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National University of Singapore

Department of Civil and Environmental Engineering

CE3116 Foundation Engineering

Semester 2 20141!

Design of Deep Foundation "art 2

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#ntrodu$tion

A test pile has been installed into the ground location of the borehole

provided in Part 1. Shaft resistance data are obtained from the test pile under

dierent working loads. With the additional information, K SP values are

compared with the assumed values in Part 1. !urthermore, the settlement ofpile is e"amined. #astl$, with another set of pile resistance data obtained

from load tests, a new design is obtained.

1% Comparison of t&e ' S"( )alues

 he K SP values calculated from the additional information are tabulated as

follow.

%epth &.'&('.)1m

*W# +

SP(

value

Kspt-+

SP(value

Kspt-+

SP(value

Kspt-+

SP(value

Kspt-+

/ /.//)0./

/ /.// &1.// /.// &.// /.// 23.// /.//)&/

/4.)

/)0./

/ 1.2 &1.// 1.4' &.// 1.)0 23.// /.014/

/'.&

/)0./

/ ).40 &1.// ).)4 &.// 1.'& 23.// 1.))'/

/03.)

/)0./

/ &.11 &1.// ).01 &.// ).4) 23.// 1.2&')/

/1/).

4/)0./

/ &. &1.// &.&/ &.// ).04 23.// 1.0/'/

/13.3

/)0./

/ /.& &1.// /.23 &.// /.4' 23.// /.&14/

/1).3

/)0./

/ /.42 &1.// /.41 &.// /.&2 23.// /.)))&/

/ ).2/)0./

/ /./' &1.// /./0 &.// /./3 23.// /./4

/

('.4

/)0./

/ ().40 &1.// ().)4 &.// (1.'& 23.// (1.))

Average 1.3) 1.2 1.&4 /.02

%epth '.)1(1).1'm*W# +

SP(value

Kspt-+

SP(value

Kspt-+

SP(value

Kspt-+

/ /.// 23.// /.// ).// /.// 44.// /.//)&/

/ 2).4/ 23.// /.') ).// )./) 44.// 1.1'

)

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4//

11&.'/ 23.// ).// ).// 4.&0 44.// ).2'

'//

12'./ 23.// ).0/ ).// .14 44.// &.&

')//

)14.4/ 23.// &.3 ).// 0.)2 44.// 4.03

'// '1.// 23.// 1./ ).// &.2/ 44.// )./3

4// )/.1/ 23.// /.&2 ).// /.33 44.// /.4

)&// 1).0/ 23.// /.)) ).// /.4' 44.// /.)'

/ 0.2/ 23.// 1.2) ).// &.&& 44.// 1.'3

Average 1. &.2 ).1

%epth 1).1'(12.1'm

*W# +

SP(value

Kspt-+

SP(value

Kspt-+

/ /.// 44.// /.// )3.// /.//)&/

/ &2.&/ 44.// /.0/ )3.// 1.&14/

/ 3.1/ 44.// 1.3& )3.// ).0)'/

/1/3./

/ 44.// ).4& )3.// &.'')/

/10.1

/ 44.// &.0) )3.// .)&'/

/1'3.1

/ 44.// 4.40 )3.// 3.&/4/

/1/&.2

/ 44.// ).&2 )3.// &.0&)&/

/ )'.3/ 44.// /.0 )3.// 1.1/

/ 0.4/ 44.// /.1' )3.// /.&1

Average ).&& &.3'

%epth 12.1'( 10.))m*W

# +

SP(

value

Kspt-+

SP(

value

Kspt-+

SP(

value

Kspt-+

/ /.// )3.// /.// )0.// /.// )0.// /.//

)&// 0.1/ )3.// /.&/ )0.// /.)' )0.// /.)'

4//

)4./ )3.// /.'1 )0.// /.00 )0.// /.00

'//

4'.)/ )3.// 1.0) )0.// 1.3 )0.// 1.3

&

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')//

4&.0/ )3.// 1.) )0.// 1.2 )0.// 1.2

'//

/./ )3.// ).)4 )0.// ).1 )0.// ).1

4//

1.1/ )3.// ).) )0.// ).10 )0.// ).10

)&// 1./ )3.// /./ )0.// /./ )0.// /./

/

(1)./

/ )3.// (/.44 )0.// (/.4& )0.// (/.4&

Average 1.&) 1.)3 1.)3

%epth 10.))()1.))m*W# + SP( value

Kspt-+ SP( value

Kspt-+

/ /.// )0.// /.// &.// /.//)&/

/)).1

/ )0.// /.3' &.// /.14/

/&/.2

/ )0.// 1./' &.// /.02'/

/43.3

/ )0.// 1.3/ &.// 1.&&')/

/22.'

