Justification for Base Resistance Formula

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 = where fcu = compressive strength of concrete at 28 days Ac = area of concretefy = yield stress of steel As = steel areaFs = factor of safety (≥ 2)

sFsyccu A f 0.75 A f 0.4

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

In practice, Wp is much Smaller compared to Qu,

Qu

WP

Qu + Wp = Qs + Qb

Qs

Qb

Qu = ultimate capacityQs = ultimate shaft resistance Qb = ultimate base resistance Wp = self weight of pile

Qu = Qs + Qb - Wp

Qu = Qs + Qb


Ultimate Shaft Resistance

The ultimate shaft resistance, Qs is generally taken as:

Qs = wherefs = ultimate unit shaft resistancedAs = local incremental shaft area of pile

For layered soil, the above equation can be rewritten as:

Qs = wherefsi = ultimate unit shaft resistance in layer iAsi = shaft area of pile in layer i

ssdAf

si

N

isiAf

1


Ultimate Base Resistance

The ultimate base resistance, Qb is generally estimated from the relationship:

Qb =

wherefb = ultimate base resistanceAp = pile base area

pbAf


Allowable Capacity

The allowable capacity is equal to the sum of ultimate shaft and base resistance divided by a suitable factor of safety:

Qa =

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.

FQQ bs )(


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.

qs max

unit shaft resistance

displacement5 - 6mm

t-z curve


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.

qb max

unit base resistance

displacement5% - 10% of pile diameter

q-z curve


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 =

where Fs is typically 1.5 to 2 and Fb is typically 3 to 4.

b

b

s

s

FQ

FQ


• 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.


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


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.


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)Soil Type Reference Adhesion

Factor, a

London Clay

Golder and Leonard (1954) 0.25 - 0.70Tomlinson (1957) and Skempton (1959) 0.30 - 0.60

Tomlinson (1957) and Skempton (1959) 0.45 (average)

Stiff Clay

Woodward et al (1961) 0.50Mohan and Jain (1961) 0.66Whitaker and Cooke (1966) 0.44

Reese and O'Neill (1988) 0.55

Stiff silty Clay Chin (1982) 0.80 - 0.85

Beaumont Clay Pearce and Brassow (1979) 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


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.


Kulhawy and Jackson (1984) reported the correlation between a and Cu based on the data of over 100 pile load test.

a = 0.21 + 0.26

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.

(or) a = 0.5

u

a

CP

a

u

u

s

PC

Cf 5.0

a

u

PC


Fleming et al (1985) proposed the following relationships.

For Cu/s'v <1,

a =

For Cu/s'v >1,

a =

where s'v is the effective vertical stress.

5.0)'/(5.0

vuC s

25.0)'/(5.0

vuC s

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:


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.


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'


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' = drained shear strengths'h = effective horizontal stress acting on pile shaftd' = effective friction angle between the pile and soils'v = effective vertical stress Ks = coefficient of horizontal stressf' = effective friction angle b = 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.


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) ( Cu/s'v)

Fleming et al (1985) proposed to use the following relationship to estimate the value of b. For Cu/s'v <1:


and for Cu/s'v > 1.0,


375.0nc

625.0

5.0nc

5.0

5.0nc

75.0


• 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).


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.


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 , Fleming, 1985).9 cN


Non-Cohesive Soil

The bearing pressure beneath a pile in a uniform deposit of non-cohesive 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


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:


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.


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)


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.


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)


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

CasePile

Diameter (mm)

Penetration (m)

Working Load (ton)

Test Load (ton)

Location Formation Casting Method

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 DetailCase 1 Test Pile

Depth (m) Soil Description SPT

0 - 2.6 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 100

15.2 - 18.0 Hard Granite 43-55% RQD

Case Studies (test pile detail)

Case 1 Test Pile


Case 2 Test Pile


0 - 1.0 fill material


2.7 - 8.0 stiff silty Clay 13-14

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 >100


Case 2 Test Pile


Case 3 Test Pile


0 - 1.0 Firm clayey Silt


2.7 - 8.5 stiff silty Clay 30-33

8.5 - 12.0 very dense sandy Silt 56

12.0 - 18.0 weathered Siltstone >100



Case 3 Test Pile


Case 4 Test Pile



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 4 Test Pile


Case 5 Test Pile



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



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.


