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Ministry of Construction, Engineering Services, Housing & Common Amenities
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GUIDELINES
FOR
INTERPRETATION OF SITE
INVESTIGATION DATA
FOR ESTIMATING THE
CARRYING CAPACITY OF
SINGLE PILES FOR
DESIGN OF BORED AND
CAST IN-SITU REINFORCED
CONCRETE PILES
•
..
Institute for Construction Training and Development
"Savsiripaya"
123, Wijerama Mawatha
Colombo 07.
Tel:2699801,2695965
•
Publication No. - ICTADI DEY115
First Edition - April, 1997
Second Edition (Revised) - January, 20 II
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MIN I TRY OF CONSTRUCTION, ENGINEERING SERVICES, HOUSING &
COMMON AMENITIES
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UIDELINES FOR INTERPRETATION OF SITE
I NVE TIGATION DATA FOR ESTIMATING THE CARRYING
APA ITY OF SINGLE PILES FOR DESIGN OF BORED AND
AST IN-SITU REINFORCED CONCRETE PILES
ICTAD PUBLICATION NO: ICTADIDEVI1S
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First Edition - April 1997
.
Second Edition (Revised) - January 2011
Published by
Institute for Construction Training and Development
'Savsiripaya'
123, Wijerama Mawatha
Colombo 07
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I
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Published by
Insti tute for Con tructi on Training and Development (ICTAD)
' Savsiripaya'
123, Wij erama Mawatha
Colombo 07.
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51
1 Edition
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April 1997
nd
-
January 2011
2 Edition (Revised)
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Copyright Reserved
No part of this publ ication shall be
reproduced or transmitted in any
form or by any means without
permission of the publ isher.
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1'1{It;Jt'
; I~ TO THE REVISED EDITION
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l'ltl. I I', t 'd ili n of this Guidelines for Interpretation of Site Investigation Data for
h' ( III HillS 1/1
arrying Capacity of Single Piles for Design of Bored and Cast in Situ
I I" lill' \ U nCl'ctc Piles has been in use since 1997. The need has arisen to revise this
10 ' 11111 n! t ) updatc it with the latest techno!ogy,.construction methods and technical data
\ " ,h II' m I' appropriate to present working environment.
,1111 'onlJ11i tt
,,
has been formed and the committee consisted of the following;
I
In
dition has then been reviewed at the ICTAD Steering Committee
I1lb r of following organizations.
I,
I
11I 11j t'
.
I r f. A S Kulathilaka
I I' f. H S Thi lakasiri
' ng. K L S Sahabandu
:ng. Parakrama Jayasinghe
2,
" II
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I.
2,
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,
7,
~,
~,
I o.
- University of Moratuwa
- University of Moratuwa
- Central Engineering Consultancy Bureau
- Consultant
Thc Institution of Engineers, Sri Lanka
,'ri Lanka Institute of Architects
'oci ty of Structural Engineers - Sri Lanka
The Association of Consulting Engineers, Sri Lanka
II~ titute of Quantity Surveyors Sri Lanka
In stitute of Project Managers Sri Lanka
National Construction Association of Sri Lanka
epartment of Buildings
National Water Supply and Drainage Board
Institute for Construction Training and Development (ICTAD)
I hop • til It thi ' publication will be a useful guide for Engineers, Designers, Consultants,
1'1 1)1 I'L D v lopers and others.
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" n u l. UANOARA AWSADAHAMY
( '1I /\lnMAN
\(" I'A n
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III
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II( U{IllW RD TO THE FIRST EDITION
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I llId I i " 'ol'pol'ate activity of developing Construction Industry related documents,
11' 1 I Jill id ntified the need for having Guidelines for Interpretation of site
III \ i ~ III Hl da ta fo r estimating the Carrying Capacity of Single Piles for Design of
III I I IIld '/I,t 111 - itu Reinforced Concrete Piles and hence initiated the formulation of
111 1 I'll 1,11 11 ",
I 1\111' til Ii U i,' d ument will help the practicing Geotechnical Engineers in designing
I II inli ll 111\ )11 "
.
lit
(I I II
1111 ' w , ' publi shed by the Technology Development Division of ICTAD and
' v' I til Ii 1 I I
a panel of experts from Sri Lanka Geotechnical Society, under the
\I 1111 ' \ or"" 'II 'xpcri enced group of professionals in the Construction Industry,
I I I IHI\ I ·tt . with grateful thanks the service of the members of the drafting panel of
" I I 1111 I I t}l' 'hn ica l Society and the Review Committee Members in the publication
1111
I\ ll ill.