/ )0.// ).// &.// 1.22'/

/4.

/ )0.// ).&1 &.// 1.3'4/

/

2./

/ )0.// ).// &.// 1.2)&/

/43.2

/ )0.// 1.3/ &.// 1.&)

/).4

/ )0.// /.'4 &.// /.3&

Average 1.2 1.)'

%epth )1.))()4.)/m*W# + SP( value

Kspt-+ SP( value

Kspt-+ SP( value

Kspt-+

/ /.// &.// /.// 23.// /.// 1//.// /.//)&/

/ /.// &.// /.// 23.// /.// 1//.// /.//4/

/10.)

/ &.// /.21 23.// /.&) 1//.// /.10'/

/4'.0

/ &.// 1.&0 23.// /.03 1//.// /.2/')/ 0./ &.// ).&' 23.// 1.21 1//.// /.0

4

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/ /'/

/''.

/ &.// ).33 23.// 1.32 1//.// 1.//4/

/33./

/ &.// ).14 23.// 1.&2 1//.// /.33)&/

/24.4

/ &.// 1.21 23.// /.'2 1//.// /.24

/ /.1/ &.// /.// 23.// /.// 1//.// /.//

Average 1.2& /.' /.22Tables 1-6 K SPT  values in each layer of soil.

Workings

 he K SP values are calculated using the following formula5

 K SPT =qs

 N 

Where +s is the shaft resistance obtained from pile load test and is the (

value obtained from borehole data.

Several representative (values in each la$er of soil are taken in order to

provide a more e"tensive comparison.

 he calculated K SP values are compared against the assumed K SP value inthe part 1 design, which is ).2.

Discussions

 he comparisons show that the K SP values calculated from the pile load tests

are generall$ lower than the assumed values in the previous design.

 herefore, the e"act stiness of the soil is lower than what was e"pected in

the previous design.

6esides, it can be observed from the pile load tests that the shaft resistance

of the pile is dependent on the working load. 7owever, the shaft resistance as

calculated from the formula +s-K SP is an ultimate resistance without the

consideration of the e"act behavior of the soil. herefore, the results obtained

from the previous design ma$ be less conservative and partial factors are

thus important to factor down the resistance.

Also, it can be observed that there is no general trend or correlation between

the magnitude of the working load and the shaft resistance.

2% "ile *ase resistan$e

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6ase resistance of bored pile under drained condition at dierent level of soil

can be correlated to the SP( value at the corresponding level of soil b$ the

following e+uation.

6ase resistance per unit area,qb= K b N    89hang : 6roms, 1''1;

where

30 45b

 K    = −

b-1./

%esign base resistance 86;,  RbB=( π 4 )×D2×30×N 

1.0=3393kN 

Case C

=93 partial factors5 >b-1.

%esign base resistance 89;,  RbC =(π 

4)×D2×30×N 

1.6=1060kN 

 he e"pected base resistance will be the same as the previous design as the

K b has not been changed.

3% Settlements

At /.2 W#-4//k, settlement of pile- 4.'0mm

At 1./ W#-')//k, settlement of pile- 1/.4mm

?a"imum allowable settlement of pile- /.1 @ ominal diameter of pile

  -/.1 @ 1)// - 1)/mm

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at ultimate compressive resistance as stated in =urocode 3.

ltimate compressive resistance - 11))4 k 8from Part 1;B')//k.

6$ e"trapolating, settlement at ultimate compressive resistance- 1).3 mm

C 1)/mm

7ence, the settlement is acceptable.

Discussions

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/ /4//./

/(

)&.&/ )0.// (/.0& &1.// (/.32 &.// (/.2 23.// (/.41

/.// 1/.&/ )0.// /.&3 &1.// /.&& &.// /.)' 23.// /.10

%epth '.)1(1).1'm

 SP(value

Kspt-+

SP(value

Kspt-+

SP(value

Kspt-+

/.// 2.'/ 23.// /.1/ ).// /.)& 44.// /.1&4//./

/ 01.4/ 23.// 1.4& ).// &.1& 44.// 1.02')//./

/))1.&

/ 23.// &.00 ).// 0.21 44.// 2./&1&0//.

//104.3

/ 23.// &.)4 ).// 3.1/ 44.// 4.)/11//.

//

)44./

/ 23.// 4.)0 ).// '.&0 44.// 2.22104//.

//))&.2

/ 23.// &.') ).// 0./ 44.// 2./0)/3//.

//)&3./

/ 23.// 4.1 ).// '.1) 44.// 2.&'104//.

//&4./

/ 23.// .&' ).// 14.// 44.// 0.)31&0//.

