The UWA05 method for prediction of axial capacity of driven piles in sand

See discussions, stats, and author profiles for this publication at: http://www.researchgate.net/publication/268590430Development of the UWA-05 Design Method for Open and Closed Ended Dr

See discussions, stats, and author profiles for this publication at: http://www.researchgate.net/publication/268590430

Development of the UWA-05 Design Method for Open and Closed Ended Driven Piles in

Siliceous Sand

CONFERENCE PAPER · OCTOBER 2007

DOI: 10.1061/40902(221)12

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Barry Michael Lehane

University of Western Australia

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James Schneider

Consulting Engineer

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Available from: Barry Michael Lehane Retrieved on: 07 August 2015

1 INTRODUCTION AND BACKGROUND

The authors, at the request of the American

Petrole-um Institute (API) piling sub-committee, recently

conducted a review of methods for the assessment of

the axial capacity of driven offshore piles in

sili-ceous sand The review, which is described in detail

in Lehane et al (2005a) and involved the

develop-ment of an extended database of static load tests,

evaluated the existing API recommendations

(API-00) and three Cone Penetration Test (CPT) based

methods namely: Fugro-04 (Fugro 2004), ICP-05

(Jardine et al 2005) and NGI-04 (Clausen et al

2005) A new design method, referred to as

UWA-05, emerged following the evaluation exercise and is

the described in this paper and in Lehane et al

(2005b)

The assessment of the predictive performance of

API-00, Fugro-04, ICP-05 and NGI-04 against the

new UWA pile test database indicated the following

trends (which are described in detail in Lehane et al

2005a):

1 All three CPT based design methods considered

(Fugro-04, ICP-05 & NGI-04) had significantly

better predictive performance than the existing

API recommendations, which were seen to lead

large under-predictions in dense sands and

be-come progressively non-conservative as the pile

length (L) or aspect ratio (L/D) increased

2 Despite the CPT based methods having a

broad-ly similar predictive performance against the

new database of load tests, their formulations

lating the pile end bearing with the cone tip

re-sistance (qc) are notably different Formulations

for shaft friction also differ significantly in

de-tail, although all assume a near-proportional re-lationship between local shaft friction (τf) and qc

and allow for the degradation of τf with distance above the pile tip (h) due to friction fatigue

3 The ICP-05 method indicated the lowest coeffi-cient of variation (COV) for calculated to meas-ured capacities (Qc/Qm) of 0.32, when an equal weighting is given to each pile test in the data-base However, the relative performance of each method for various categories within the data-base is less clear For example, NGI-04 predic-tions appear best for open-ended piles in com-pression while Fugro-04 and ICP-05 provide comparable predictive accuracies for open-ended piles in tension

4 When account was taken of the relative reliabil-ity of the pile test data (using a carefully de-signed weighting procedure), the methods listed below for each category of pile lead to the low-est probability of failure:

 API-00: closed-ended piles in compression

 Fugro-04: closed-ended piles in tension

 ICP-05 & NGI-04: open-ended piles in com-pression

 ICP-05 & Fugro-04: open-ended piles in ten-sion

5 API-00 gives the lowest probability of failure for closed-ended piles in compression partly be-cause the method generally under-predicts the capacity of the database piles to a significant de-gree However, while the same average level of under-prediction also applies to API-00 predic-tions for closed-ended piles in tension, the esti-mated probability of failure is larger than the three alternative CPT design methods

The UWA-05 method for prediction of axial capacity of driven piles in sand

B M Lehane, J.A Schneider and X Xu

The University of Western Australia (UWA), Perth

ABSTRACT: This paper describes a new method for evaluating the axial capacity of driven piles in siliceous sand using CPT qc data The method is shown to provide better predictions than three other published CPT based methods for a new extended database of static load tests The design expressions incorporate the most important features currently accepted as having a controlling influence on driven pile capacity at a fixed time after installation (e.g the effects of soil displacement, friction fatigue, sand-pile interface friction, dilation at the shaft and loading direction) and are seen to reduce to a simplified form for typical (large diameter) off-shore piles

