Transcript Axial Capacity of Bored Pile

Case History Evaluation of the Axial Behaviour
of Bored Pile from SPT
Aung Naing Moe
August 2014
Outline
- Introduction
- Axial Capacity of Bored Pile
- Case Studies
- Conclusions & Discussions
Introduction
• Since 1967, there have been a significant increase in the use
of bored piles as foundation in Singapore.
• Reported by Chang and Broms (1990), approximately
200,000-400,000 m of bored piles is installed each year. The
diameter of Bored piles varies from 500 mm to 1800 mm.
• Until late 1970s, the design procedure for bored piles was
essentially empirical and the capacity was very often
underestimated.
Introduction
• As a result, the designs were often conservative. One of the
most valid reasons for conservative design procedure is the
lack of understanding of the behaviour of bored piles in local
residual soils and weathered rocks.
• For the design verification purpose, proof load tests were
conducted. Although test piles were occasionally loaded to
failure, they were often not instrumented.
• As a result, only load-displacement behaviour of pile could be
determined and test data did not provide the information on
the load distribution and the load-transfer characteristics of
pile.
Introduction
• To develop the design of bored piles in residual soils and
weathered rocks of Singapore, number of studies on
instrumented bored piles have been carried out since early
1980s.
• These studies show that the load transfer is primarily through
the shaft resistance and the mobilized point resistance is very
small at the working load.
• The results of these studies were reported by Yong et al
(1982), Chin (1982), Chin et al (1982), Buttling (1986) and
Buttling & Robinson (1987).
Introduction
• In late 1980s and early 1990s, similar studies were carried out
and the results were reported by Chang & Goh (1988) and
Chang & Broms (1991).
• The design recommendations were given on the unit shaft
friction, critical displacement and load transfer curve.
• The more comprehensive study was carried out by Chang &
Zhu (2002) and the report was focused on a better
understanding of the interaction mechanism between pile
shaft and the surrounding soil and the construction effects on
the pile performance.
Axial Capacity of Bored Pile
• The function of piles is to transfer the load to the stronger
layers of the ground which are capable of supporting the load
with an adequate factor of safety and without settling at the
working load by an amount detrimental to the structure that
they support.
• At all times, it is important that the stress induced in both pile
material and supporting soil is kept within an allowable limit.
Axial Capacity of Bored Pile (structural)
• Structural Capacity
For nominally reinforced bored pile, as recommended in BS 8004
and SS CP4 (2003), the allowable structural capacity can be
computed as:
Qst = 0.25 fcu Ac
where Ac = area of concrete and 0.25 fcu should not exceed 7.5
N/mm2.
Axial Capacity of Bored Pile (structural)
For rock socketed reinforced bored piles with full length
steel reinforcement, the allowable structural capacity
may be determined as axially loaded short columns in
accordance with SS CP65 and can be taken as:
Qst =
0.4 f cu A c  0.75 f y As
Fs
where
fcu
= compressive strength of concrete at 28 days
Ac
= area of concrete
fy
= yield stress of steel
As
= steel area
Fs
= factor of safety (≥ 2)
Axial Capacity of Bored Pile
• Geotechnical Capacity
A pile subjected to the axial load will carry the load partly by
shear generated along the pile shaft, and partly by normal stress
generated at pile base.
The ultimate capacity is equal to the sum of ultimate shaft and
base resistance.
Axial Capacity of Bored Pile (geotechnical)
Force Diagram
Qu
Qu + Wp = Qs + Qb
Qu = Qs + Qb - Wp
WP
In practice, Wp is much
Smaller compared to Qu,
Qs
Qu = ultimate capacity
Qs = ultimate shaft resistance
Qb = ultimate base resistance
Wp = self weight of pile
Qu = Q s + Q b
Qb
Axial Capacity of Bored Pile (geotechnical)
Ultimate Shaft Resistance
The ultimate shaft resistance, Qs is generally taken as:
Qs =
where
fs
dAs
 f dA
s
s
= ultimate unit shaft resistance
= local incremental shaft area of pile
For layered soil, the above equation can be rewritten as:
N
Qs =  f si Asi
where
fsi
Asi
i 1
= ultimate unit shaft resistance in layer i
= shaft area of pile in layer i
Axial Capacity of Bored Pile (geotechnical)
Ultimate Base Resistance
The ultimate base resistance, Qb is generally estimated from the
relationship:
Qb = f b Ap
where
fb
= ultimate base resistance
Ap
= pile base area
Axial Capacity of Bored Pile (geotechnical)
Allowable Capacity
The allowable capacity is equal to the sum of ultimate shaft and
base resistance divided by a suitable factor of safety:
Qa =
(Qs  Qb )
F
A single global factor of safety (F) of 2.0 to 3.0 is commonly used
to evaluate the allowable capacity of single piles.