//&2/.3

/ 23.// .12 ).// 1&.4' 44.// 3.'3')//./

/)'1.2

/ 23.// 2.11 ).// 11.)1 44.// .&4//./

/

&21./

/ 23.// .1 ).// 1&.2/ 44.// 3.'0

/.//

(1/).&

/ 23.// (1.3' ).// (&.'& 44.// ().&&

%epth 1).1'(12.1'm

 SP(value

Kspt-+

SP(value

Kspt-+

/.// .&/ 44.// /.14 )3.// /.)&4//./

/ 22.1/ 44.// 1.)2 )3.// )./4')//./

/104.)

/ 44.// 4.1' )3.// .0)1&0//.

//10).2

/ 44.// 4.12 )3.// .311//.

//&)/./

/ 44.// 3.)3 )3.// 11.02104//.

//&41.

/ 44.// 3.3 )3.// 1).2

0

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)/3//.//

413.4/ 44.// '.4' )3.// 12.4

104//.//

1''.0/ 44.// 4.24 )3.// 3.4/

1&0//.// 4/.0/ 44.// /.'& )3.// 1.21

')//.// (03.4/ 44.// (1.'' )3.// (&.)4

4//.//

(123./

/ 44.// (&.23 )3.// (2.01

/.//

(14.)

/ 44.// (&.3& )3.// (./0

%epth 12.1'(10.))m

  SP(value Kspt-+ SP(value Kspt-+

/.// 3.4/ )3.// /.)3 )0.// /.)4//./

/ &'.3/ )3.// 1.43 )0.// 1.4)')//./

/ 4.1/ )3.// 1.31 )0.// 1.21&0//.

// 3&./ )3.// ).3& )0.// ).&11//.

// 4.)/ )3.// ).&0 )0.// ).)'

104//.

// 2'.)/ )3.// ).1' )0.// ).11)/3//.

// 23.'/ )3.// ).14 )0.// )./3104//.

// (&.&/ )3.// (1.&4 )0.// (1.&/1&0//.

// (&4.)/ )3.// (1.)3 )0.// (1.))')//./

/ (44.0/ )3.// (1. )0.// (1./4//./

/ (&'.3/ )3.// (1.43 )0.// (1.4)

/.// (4&.'/ )3.// (1.& )0.// (1.23

%epth 10.))()1.))m

 SP(value

Kspt-+

SP(value

Kspt-+

/.// &.3/ )0.// /.1& &.// /.1/4//./

/ &&.3/ )0.// 1.)/ &.// /.'4

'

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')//.// 1'.// )0.// /.0 &.// /.2&

1&0//.// ).&/ )0.// /.'4 &.// /.3&

11//.// )2.0/ )0.// /.') &.// /.3)

104//.// 1.2/ )0.// /.2' &.// /.4

)/3//.// 43.// )0.// 1.0 &.// 1.&1

104//.// (0).&/ )0.// ().'4 &.// ().)'

1&0//.//

(1/&.4

/ )0.// (&.' &.// ().03

')//./

/

(112.4

/ )0.// (4.1) &.// (&.)1

4//.//

(11'.4

/ )0.// (4.) &.// (&.&)

/.//

(11'.&

/ )0.// (4.) &.// (&.&1

%epth )1.))()4.)/m

 SP(value

Kspt-+

SP(value

Kspt-+

SP(value

Kspt-+

/.//42.)

/ &.// 1.) 23.// /.3'1//./

/ /.424//.

//0.)

/ &.// 1.0' 23.// 1.)/1//./

/ /.0')//.

//13.

/ &.// 4.'1 23.// &.1/1//./

/ 1.331&0//

.//)).

3/ &.// 3.&/ 23.// 4.11//./

/ ).&11//

.//)02.

4/ &.// 3.'& 23.// 2./11//./

/ ).02104//

.//&/0.

1/ &.// 0.2 23.// 2.411//./

/ &./0

)/3//.//

&/0.4/ &.// 0.23 23.// 2.41

1//.// &./0

104//.//

&'0.3/ &.// 11./0 23.// .''

1//.// &.''

1&0//.//

&&'.3/ &.// '.44 23.// 2.'

1//.// &.4/

')//.//

)).3/ &.// 3.&/ 23.// 4.1

1//.// ).&

1/

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4//.//

)44.2/ &.// .3' 23.// 4.)'

1//.// ).42

/.//)4/.

// &.// .3 23.// 4.)11//./

/ ).4/

Kb of Pile 6ase Fesistance

#oad +6 SP( Kb

/ ))).1 23&.0'4'

1

4// '&& 231.&04

)

')//1)00.

4 23))./&2

11&0/

/10.