6 The ICP-05 method displays a tendency to

under-predict pile base capacities (when assuming

ca-pacity solely from annular end bearing) and to

be-come potentially non-conservative for tension

ca-pacity as the pile aspect ratio (L/D) increases The

Fugro-04 method indicates a tendency to

under-predict compression capacities for long piles and

to over-predict base capacities in loose sand

The examination of the three CPT based methods

coupled with a review of their various deficiencies

and a careful examination of the new extended

data-base of static load tests prompted the authors to

pro-pose the UWA-05 method presented here This

method is believed to represent a significant

im-provement on Fugro-04, ICP-05 and NGI-04

meth-ods Particular comparisons are made with ICP-05,

which Lehane et al (2005a) adjudged to have a

marginally better predictive performance than the

other two CPT based methods

2 THE UWA-05 DESIGN METHOD FOR PILES

IN SAND

2.1 End Bearing

Factors that were considered in the development of

the UWA-05 proposals for base capacity evaluation

of closed and open-ended piles are listed in the

fol-lowing These proposals are based on the analyses

reported in Xu & Lehane (2005) and Xu et al

(2005) The base capacity is defined as the pile end

bearing resistance at a pile base movement of 10%

of the pile diameter, qb0.1

2.1.1 Closed-ended piles

 The strong direct relationship between the end

bearing resistance of a closed-ended driven pile

and the cone tip resistance, qc, has been

recog-nised for many years and arises because of the

similarity between their modes of penetration

 Given the difference in size between a pile and a

cone penetrometer, a correlation between qb0.1

and qc requires use of an appropriate averaging

technique to deduce an average value of qc Xu

& Lehane (2005) show that, for many

stratigra-phies encountered in practice, qc may be taken

as the average qc value taken in the zone 1.5 pile

diameters (D) above and below the pile tip

 Xu & Lehane (2005), however, also show that

when qc varies significantly in the vicinity of the

pile tip (i.e within a number of diameters), the

Dutch averaging technique (Van Mierlo &

Koppejan 1952, Schmertmann 1978) provides the

most consistent relationship for end bearing and

should be employed to calculate qc

 A simplified (and conservative) means of

deter-mination of the Dutch qc value is provided in

Lehane et al (2005b), which may be more

practi-cal when using CPT data collected offshore, which are often not continuous

 The values of qb0.1 for driven piles are less than

c

q because the displacement of 0.1D is insuffi-cient to mobilise the ultimate value (of qc)

 The findings of Randolph (2003), White & Bol-ton (2005), and others, are consistent with the UWA-05 proposal to adopt a constant ratio of

qb0.1/qc for driven closed-ended piles

The UWA-05 design equation for the end bearing of

a closed-ended pile, with diameter D, is given as:

2 1 0

4 q

where qb0.1/qc= 0.6 (1)

2.1.2 Open-ended piles

 Salgado et al (2002), Lehane & Gavin (2001, 2004), and others, have shown that a relatively consistent relationship between qb0.1 for a pipe pile and the CPT qc value becomes apparent when the effects of sand displacement close to the tip during pile driving are accounted for This installation effect is best described by the incre-mental filling ratio (IFR) measured over the final stages of installation- and is referred to here as the final filling ratio (FFR) As the FFR ap-proaches zero, qb0.1 approaches that of a closed-ended pile with the same outer diameter

 The displacement induced in the sand in the vi-cinity of the base is most conveniently expressed

in the terms of the effective area ratio Arb*, de-fined in Equation 2c This ratio depends on the pile’s D/t (diameter to wall thickness) ratio and the FFR value, varying from unity for a pile in-stalled in a fully plugged mode to about 0.08 for a pile installed in coring mode with D/t of 50

 Lehane & Randolph (2002), and others, have shown that, if the length of the soil plug is greater than 5 internal pile diameters (5Di), the plug will not fail under static loading, regardless of the pile diameter