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I • .,1, J) Int hll
1111111111
III (1111
J
Wijeyesekera
nI' 'onstruction Training and Development
IIIII
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,
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IV
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)'he members involved in drafting the first edition in •1997
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I' 'nnakoon
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University of Moratuwa
I ), , . ,' I ul thilaka
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University of Moratuwa
l it . l'IIlIill . iva
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Soil Engineering and Deepwells Ltd.
tv! ' . I., I.
th abandu
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Central Engineering Consultancy Bureau
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National Building Research Organisation
Member-SLGS
1111 I: II I
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"
V Il'IIj
f\ 11
las
f\h .
W Wlj undara
f\ I, I
I I, I'f)undo
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I, l\ nll' luu l unas inghe
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Member-SLGS
f\ ll , ,' I' I{u na inghe
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Member-SLGS
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Member-SLGS
Reviewing Panel for the first edition in 1997
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Institution of Engineers Sri Lanka
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University of Peradeniya
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University of Moratuwa
I n nlharan
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Central Engineering Consultancy Bureau
f\ I,. 1\ I) Wi I ramasinghe
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Dept. of Buildings
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Association of Consulting Engineers
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Institute for Construction Training and Development
I ' l l I. N
I Alwis
111111. II N .' !1 wil'atne
I 1\ I. l'tI 1111
f\ II
f\I, .
,a
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M S 'Piyadasa
f\ II ,, ', '" ,'um anasekara
...
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Caution: This document should only be used by persons trained in Geotechnical
Engineering
as at several places engineering judgment is both crucial and critical.
,
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DISCLAIMER
Whilst all care and efforts have been made to ensure the proper production of this
document and its contents, neither does the Drafting Committee nor the rCTAO accept
any liability for problems, Difficulties or Controversy arising from the usage of these
guidelines.
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VI
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CONTENTS
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Page
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II II II If
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11 \
nl
1
I
II ~'
til"
• I P I( IL ~
r Pi I s
1
2
I I II! I' III! 111'1 )1I !l 1:lId ea ring Capacity
r l Itim t Unit Skin Frictionai Resistance (fu)
I I nlll IH
r II
II
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3
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6
1' 1 Ii I,' "i ti on
I I III I' tIn,
r. 'lll 'l11 nt
7
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7
I I I II
8-10
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Vll
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C ll lJ)I I. L I NIT~ S II O R INTERPREATION OF SITE INVESTIGATION DATA FOR
I. II 1A' I IN(i T IE ARRYING CAPACITY OF SINGLE PILES FOR DESIGN OF
110 1U ;O AND CAST IN-SITU REINFORCED CONCRETE PILES
I II
I 1'1« )I)tJ 'TiON
,
110 I' I Ill ' 'ul1 siderable progress that has been made during the last few decades in
I IId\ I 1111 1111 1' Ihe interaction of a pile and the surrounding soil, the design of pile
hilI! d II IHI uti li very much an art with empirical methods based on experience still being
\I \ I II I llmllling the carrying capacity of piles. Therefore the methods given in this
11 11\ 111 11 III 1', I' th ' in terpretation of site investigation data should merely serve as guidelines.
1\
\I I ' II I that the actual capacity of piles is confirmed by load tests on preliminary test
III I II III HI
other fo undation, pile foundations should be such that,
(,
ill
h IV 'uflic ient safety against bearing capacity failure;
I
til '
10 Il Ot un dergo excessive settlements.
1111 111 111
' hill n load of a structure is carried by a group of piles.