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.




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.


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.


sister barstrain gauge

extensometerprotective pipe



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.


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.


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 ChangeLayer 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


0.0 200.0 400.0 600.0 800.0 1000.0 1200.0 1400.00.80

1.20

1.60

2.00

2.40

2.80

f(x) = − 0.000693885373310023 x + 2.64765294944199

Strain vs Elastic Modulus

Strain (10-6)

E (t

on/m

m2)

Test Load 189 377 566 754 943 1131

Ep (t/mm2) 2.54 2.44 2.31 2.17 1.98 1.77


With a proper elastic modulus, the load distribution at each layer of stain gauges can be calculated from the following relationship.

e =

sp =

X = e

P = e Ep Ap

p

p

Es

pAP

pAP

pE1

e = axial strainsp = axial stressEp = elastic modulus of pileAp = area of pile


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.


0 150 300 450 600 750 900 1050 12000

2

4

6

8

10

12

14

16

18

20

22

24

26

189 tons

377 tons

566 tons

754 tons

943 tons

1131 tons

Axial Load (ton)

Dep

th (m

)

load distribution diagram


qs max

unit shaft resistance

displacement5 - 6mm

t-z curve


qb max

unit base resistance

displacement5% - 10% of pile diameter

q-z curve

Case Studies (load distribution & pile capacity)• Test Results Case 1 test pile




Case 2 Test Pile







Case 3 Test Pile



Case Studies (load distribution & pile capacity)Case 4 Test Pile






Case Studies (load distribution & pile capacity)Case 5 Test Pile






Case Studies• Conclusions & Discussions

Case No.

Depth (m)

SPT N Value(blows/300 mm)

Shaft Resistance

fs, (kPa)fs/N

1

4.3 - 7.3 9 53 5.97.3 - 10.3 14 96 6.9

10.3 - 13.3 18 198 11.013.3 - 16.3 100 298 2.9

2

0.0 - 10.7 23 52 2.310.7 - 13.7 72 210 2.913.7 - 16.7 98 168 1.716.7 - 18.7 111 260 2.3

3

3.5 - 6.5 56 112 2.06.5 - 9.5 100 247 2.59.5 - 12.5 100 213 2.1

12.5 - 15.5 150 388 2.615.5 - 18.5 167 429 2.6

4

0 - 6.5 11 39 3.56.5 - 9.5 11 23 2.19.5 - 12.5 25 24 1.0

12.5 - 15.5 25 26 1.015.5 - 18.5 25 26 1.018.5 - 21.5 42 168 4.021.5 - 24.5 150 313 2.124.5 - 27.5 150 210 1.4

53.5 - 6.5 63 208 3.36.5 - 9.5 107 175 1.69.5 - 12.5 150 482 3.2 summary of mobilized

shaft resistance

Case Studies (conclusions & discussions)

Case No.

SPT N Value

(blows/300 mm)Base Resistance

fb, (kPa)fb/N

1 100 6468 64.7

2 111 11033 99.4

3 167 9101 54.5

4 150 5796 38.6

5 150 3782 25.2

summary of mobilized base resistance


Case No.

Depth (m)

SPT N Value(blows/300

mm)

Shaft Resistance

fs, (kPa)

Critical Displ-

acement (mm)

2

7.7 - 10.7 23 52 N.A10.7 - 13.7 72 210 3.013.7 - 16.7 98 168 5.016.7 - 18.7 111 260 5.0

4

0 - 6.5 11 39 N.A6.5 - 9.5 11 23 N.A9.5 - 12.5 25 24 5.0

12.5 - 15.5 25 26 8.615.5 - 18.5 25 26 8.618.5 - 21.5 42 168 N.A21.5 - 24.5 200 313 7.324.5 - 27.5 200 210 11.8

53.5 - 6.5 61 208 5.06.5 - 9.5 107 175 5.09.5 - 12.5 150 482 N.A

summary of critical shaft displacement


Relationship between unit shaft resistance & SPT (N)


Relationship between fs/N & SPT (N)


Relationship between unit base resistance & SPT (N)


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.


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.

Date post:	14-Jul-2016
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Justification for Base Resistance Formula

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