The lower value is often used when the ultimate capacity is
determined from load tests and the higher value when the
capacity is estimated from a static formula.
Axial Capacity of Bored Pile (geotechnical)
Another important factor, the settlement of the pile under the
working load should not exceed the specified limit. In Singapore,
the maximum settlement of bored pile should not exceed 25 mm
at 2 times working load (Public Works Department, Housing and
Development Board & SS CP4 2003).
In the 1st Phase MRT construction, the Mass Rapid Transit
Corporation of Singapore specifies that the maximum settlement
should not exceed 6-9 mm at working load and 9-20 mm at 1.5
times working load (Buttling and Robison, 1987).
Axial Capacity of Bored Pile (geotechnical)
The axial displacement that is required to fully mobilize the shaft
resistance for bored piles is usually small, typically 5-6 mm
(Whitaker and Cooke 1966, Aurora and Reese 1977, Horvath and
Kenney 1979) or 5-10 mm (O’Neill and Reese 1972). Based on the
findings by local investigators, 4-9 mm of pile shaft movement is
required to fully mobilize the shaft resistance.
unit shaft resistance
t-z curve
qs max
5 - 6mm
displacement
Axial Capacity of Bored Pile (geotechnical)
In contrast a relatively large displacement, approximately 5 %
(Aurora and Reese 1977) or 10 % (Woodward et al. 1972) of the
pile diameter, is required to fully mobilize base resistance. Thus at
the working load, the shaft resistance plays an important role.
unit base resistance
q-z curve
qb max
5% - 10% of pile diameter
displacement
Axial Capacity of Bored Pile (geotechnical)
This difference in the required displacement for fully
mobilization of resistance and its effect on pile behaviour are not
taken into account in the traditional design approach in
Singapore.
Since the different displacements are required for fully
mobilization of the two resistance components, the use of
different partial factor of safety for the shaft resistance and base
resistance is recommended in the improved traditional design
method. The allowable pile capacity can be expressed as:
Qa = Qs  Qb
Fs
Fb
where Fs is typically 1.5 to 2 and Fb is typically 3 to 4.
Axial Capacity of Bored Pile (geotechnical)
• Estimation of Unit Shaft Resistance
The load transfer mechanism in the design of bored pile shaft
resistance is similar to that used to analyze the resistance to a
sliding of a rigid body in contact with soil.
Two methods of analysis, one for cohesive soil and the other for
non-cohesive soil, can be used to estimate the ultimate shaft
resistance of bored pile.
Axial Capacity of Bored Pile (geotechnical)
a-Method
This method is commonly used to estimate the ultimate unit
shaft resistance of piles in clay soil subjected to an undrained
loading condition (total stress analysis).
The skin resistance is evaluated from the undrained shear
strength (Cu) as determined by field or laboratory tests.
Tomlinson (1957) recommended the a-method to determine the
unit shaft resistance as follows:
fs = a Cu
Cu
= undrained shear strength and a = adhesion factor
Axial Capacity of Bored Pile (geotechnical)
Evaluation of a
Number of studies have been carried out to determine the
adhesion factor (a) for stiff and hard clays and weathered rocks.
Generally, the a value decreases with increasing undrained shear
strength.
The value of a for a given pile at a given site should be
determined from a pile load test.
Axial Capacity of Bored Pile (geotechnical)
However, it is impossible and therefore many attempts have
been made to establish the correlation between Cu and a.
Typically, the value of a ranges from 0.25 for very stiff to hard
clay to 1.0 soft clay.
Some a values suggested by researchers based on the intensive
studies in different soils are summarized in following Table.
