00

10 0 1491.19 0.00

CASE (i) Pw = 944.526 kN =>PILE OKCASE (ii) Pw = 994.128 kN =>PILE OKCASE (ii) Pw = 426.055 kN =>PILE OK

USE H PILE SECTION: H 300 x 300 x 94kg/m

Description of Soil layers


66

Example 3

Calculation of structural and geotechnical capacity of timber or wood pile

1. PILE DATAPILE SECTION kg/m Tanalised 175 x 175 Kempas

Compression parallel to grain N/mm2 10.69

Properties : sectional area, Ag mm2 30625flange width, B mm 175section depth, D mm 175

2. PILE STRUCTURAL CAPACITYPile Working load :

Compression Load, N = 135 kNTension Load, T = 0 kN

Pile Structural Capacity, Qs = fy Ag kN= 327 kN =>OK

3. PILE GEOTECHNICAL CAPACITY

Ultimate Frictional Resistance, Qs = 2* N* As kNUltimate Capacity of Base, Qb = 200* N* Ab kNUlitmate bearing Capacity, Qult = Qs + Qb kN

Case (i) Pile Working Load, Pw = (Qs + Qb) / 2.5Case (ii) Pile Working Load, Pw = Qs/1.5 Qb=0Case (iii) Pile Working Load, Pw = Qs/3.5 Qb=0

BOREHOLE REFERENCE BH 116GROUND LEVEL OF SOIL INVESTIGATION (m) 103.5WORKING PLATEFORM LEVEL (m) 103.5CUT-OFF-LEVEL (m) 102.5

Sectional area , Ab = mm2

Perimeter , P = mm

Depth Effective SPT qs qb Qs SQs QbFrom To Depth N

(m) (kN/m2) (kN/m2) (kN) (kN) (kN)

1 G.L. to C.O.L 0 1.0 1 0 0 02 Peaty CLAY 1 9 8 0 0 0.003 Silty CLAY 9 12 3 16 67 67.204 Sandy CLAY 12 15 3 11 46 113.405 Sandy Silt 15 17 2 60 168 281.406 GRANITE 0 0 0 100 0 281.40 612.50

WOOD PILE DESIGN

Description of Soil layers

30625700

B

D

Assume mode of failure

7 0 0 281.40 0.008 0 0 281.40 0.009 0 0 281.40 0.00

10 0 0 281.40 0.00

CASE (i) Pw = 357.56 kN =>PILE OKCASE (ii) Pw = 187.6 kN =>PILE OKCASE (ii) Pw = 80.4 kN =>PILE OK

USE : Tanalised 175 x 175 kg/m


67

Example 4

Design of a pile cap for 6-pile group

Project Title :Revision :Designed By : Date :Checked By : Date :Approved By : Date :

TO SS CP 65: PART 1 : 1999THIS VERSION DATED 27TH JUNE 2001

(A) PILE DETAILS

Pile Type H-pile = kg/mPile Width B = mmPile depth D = mm

Pile x-area Ast = mm2py = N/mm2

pile capacity P = kN P =0.3 py AstPile Compression Load Pw = kN

Pile tension Load PT = kNpile spacing // to l Sl = mm < 3Bpile spacing // to b Sb = mm > 3D

Pile embedment e= mmPileedge to pilecap overhang, Ov = mm

(B) COLUMN DETAILS

Column Load F = kNColumn Length C1 = mmColumn Width C2 = mm

(C) PILECAP DETAILS

Pilecap Length l = mmPilecap Width b= mmPilecap Depth h = mm

Concrete Cover c = mmEffective depth d = mm

d = h - c -e -fbar/2

(D) MATERIAL STRENGTH

Concrete characteristic strength fcu= N/mmMain Bar characteristic strength fy= N/mm

(E) PUNCHING SHEAR AROUND COLUMN PERIMETER

Column perimeter Pr = mm Pr = 2*(C1 + C2)Shear force V = kN V = 1.5 * FShear Stress v = N/mm v = V / (d * Pr)

O.K. Max concrete Stress vmax = N/mm vc =min (5, 0.8 *sqrt(fcu))

(F) SHEAR ALONG CRITICAL SECTION OF 0.2D INSIDE FACE OF PILE

// to pilecap length avl = mm av =Sl-B/2-C2/2// to pilecap width avb = mm av =Sb/2-D/2-C1/2

100As/bd= 100As/bd = 1.59400/d = 400/d = 0.63

Concrete stress vc = N/mm Table 3.9Enhance. factor f = f = 2d/avb

O.K. Enhance stress allow vc = N/mm allow vc = min (5, 2vc* f)Shear Stress v = N/mm v = (1.5 *3Pw) / (b *h)

300*300*84.5302294

10770265856800360900

6-pile group

1400

103

900900

2400200080050

150100

3230

637.5

40460

360048452.115.00

1.59

299

10.864.263.682.25

DESIGN OF PILECAP FOR 6-PILE GROUP

D

B

Length,l

Width,b

C1

C2sb

sl sl


68

(G) MAIN REINFORCEMENT PARALLEL TO LENDTH OF PILECAP

BOTTOM STEELM= kNm M = 1.5*3 Pw *(Sl/2)/1000

M/(fcu*bd)= < 0.157 CL3.4.4.4z= d z = 0.95 d but not greater than 0.95d

As required (min)= mm Table 3.27 As(min)= 0.13% bhAs required= mm As required = M/ (0.87fy z)

Provide : First Row 14 T 25Seceond Row 0 T 25

Third Row 0 T 25

O.K. As provided = mm 100As/bd = 0.54Spacing = mm Spacing = (b - 2c -fbar)/(n-1)

O.K. Clear spacing = mm Table 3.30 Max Spacing = 155 mm

TOP STEELM= kNm M = 1.5*3 PT*(Sl/2)/1000

M/(fcu*bd)= < 0.157 CL3.4.4.4z= d z = 0.98 d but not greater than 0.95d

As required (min)= mm Table 3.27 As(min)= 0.13% bhAs required= mm As required = M/ (0.87fy z)


Third Row 0 T 20

O.K. As provided = mm 100As/bd = 0.34Spacing = mm Spacing = (b - 2c -fbar)/(n-1)


(H) MAIN REINFORCEMENT PARALLEL TO WIDTH OF PILECAP

BOTTOM STEELM= kNm M = 1.5*2 Pw *(Sb)/1000

M/(fcu*ld)= < 0.157 CL3.4.4.4z= d z = 0.9 d but not greater than 0.95d

As required (min)= mm Table 3.27 As(min)= 0.13% lhAs required= mm As required = M/ (0.87fy z)


Third Row 0 T 32

O.K. As provided = mm 100As/bd = 1.05Spacing = mm Spacing = (l - 2c -fbar)/(n-1)


729

0.90

16085

83

140115

16200.0498

6684

6872

2080

2080

4398

3360

249614633

0.95

0.95

120

0.0224

140

115

0.0861

3008


69

(H) MAIN REINFORCEMENT PARALLEL TO WIDTH OF PILECAP

TOP STEELM= kN

Design Working Manual Part1

Documents

design manual

design of foundations

conceptual design

design process

civil design handbookdesign

easy referenceduring

index parameters

cohesionless soil