" 1\ II I II pi I
. are founded on sands or clays, the behavior of the group is different from
lit II I I' I ill I pile. Therefore, in such cases, group effect should be considered.
c' \ I ~ H IN G AP ACITY OF PILES
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. Ultima te Carrying Capacity of Piles
In pile fo undations the pile load is carried both by skin friction and end bearing.
'I'll refore, from a static analysis, the ultimate carrying capacity of a single pile can
b obtained as;
Where
-
Ultimate carrying capacity of pi Ie;
-
Ultimate end resistance per un it area of base;
-
Ultimate skin friction resistance per unit surface area
of the shaft;
= Area of the base, and surface area of• the shaft
respectively .
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1
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2.2
Allowable Carrying Capacity of Piles
Allowable Carrying Capacity ofa single pile can be obtained by;
Where Pallowable = allowable' carrying capacity cif a single pile; and
F b, is the safety factor with respect to end bearing & Fs is the safety factor with
respect to skin friction .
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Pallowable should be taken as the lesser of the values computed using,
(i)
Fb = Fs = 2.5; and,
(ii)
Fb = 3 ,
Fs = 2,
However above values of factor of safety may be changed based on experience of
the designers, extent of the site investigation process & construction quality
control procedures adopted during Construction.
3.0
ESTIMATION OF UNIT END BEARING CAPACITY
3.1
Estimation of Ultimate End Bearing Capacity qu in Sands
3.1.1
Estimation of qu Based on Angle of Internal Friction 0
Where
Po
= Effective vertical stress at base level of foundation
determined as described in (i) and (ii) below;
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Nq
= Bearing capacity factor.
Some typical values of Nq are given in Figure I.
In the case of Po the recommendation made in Ref (1) is that for
practical design purposes that
(i)
linearly with depth up to pile lengths o.f 10 to 15
diameters; and
Po increases
(ii) Below the pile lengths given above Po is constant with depth.
Alternatively the Ref. (2) may also be used to estimate Po .
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2
I
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• 1.2 Estimation of qu Based on SPT N Value
qnetult
where
nd
ith
Y
L
=
q u - YL
-
Average unit weight of soil along the pile shaft;
Embedded length of the pile.
=
•
It j , recommended to use
•
q net ult
= 40. N
kN/rn z
II' Bild N used in 3.1.1 respectively should be those obtained at the layer
II 1\ lit pll ' are founded and averaged with in the zone of influence, which is
III I III tI II ' R limes pile diameter above the pile base level and 3 times the pile
1 11111 I I h I w the pi Ie base level.
' 1111
I
)f
ty
i
I I 111111 1111 of Ultimate Unit End Bearing Capacity qu in Clays
q net ult. =9C u
WI1 /'
Cu
= Undrained shear strength of the clay
I III d 1111 {If Allowable Unit End Bearing Capacity qallow in Rocks
I h' I I III b ul'in ~ capacity of a pile resting on rock can be obtained from Figure 2
II II 1111 I ill l . mpressive strength of rock cores obtained at the founding level of a
" I Hili til I [of the rock below the founding level of the pile.
n
II. . II \ (1I1i11 1 d d that above mentioned rock parameters shall be determined to a
dl plh II I 1\ ,t throe (3) times a pile diameter below the founding level.
II I \ I If)N OF ULTIMATE UNIT SKIN FRICTIONAL RESISTANCE (fu)
II
I
r
.,'11 ' I I I
I II I 'II m that is developed on a pile shaft is influenced considerably by the
lit
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II fitll 1 111 I I 1\ 'tru ti n. Therefore the estimate given in this section should only
)
I I
I
,
!I I
I lin; fo r des ign.
j '" " 1111 111' 1" ill
"
II, I
..
a nds
III (,11 of fu Based on Angle ofInternal Friction 0
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3
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(i)
•
Ks -Pez)
-0
where
(a)
-
coefficient of lateral earth pressure on shaft;
vertical effective stress at any depth;
angle of friction between concrete and sand.
Estimation of Ks as per Ref (1)
For bored and cast in-situ pi les, Ks is related to the Ko value as follows:
Ks/Ko = 0.70 to 1
Ks= Coefficient of lateral pressure at rest.
where
Typical values for Ko are given below:
Relative density
Ko .