Axial Capacity of Bored Pile (geotechnical)
Golder and Leonard (1954)
Adhesion
Factor, a
0.25 - 0.70
Tomlinson (1957) and Skempton (1959)
0.30 - 0.60
Tomlinson (1957) and Skempton (1959)
0.45 (average)
Soil Type
London Clay
Stiff Clay
Stiff silty Clay
Beaumont Clay
Reference
Woodward et al (1961)
0.50
Mohan and Jain (1961)
0.66
Whitaker and Cooke (1966)
0.44
Reese and O'Neill (1988)
0.55
Chin (1982)
Pearce and Brassow (1979)
0.80 - 0.85
0.60
Kenny Hill Formation,
Malaysia
Toh, C.T et al (1989)
0.50 - 0.54
Silt Stone
(highly weathered)
Davies et al (1979)
0.65 - 0.71
Axial Capacity of Bored Pile (geotechnical)
Weltman and Healy (1978) studied the ultimate shaft resistance
of bored piles in boulder clay and other glacial tills and
introduced the a verses Cu curve.
Axial Capacity of Bored Pile (geotechnical)
Kulhawy and Jackson (1984) reported the correlation between a
and Cu based on the data of over 100 pile load test.
Pa
a = 0.21 + 0.26
Cu
where Pa is the atmospheric pressure, 101 kPa. The a value and
Cu/Pa should not exceed 1 and 3, respectively.
Based on the comprehensive study, Kulhawy and Phoon (1993)
found that both the unit shaft resistance and the adhesion factor
vary linearly.
C
fs
C
 0.5 u (or)
a = 0.5 Pu
Cu
Pa
a
Axial Capacity of Bored Pile (geotechnical)
Fleming et al (1985) proposed the following relationships.
For Cu/s'v <1,
a = (C
0.5
0.5
u / s 'v )
For Cu/s'v >1,
a =
0.5
(Cu / s ' v ) 0.25
where s'v is the effective vertical stress.
Axial Capacity of Bored Pile (geotechnical)
Semple and Rigden (1984) proposed the value of a as a function of
Cu/s'v and L/d. The a value can be taken as:
a = F ap
where F is the length factor and ap is peak friction coefficient. The
values of F and ap can be obtained from followings:
Axial Capacity of Bored Pile (geotechnical)
The back calculated a value from results of load tests is subject
to soil disturbance, constriction effects and rate of loading.
Moreover, the undrained shear strength is not a fundamental
soil parameter. It depends on various factors, such as the stress
history, the effective overburden stress, the effective friction
angle, the water content and the testing method.
Therefore the care should be taken when using a method for the
estimation of shaft resistance.
Axial Capacity of Bored Pile (geotechnical)
b-Method
The effective stress analysis is commonly used to estimate the
ultimate unit shaft resistance of pile in coehionless soil or
cohesive soil which is subjected to a drained loading condition
(effective stress analysis). In this method, the skin friction
resistance is related to the effective overburden pressure s'v:
fs = c' + s'h tan d'
since s'h = Ks s'v, tan d' = tan f' and the above equation becomes:
fs = c' + Ks s'v tan f'
Axial Capacity of Bored Pile (geotechnical)
In practice, due to the soil disturbance associated with pile
installation, the drained shear strength is commonly neglected.
fs = Ks s'v tan f’
(or)
fs = b s'v
Where:
c'
s'h
d'
s'v
Ks
f'
b
= drained shear strength
= effective horizontal stress acting on pile shaft
= effective friction angle between the pile and soil
= effective vertical stress
= coefficient of horizontal stress
= effective friction angle
= Ks tan f'
Axial Capacity of Bored Pile (geotechnical)
There is a relationship between the coefficient Ks and the coefficient of
earth pressure at rest K0. Kulhawy (1984) recommended Ks = 0.7 - 1.0 K0
and also suggested that d' = 1.0 f' for cast-in-place piles in sand.
For cohesive soil, the value of b ranges typically from 0.25 to about 0.40
depending on the over consolidation ratio (OCR).
b = 0.25 (OCR)0.5
An equivalent can be estimated for residual soils and weathered rocks
from the following relationship.
OCR = Cu/Cnu
where Cnu is the undrained shear strength of the normally consolidated
clay which can be estimated from the c/p ratio. If no test data is available,
the widely accepted c/p ratio of 0.22 can be used.