1 23 &).3&011/

/

)1&).

0 23

&3.4132

4104/

/ )&22 2341.&123

')/3/

/)3''.

2 234'.114/

4104/

/)3''.

2 234'.114/

41&0/

/)44&.

' 234).0324

4

')// 1'22 23&4.)'0)

2

4//

13&).

0 23 &/.4

/13&).

0 23 &/.4

 average

&).24210

Tables 7-13. New K SPT  and K b values.

Workings

With the additional information, new K SP values can be calculated using the

same method as section 1.

With the new K SP values, new values of shaft resistance can be calculatedusing the formula +s-K SP.

 he new average K SP values that are used for the new design are tabulated

as follow.

e!"h #$% K  SPT #avera&e%&.'& ( '.)1 &.1/

11

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'.)1 ( 1).1' 4.431).1' ( 12.1' 4.'212.1' ( 10.)) ).1/10.)) ( )1.)) /.30)1.)) ( )4.)/ ).2Table 1'. esi&n values of K SPT  in each layer of soil.

As larger K SP values are generall$ obtained from the additional information,

larger values of shaft resistance are obtained, thus the design length of the

pile can be reduced. his indicates that the soil behaviour is stier than what

was previousl$ assumed, thus providing more shaft resistance.

Also, using the additional information obtained from pile load tests, new

values of K b can be obtained using the formula K b-+b 8as shown in able

1&;, where is 23 at )4.)/m according to borehole data.

 he obtained result of K b, &).242, is larger than the assumed value of &/. he

larger value of K b results in a larger base resistance. herefore, the pile lengthcan be further reduced as more load can be taken b$ pile base resistance.

 he shaft resistance and base resistance can be determined using the

following formulas.

 Rs¿∑(πD Li)× K SPT −i × N i

γ 

Where #i, K SP(i, i are the length of pile, K SP, values respectivel$ in i(th soil

la$erG %-1)///mmG >6-1./G >9-1.&.

 Rb=( π 4 )×D2×K b×N 

γ 

Where K b-&&.242G is the value at the pile baseG %-1)//mmG >6-1./G

>9-1..

Case B

# -) m

Fs6 -1/ ''4 k

Fb6 -& 01 k

F6 -14 32 k

 he pile length has been reduced to )m from )0m in the previous design.

1)

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Case C

# -)0 m

Fs9 -' &)3 k

Fb6 -) &// k

F9 -11 )3 k

 he pile length has been reduced to )0m from &4m in the previous design.

Furt&er Dis$ussions and Con$lusion

After some anal$ses of the data from Anne" 9, we found that there is load

redistribution when the load increases. !or e"ample, at the depth of &.'& H

'.)1m, the K SP value increases from substantiall$ from /.01 to .)) when the

load increases from ')//k to 1&0//k while at the depth of '.)1 H 1).1'm,

the K SP  value decreases from &.00 to &.)4 when the load increases from

')//k to 1&0//k. his shows that when the load increases, the load is

redistributed to other level of soil when certain level of soil has reached its

ma"imum resistance.

 he dierent K SP values when the same load is applied before and after the

ma"imum load implies that the stress strain curve of soil is non(linear and the

soil e"hibits plastic behavior. !rom the data, we can observe that the soil is

generall$ stier after the ma"imum load is applied. his is coincident with the

theor$ we have learnt in the class that the soil follows a dierent stress strain

curve when it loads be$ond its $ield stress.

6esides, from the new data, we can observe that K SP value can be up to

12.4 while in the previous design, we had chosen the K SP value to be ).2.

 he chosen value of ).2 is +uite conservative as we can observe from the

data, nearl$ all la$ers of soil has a K SP value larger than ).2 e"cept the la$er

that is near to the base. Some of the reasons that the la$er near to the base

has a small K SP value are the e"istence of the base resistance at that la$er of 

soil and also there is some anchorage length re+uired for the pile to build up

resistance that is similar to the case of steel in the reinforced concrete.

7ence, the chosen value of ).2 is +uite safe for design of pile but for most of the cases, this will result in over(designing of the pile and introduce large

economic cost.

!urthermore, it can be observed from the data in Anne" 9 that the higher the

working load, the higher the mobilised base resistance. his is due to the fact

that when working load e"ceeds the shaft resistance, more of the load will be

taken b$ the base.

1&

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,eferen$es

=urocode 3 8)//';. (eo"echnical desi&n - Par" 1) (eneral rules.

!arrel : revor 81''';. (eo"echnical esi&n "o *urocode 7.

 an I.9. : 9how 9.?. 8)//&;. esi&n and cons"ruc"ion of bored !ilefounda"ion.

12