Loose
0.5
Medium dense
0.45
Dense
0.35
Alternatively Ks may be determined as outlined in Ref. (2) .
•
(b) Estimation of Pz
.
.
Pz is estimated following the guidelines given in section 3.1.1.
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(c) Estimation of;)
Values of 0 are related to the friction angle 0.
Ref. (1) gives the recommendations of the following researchers:
Kulwahy
:(0/0) = 1.0
Brooms
:(0/0) = 0.75
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4.2.2 Estimation of fu Based on SPT N Value
.
It is recommended to use,
,
fu
= 1.3.N kN/m z
Ref. (1) gives a limiting value of fu = 100 kN/mz .
•
Note 1:
;
Note 2:
4.3
The pile length should be divided into several elements based on the existing soil
layers and values of 0 and N used in sections 4.2.1 & 4.2.2 respectively, should
correspond to those in each of the element.
The friction that can be mobilized is highly dependent on the method of pile
construction. Special care is necessary when pile groups are used to carry a column
load to ensure that pile construction method does not disturb the soil adjacent to
adjoining piles.
Estimation of fu in Clays
•
Where
Ca
=
Adhesion between pile and the soil;
Cu
=
Undrained shear strength of the clay.
The coefficient a is a function of the stress history of the clay.
For soft normally consolidated (nc) clays and peat, a = l',
For stiff over consolidated (oc) clays, a < 1.
: For the estimation of a, Figure 3 given in Ref. (3), may be used.
The value of a may also be determined from pull out tests and experience.
4.4
Estimation of fu in Weathered Rock
This may be estimated using SPT value N.
It is recommended to use,
fu = 2.N
kN/m2.
2
With the further condition that fu should not exceed 200 KN/m .
•
4.5
Estimation of fu in Bed Rock
Generally, the design of rock socket friction is a function of the surface roughness of
rock sockets, the unconfined compressive strength of intact rock, the confining
stiffness around the rock socket in relation to fractures of rock mass and socket
diameter, and the rock socket length-to-diameter ratio. It is very complicated to
quantify all of these aspects in rock socket design and hence, some simplified
5
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semi -empirical methods are used in practise. In this respect, the method specified in
rcf (l) is given below:
Where the rock socket reduction factor a and rock socket correction factor p, can be
obtained from the Figures 4 and 5 respectively. The mass factor j can be obtained
from the guidelines proposed in Table I.
Table 1 - Guidelines to Estimate the Mass Factor j
RQD (%)
Fracture frequency per
meter
0- 25
25 - 50
50- 75
75 - 90
90 -100
Mass factor j
0.2
0.2
0.2 - 0.5
0.5 - 0.8
0.8 - 1.0
15
15 - 8
8-5
5 -1
1
The mobilized rock socket friction depends on the construction methodology and in
general 25% of the estimated rock socket friction is assumed to be mobilized, if
bentonite slurry is used to stabil ize the pi Ie bore .
•
5.0
NEGATIVE SKIN FRICTION
When the soil is settling downwards relative to a pile, the settling soil causes a down drag
force on the pile which is termed as negative skin friction.
Calculation of the magnitude of the negative skin friction is a complex problem in which
several : factors are involved. However in practice, following simplified calculation
procedures may be adopted.
As outl ined in Ref. (1), unit negative skin friction resistance 1:5 can be given as
1:5
Where
=
P . p(z)
=
Skin friction factor, given in Figure 6;
•
and
p(z) = Vertical effective stress at any depth, and computed as indicated
previously in Section 3.1.1.
,/!'fo'
Alternatively for clay soils, the method in Section 4.3 may also be used .
•
•
6
6.0
ESTIMATION OF SETTLEMENTS
The settlement of a single pile varies depending on the appli ed load, pile di a meter, elastic
properties of the pile material and the surrounding medium.
The method outlining ref. 2 may be used to estimate the settlcmcnt of a s ingle pile, if
reasonable value of elastic properties of the surrounding medium co'uld be obtained.