Axial Capacity of Bored Pile (geotechnical)
Number of studies have been carried out to determine the b value.
Wong (2005) recommended the following relationship to estimate the
b.
b = (Cu/s'v)0nc.375 ( Cu/s'v)0.625
Fleming et al (1985) proposed to use the following relationship to
estimate the value of b. For Cu/s'v <1:
b = (Cu/s'v)0nc.5
( Cu/s'v)0 .5
and for Cu/s'v > 1.0,
b = (Cu/s'v)0nc.5
( Cu/s'v)0.75
Axial Capacity of Bored Pile (geotechnical)
• Estimation of Unit Base Resistance
The base resistance normally depends upon the shear strength
properties of soil within the vicinity of the pile base.
Large amount of displacement is required to fully mobilize the
base resistance.
The mobilized base resistance at the working load is usually
small (Chang and Wong 1987).
Axial Capacity of Bored Pile (geotechnical)
Cohesive Soil
The drained end bearing capacity of bored pile in clayey soil is
larger than the undrained. However, the displacement required
to mobilize the drained capacity would be too large to be
tolerate by most of structures.
For this reason, the ultimate base resistance of piles in clay is
calculated as a function of undrained shear strength (Cu) and
bearing capacity factor (Nc). The unit base resistance can be
estimated from the following relationship.
Axial Capacity of Bored Pile (geotechnical)
fb = Nc Cu
The value of Nc is usually taken as 9 (Skempton, 1951) if the pile
tip penetrates into the bearing stratum by 3 times pile diameter
or more.
However when the ratio of the embedment depth in the bearing
stratum, to the diameter of pile base is less than 3, a linear
interpolation is necessary for the adoption of the value of Nc
(6  N c  9 , Fleming, 1985).
Axial Capacity of Bored Pile (geotechnical)
Non-Cohesive Soil
The bearing pressure beneath a pile in a uniform deposit of noncohesive soil is directly proportional to the vertical effective
stress.
From the general bearing capacity equation, the unit base
resistance can be express in the terms of the effective vertical
stress (s'v) and bearing capacity factor (Nq).
fb = Nq s'v
Axial Capacity of Bored Pile (geotechnical)
Berezantzev et al (1961) recommended the value of Nq as a
function of friction angle f'. The relationship between frictional
angle f' and bearing capacity factor Nq is shown in Figure below:
Axial Capacity of Bored Pile (geotechnical)
Estimation of Pile Capacity from Standard Penetration Test (SPT)
The soil parameters derived from laboratory tests are used in
traditional method of design for piles.
However for stiff cohesive soil, the determination of the
undrained shear strength and deformation parameters from
laboratory tests is not reliable due to difficulty in “undisturbed”
sampling and sample disturbance.
Also, obtaining of undisturbed sample in cohesionless soil is very
difficult.
Axial Capacity of Bored Pile (geotechnical)
As a result, in-situ tests are commonly used to calculate the
geotechnical capacity of bored piles.
The standard penetration test (SPT), developed around 1927, is
currently the most widely used in-situ test in many countries
around the world.
The test method has been standardized as ASTM 1586 since
1958 with periodical revision to date.
The reason for preference for SPT test is probably because it is
easy to use, inexpensive and the long experience accumulated
with interpretation.
Axial Capacity of Bored Pile (geotechnical)
Estimation of Unit Shaft Resistance
As presented earlier, the unit shaft resistance of bored piles is
normally estimated by the a method. However it should be
highlighted that it is difficult to determine the undrained shear
strength from unconfined compression tests or triaxial
undrained tests (UU tests) due to sample disturbance.
Therefore it is preferable to correlate the Cu from penetration
tests. For residual soils of Singapore, as recommended by Stroud
(1974), the relationship between the standard penetration
resistance or N value and the undrained shear strength is:
Cu = 5 - 6N (kPa)
Axial Capacity of Bored Pile (geotechnical)
using a value of 0.45, as recommended by Skempton (1959), the
relationship between ultimate unit shaft resistance and standard
penetration resistance (N) can be taken as:
fs = 2.45N (kPa)
Meyerhof (1976) suggested that the ultimate unit shaft resistance of
bored piles can be estimate directly from the standard penetration
resistance (N).
fs = N (kPa)
A well known relationship fs = 2N (kPa), proposed by Meyerhof (1976) for
driven piles in sand, is often used for the design of bored piles in residual
soils in Singapore (Broms et al. 1988).
Axial Capacity of Bored Pile (geotechnical)
Based on the extensive studies of instrumented pile tests in residual soil of
Singapore, Chang & Goh (1988) and Chang & Broms (1991) recommended the
following relationship to evaluate the ultimate unit shaft resistance of bored
piles.
fs = 2N (kPa)
The Singapore code for foundation, SS CP4 (2003) recommended the following
empirical relationship to estimate the ultimate shaft resistance.
fs = Ks N (kPa)
where Ks is the skin friction coefficient and value depends very much on the
local experience. For soil of Bukit Timah Granite, a value of Ks between 1.5 to
2.5 may be adopted. For dense or hard cemented soil in the Old Alluvial, a
value of Ks between 2 and 3 can be adopted.
Axial Capacity of Bored Pile (geotechnical)
Estimation of Unit Base Resistance
As discussed, the unit base resistance of bored piles is normally
estimated from bearing capacity equation, fb = Nc Cu.
Using cu = 5 - 6N based on Stroud (1974) and Nc = 9 as
recommended by Skempton (1951), the ultimate unit base
resistance can be taken as:
fb = 45N (kPa)
Axial Capacity of Bored Pile (geotechnical)
Meyerhof (1976) suggested that the ultimate unit base
resistance of bored piles can be estimate directly from the
standard penetration resistance (N).
fb = 120Ncorr (kPa)
where Ncorr can be taken as:
Ncorr = CN N60
where CN is SPT overburden correction factor and N60~N.
CN = 10 (1/s'v)0.5
Axial Capacity of Bored Pile (geotechnical)
Based on the extensive studies of instrumented pile tests in residual soil of
Singapore, Chang & Broms (1991) recommended the following relationship
to evaluate the ultimate unit base resistance of bored piles.
fb = 30 - 45N (kPa)
The SS CP4 (2003) recommended that qu may be related to the SPT N-value
as:
fb = Kb 40N (kPa)
where Kb is coefficient and value depends on the depth of embedment in
bearing stratum, effect of loosing of soil at pile base, effect of softening of
soil due to ingress of ground water and cleanness of pile base. A Kb value of
between 1 and 3 may be adopted with limiting value of fb = 10 MPa, unless
otherwise verified by load test.
Case Studies
The main objective is to study the results and performances of
load tests conducted on the instrumented bored piles.
The piles under this study were located at various sites around
Singapore and were installed in different soil conditions and
geological formations.
The results of 5 instrumented load test data were used in this
chapter. The details of the test piles and their locations are
summarized in following Table.
Case Studies
Pile
Working Test
Penetration
Casting
Load
Load Location Formation
Case Diameter
(m)
Method
(mm)
(ton)
(ton)
1
600
16.8
180
558
Senja
Road
Bukit
Timah
Tremie
2
600
19.2
212
742
Balestier
Road
Old
Alluvial
Dry
3
1400
19.0
1000
3295
Bukit Ho
Swee
Jurong
Dry
4
1000
28.0
580
1740
Boon Lay
Way
Jurong
Tremie
5
900
13.0
180
610
Jalan
Kilang
Jurong
Dry
Case Studies
• Test Pile Detail
Case 1 Test Pile
Depth (m)
0 - 2.6
Soil Description
SPT
fill material
6
2.6 - 4.0
medium stiff silty Clay
7
4.0 - 8.0
loose clayey Silt with medium coarse sand
9
8.0 - 15.0
medium dense clayey Silt with coarse sand
14-18
15.0 - 15.2 Very dense Silt with decomposed Granite
15.2 - 18.0
Hard Granite
100
43-55% RQD
Case Studies (test pile detail)
Case 1 Test Pile
Case Studies (test pile detail)
Case 2 Test Pile
Depth (m)
0 - 1.0
Soil Description
SPT
fill material
1.0 - 2.7
medium stiff silty Clay
6
2.7 - 8.0
stiff silty Clay
8.0 - 11.5
very dense clayey Sand
60
11.5 - 14.0
hard clayey sandy Silt
77
14.0 - 18.4
very dense to hard clayey silty Sand
13-14
>100
Case Studies (test pile detail)
Case 2 Test Pile
Case Studies (test pile detail)
Case 3 Test Pile
Depth (m)
0 - 1.0
Soil Description
SPT
Firm clayey Silt
1.0 - 2.7
medium stiff silty Clay
2.7 - 8.5
stiff silty Clay
8.5 - 12.0
very dense sandy Silt
8
30-33
56
12.0 - 18.0 weathered Siltstone
>100
18.0 - 28.0 weathered Siltstone