When a group of piles is used, the settlements that take place are th ose of the pi Ie group
and not of the individual pi Ie.
.
{The settlement of a pile group may be determined using appropriate th eory ass umin g that
the pile group behaves as an equivalent raft.
The diameter and the depth of the equivalent raft may be obtained from the method given
in Ref. (1).
7.0
REFERENCES
01.
Tomlinson, MJ. (1987); "Pile Design and Construction Practice". Viewpoint
rd
Publication, London; 3 Edition.
02 .
Poulos H . G. and Davis, E. H., 1980, Pile Foundation Analysis and Deasign, John
Wiley, New York.
1h
03.
Bowles, 1. E., "Foundation analysis and design", 1996, 5 Edition. McGraw-Hill.
04.
BS 8004, 1986 Code of practice for Foundations.
•
•
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•
•
7
,
200
..
.
.
v
.
•
.
.
~ ISO
-.. -
'1/
....o '
Depth/leas width ratio
--. ~ . ' . J
.
1/ 1
.
.
I
•
•
,...
OfB~5
,,0
.. .
.
- ~
1°
\
o-
.
.. .
,
30
35
40
45 '
Angfe ofsfleatfng resistance, degrC'es
Figure 1 - Bearing capacity factor of Beresantive et al. 4,25 - Reference (1)
Uniaxial compressive strength (MN/ml)
HOd \110"9
tQ1--
Hod
,"uk
I Group
I rocks
I
,
I
!
I
I
I
I
I
l
I
I
RQD> 90
I
I
I
I
I
I
Widely spaced
discontinuitaes
{ thick beddln9 1
1.0 III
I
I
•
t
I
I
RQD
(75, 90)
bOOmm
Allowable
bearmg
pressures
Medium spaced
<its con t inuil ies
(medium beddmg)
. I
Closely spaced
discontlnulhes
mm
L----L_-=--'--'-'~---1...4-~'----"-L...... ,..,..-
(b)
- ' ' !thin
60mm
bedding I
,
RQO < 75
Figure 2 - Allowable end bearing capacity for igneous and metamorphic rock
(BS 8004, 1986)
8
•
1.2
....
l.J
g 1.0 1-0...
\~
1
•
--....- -.1- -+--+1----1-1--lr---+I--4---+1-
c-·
-' ......... ..
0.9
~
0.8 •
O.," I
•
API ( 19::l4)
- - - - 1\ v"rage froll
.
1---,=
Pc,," O: l .t! ( I I I I
- - l\ulhur
•
0 ..6
0.5
.
,
:wo
150
1/
250
t 1111. It II b -tween undrained shear strength (Su) and { • 1111111'
Figure 3 - l{
10
I' I
~
I
1
t
' .
.
,I
:
.
\ 1
•
Rock socket skin fl .clion
--fj .. o<f3?iUG
\
1
- Williams g Pel/5
"'"
.
.
\
.
- "
Rosenberg&.
Joumeaux
.4-
2
0
' ... .........
~
""
.....
-.
Horvath - --~.... - .. - , J II ,i l
I
I , -,, . i t '-,..
1
, \ , \ I
"
f
10
Unconfined comprf!ssion strength -ql/' , MN/m 2
,
o
Figul' - 4 - Ro J<'ocket reduction factor (Reference (1»
•
9
•
•
,
,..
1.0 •
.
•
.
-~
~
....
.....0\}
Of]
...
.
~
c::
.- 0.6
0
~
"
.
•
.
~0
•
u
.
II
•
Q4
~
v
~
.
0
Q:
.
,
•
•
:x
~
.
.
02
o
o
0.2
•
04
0.6
MaSS factor j
1.0
08
Figure 5 - Rock socket correction factor (Reference (1))
•
0.
.
f0
,
/I
I
I
•
•
•
G-
.
s0
60. •
o
0..1
0.2
0.3
0.4-
•
0.5
0.6
Skin friction fJctor, f3
Figure 6 - Negative skin friction for piles driven into spft to firm clays
(Meyerhof, 4.3 (Reference (1))
10