>100
Case Studies (test pile detail)
Case 3 Test Pile
Case Studies (test pile detail)
Case 4 Test Pile
Depth (m)
0 - 1.4
Soil Description
SPT
fill material
1.4 - 6.3
loose to medium dense sandy clayey Silt
7-11
6.3 - 14.5
medium dense to dense sandy Silt
25-51
14.5 – 33.5 hard sandy Silt
>100
Case Studies (test pile detail)
Case 4 Test Pile
Case Studies (test pile detail)
Case 5 Test Pile
Depth (m)
0 - 0.8
Soil Description
SPT
fill material
0.8 - 3.0
Stiff clayey Silt
11
3.0 - 5.8
hard clayey Silt
40-63
5.8 - 7.6
hard clayey Silt
>100
7.6 -11.2
weathered Siltstone
>100
11.2 - 17.0 weathered Siltstone
>100
Case Studies (test pile detail)
Case 5 Test Pile
Case Studies
• Load Distribution & Pile Capacity
The load distribution curves provide the information of axial load
variation along pile shaft and at pile tip.
The magnitude of load distribution at each soil layer is calculated
from the measured strain changes, pile geometry and suitable
elastic modulus of pile.
The load distribution curves along a pile allow an evaluation of
the load transferred to each geological stratum and the
corresponding mobilized resistance value at each stage of
loading.
Case Studies (load distribution & pile capacity)
The pile capacity is mobilized by the movement of pile in relation
to the surrounding soil.
The ultimate capacity, which is the maximum load, is carried by
the pile without excessive settlement or failure.
For those cases in which the test loads are not high enough to
fully mobilize the ultimate capacity, the Chin method of analysis
is introduced to estimate the ultimate pile capacities.
Case Studies (load distribution & pile capacity)
Load Distribution Curves
To obtain a greater understanding of the pile-soil interaction
behaviour, it is desirable to install further instrumentations in the
test piles.
The load distribution along the pile shaft and at the pile toe can
be measured using vibrating wire strain gauges (VWSGs).
The VWSGs measured the axial strain changes in pile shaft and at
the pile toe.
Case Studies (load distribution & pile capacity)
Case Studies (load distribution & pile capacity)
Case Studies (load distribution & pile capacity)
The VWSGs are installed on sister bars (approximately 1.0 m
long). Each strain gauge assembly (sister bar) is tied to the pile
reinforcement cage at the specified intervals as indicated in test
pile detail 1-5.
Based on the current construction practice, the maximum
interval between two layers of strain gauges is 3.0 m. The signal
cable from the VWSG is routed to the readout unit which is
stationed near the pile head.
The function tests are conducted before the installation of
reinforcement cage into the borehole and upon the completion
of concreting. The strain changes under each stage of loadings
are measured and stored in the readout unit.
Case Studies (load distribution & pile capacity)
The axial deformation of pile may be measured using a simple
rod extensometer.
The extensometer consists of a stainless steel rod attached to a
fixed anchor point in the pile and placed within a protective
pipe.
The entire assembly is cast in the bored pile. As the pile undergo
compression, the steel rod remains free in the protective pipe
which undergoes compression with the pile.
A linear transducer is used to measure the axial movement of
the steel rod.
Case Studies (load distribution & pile capacity)
sister bar
strain gauge
extensometer
protective pipe
Case Studies (load distribution & pile capacity)
Case Studies (load distribution & pile capacity)
Load Distribution Calculations from Instrumentation Data
Based on the reading of VWSGs and the extensometers, both the
load distribution along the pile shaft and the load-transfer curves
can be derived.
First a suitable elastic modulus of the pile, Ep, is adopted. The
suitable elastic modulus value is back-calculated from the axial
strain measurement of strain gauges at the first layer.
With a proper elastic modulus, the load distribution at each layer
of stain gauges can be calculated.
Case Studies (load distribution & pile capacity)
Adoption of Suitable Elastic Modulus of Pile
In general, the elastic modulus is not constant and its value
depends on the quality of concrete, amount of axial strain and
methods of testing.
The results of instrumented load test piles located in NIE site at
NTU campus indicate that elastic modulus decreases as axial
strain increases (Chang & Zhu, 2002).
In this case study, where possible, this modulus degradation was
considered in the adoption of suitable modulus value for load
distribution calculations.
Case Studies (load distribution & pile capacity)
The elastic modulus of pile (Ep) was back-calculated from the
axial strain measurement of strain gauges at the first layer. Below
is the sample of average strain change and back-calculated Ep
from first layer strain gauges data.
Average Strain Change
Layer
Depth
Average Axial Strain Change
Ref
(m)
(10-6)
189
377
566
754
943
1131
A
2.35
148.0
307.1
487.3
692.0
948.1
1268.5
C
5.85
146.5
306.6
484.9
687.4
931.5
1263.0
E
11.85
144.5
303.8
479.7
677.3
913.2
1222.2
F
14.85
139.6
295.9
464.8
641.9
854.2
1091.9
G
17.85
131.7
279.1
427.9
566.4
725.7
886.6
H
19.85
119.7
260.5
386.6
498.4
624.3
741.2
I
21.55
104.0
232.4
331.7
426.1
534.8
612.4
Case Studies (load distribution & pile capacity)
E (ton/mm2)
Strain vs Elastic Modulus
2.80
2.40
2.00
1.60
y = -0.0007x + 2.6477
1.20
0.80
0.0
200.0
400.0
600.0
800.0
1000.0
1200.0
1400.0
Strain (10-6)
Test Load
189
377
566
754
943
1131
Ep (t/mm2)
2.54
2.44
2.31
2.17
1.98
1.77
Case Studies (load distribution & pile capacity)
With a proper elastic modulus, the load distribution at each layer
of stain gauges can be calculated from the following relationship.
sp
 =
Ep
sp =
P
Ap

sp
Ep
Ap
= axial strain
= axial stress
= elastic modulus of pile
= area of pile
X
1
Ep
P
Ap
= 
P =  Ep Ap
Case Studies (load distribution & pile capacity)
Secondly, the calculated loads at various levels are plotted and
load distribution curves at different applied load are obtained.
The load distribution curves along a pile allow a calculation of the
load transferred to each soil stratum and the corresponding
mobilized resistance value at each stage of loading.
Based on the pile head movement and the axial strain measured,
the relative displacement in the middle of each soil layer between
the pile and its surrounding soil or at the pile toe can be
computed. A plot of the mobilized shaft resistance verses the
relative shaft displacement or the mobilized point resistance
versus the tip movement can be obtained for each supporting
stratum to reflect the complete load transfer characteristic of the
stratum.
Case Studies (load distribution & pile capacity)
Axial Load (ton)
0
150
300
450
600
750
900
1050
1200
0
2
4
6
8
10
Depth (m)
12
14
16
189 tons
377 tons
18
20
566 tons
754 tons
943 tons
22
1131 tons
24
26
load distribution
diagram
Case Studies (load distribution & pile capacity)
unit shaft resistance
qs max
5 - 6mm
displacement
t-z curve
Case Studies (load distribution & pile capacity)
unit base resistance
q-z curve
qb max
5% - 10% of pile diameter
displacement
Case Studies (load distribution & pile capacity)
• Test Results
Case 1 test pile
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Case 2 Test Pile
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Case 3 Test Pile
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Case 4 Test Pile
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Case 5 Test Pile
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Case Studies
• Conclusions & Discussions
Case No.
1
2
3
4
5
Depth
(m)
4.3 - 7.3
7.3 - 10.3
10.3 - 13.3
13.3 - 16.3
0.0 - 10.7
10.7 - 13.7
13.7 - 16.7
16.7 - 18.7
3.5 - 6.5
6.5 - 9.5
9.5 - 12.5
12.5 - 15.5
15.5 - 18.5
0 - 6.5
6.5 - 9.5
9.5 - 12.5
12.5 - 15.5
15.5 - 18.5
18.5 - 21.5
21.5 - 24.5
24.5 - 27.5
3.5 - 6.5
6.5 - 9.5
9.5 - 12.5
SPT N Value
(blows/300 mm)
9
14
18
100
23
72
98
111
56
100
100
150
167
11
11
25
25
25
42
150
150
63
107
150
Shaft
Resistance
fs, (kPa)
53
96
198
298
52
210
168
260
112
247
213
388
429
39
23
24
26
26
168
313
210
208
175
482
fs/N
5.9
6.9
11.0
2.9
2.3
2.9
1.7
2.3
2.0
2.5
2.1
2.6
2.6
3.5
2.1
1.0
1.0
1.0
4.0
2.1
1.4
3.3
1.6
3.2
summary of mobilized
shaft resistance
Case Studies (conclusions & discussions)
Case
No.
1
2
3
4
5
SPT N Value
(blows/300 mm)
Base Resistance
fb, (kPa)
fb/N
100
6468
64.7
111
11033
99.4
167
9101
54.5
150
5796
38.6
150
3782
25.2
summary of mobilized
base resistance
Case Studies (conclusions & discussions)
Case No.
2
4
5
Depth
(m)
7.7 - 10.7
10.7 - 13.7
13.7 - 16.7
16.7 - 18.7
0 - 6.5
6.5 - 9.5
9.5 - 12.5
12.5 - 15.5
15.5 - 18.5
18.5 - 21.5
21.5 - 24.5
24.5 - 27.5
3.5 - 6.5
6.5 - 9.5
9.5 - 12.5
SPT N Value
(blows/300
mm)
23
72
98
111
11
11
25
25
25
42
200
200
61
107
150
Shaft
Resistance
fs, (kPa)
52
210
168
260
39
23
24
26
26
168
313
210
208
175
482
Critical
Displacement
(mm)
N.A
3.0
5.0
5.0
N.A
N.A
5.0
8.6
8.6
N.A
7.3
11.8
5.0
5.0
N.A
summary of critical
shaft displacement
Case Studies (conclusions & discussions)
Relationship
between
unit
shaft
resistance &
SPT (N)
Case Studies (conclusions & discussions)
Relationship
between
fs/N & SPT
(N)
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Relationship
between
unit
base
resistance &
SPT (N)
Case Studies (conclusions & discussions)
As discussed earlier, the value of elastic modulus decreased with
increased in axial strain.
The skin resistance increased with increased in standard
penetration resistance.
As presented, the relationship between the unit skin friction and
the standard penetration resistance (N) was 2.4N.
The other relationship, the unit end bearing and the respective N,
was found to be 46N.
Case Studies (conclusions & discussions)
Another important parameter, the critical shaft displacement (zs)
to fully mobilize the skin resistance was varied between 3.0 mm
and 11.8 mm. In most cases, zs = 3.0 - 8.7 mm which is
irrespective of standard penetration resistance, the diameter and
the length of piles.
Based on the finding from the results of the instrumented test
pile reported in this study, the following conclusions can be
drawn:
a) The adoption of elastic modulus value is very important for the
evaluation of load distribution curves which significantly effects
the estimation of fs and fb. The modulus degradation and the
relationship between Ep and value of e should be considered in
the calculation of load distribution. A constant Ep value should not
be adopted especially for the case when the Ep value is much
lower than the theoretical value.
Case Studies (conclusions & discussions)
b) For the design of bored pile in residual soil of Singapore, a possible
approximate relationship between fs and N is as follows:
fs = 2N (kPa)
A higher value of fs may be adopted if the soil parameters or the
important relationships are available from the load test result.
c) For design applications, the unit end bearing value fb can be related
to the penetration resistance, N, as follows:
fb = 45N (kPa)
The higher fb value may be adopted if the debris from the pile bottom is
properly removed and pile base is cleaned.
Case Studies (conclusions & discussions)
d) The test results suggested that the critical shaft displacement,
zs = 3.0-9.0 mm for the bored pile in residual soil of Singapore.
However, it is expected that similar correlations can be derived for
other soil conditions.
e) Due to inadequate data, no conclusion could be made on the
estimation of the critical tip displacement, zp value. If there is lack
of data, it is suggested that the zp value be selected as 5% to 10%
of the pile diameter.
Thank You.