 Experimental data and numerical analysis indi-cate that the resistance that can develop on the tip annulus at a base movement of 0.1D varies be-tween about 0.6 and 1.0 times the CPT qc value (e.g Bruno 1999, Salgado et al 2002, Lehane & Gavin 2001, Paik et al 2003, Jardine et al 2005)

 Lehane & Randolph (2002) suggest that the base resistance provided by the soil plug for a fully coring pile (with FFR =1) is approximately equivalent to that of a bored pile

 Recommended values of qb0.1/qc for bored piles range from 0.15 to 0.23 (Bustamante & Gianeselli 1982, Ghionna et al 1993) These

rati-os are not dependent on the pile diameter

 The value of qc should be evaluated in the same way as that employed for closed-ended piles, but

using an effective diameter (D*) related to the

ef-fective area ratio, Arb* i.e D* = D × Arb*0.5

 There are relatively few documented case

histo-ries that report the incremental or final filling

ra-tios In the absence of FFR measurements, a

rough estimate of the likely FFR may be obtained

using equation 2d (see Xu et al 2005)

The UWA-05 proposal for end bearing of driven

pipe piles is provided in Equation (2) This proposal

is developed in Xu et al (2005) and shown to

com-pare favourably with the existing database of base

capacity measurements for open-ended piles

2 1

.

0

4

q

(2a)

* rb c

1

.

q   (2b)















*

D FFR

1

A (2c)































2 0 i

m 5 1

) m ( D ,

1

min

where Di is the inner pile diameter

2.2 Shaft Friction

Factors that were considered in the development of

the UWA-05 method for shaft friction are discussed

in Schneider & Lehane (2005) and Lehane et al

(2005a) These are now summarised as follows:

 Local shaft friction (τf) shows a strong correlation

with the cone tip resistance (qc) This correlation,

which has been observed directly in instrumented

field tests has been employed successfully in well

known design methods, such as that proposed by

Bustamante & Gianiselli (1982)

 The shaft friction that can develop on a

displace-ment pile is related to the degree of soil

dis-placement imparted during pile installation The

higher capacity developed by the new generation

of screw piles compared to that of a bored and

continuous flight auger piles is just one example

of this effect

 The degree of displacement imparted to any

giv-en soil horizon is related to the displacemgiv-ent

ex-perienced by that horizon when it was located in

the vicinity of the tip This level of displacement

can conveniently be expressed for both closed

and open-ended piles in terms of an ‘effective

ar-ea ratio’, Ars*, which is unity for a closed ended

pile and, for a pipe pile, includes displacement

due to the pile material itself and the additional

displacement imparted when the pile is partially

plugging or fully plugged during driving White

et al (2005) use a cavity expansion analogy to

deduce that the equalized lateral effective stress

is likely to vary with the effective area ratio raised to a power of between 0.30 and 0.40

 The incremental filling ratio (IFR) is a measure

of soil displacement near the tip of a pipe pile and depends on a number of different parameters, in-cluding soil layering, pile inner diameter, pile wall thickness, plug densification or dilation, and installation method For the (limited) database of IFRs reported, the mean IFR over the final 20D

of penetration (where most friction is generated) can be reasonably approximated using Equation (3e) for relatively uniform dense to very dense sands in the database

 After displacement of the sand near the tip in a given soil horizon and as the tip moves deeper, the radial stress acting on the pile shaft (and hence the available τf value) in that horizon re-duces This phenomenon, known as friction fa-tigue, is now an accepted feature of displacement pile behaviour (e.g see Randolph 2003)

 The rate of radial stress and τf reduction with height above the tip (h) depends largely on the magnitude and type of cycles imposed by the in-stallation method White & Lehane (2004) show that the rate of decay is stronger for piles experi-encing hard driving and much lower for jacked piles, which are typically installed with a

relative-ly low number of (one-way) installation cycles

 White & Lehane (2004), and others, also show that the rate of degradation with h is greater at higher levels of radial stiffness (4G/D) and there-fore τf at a fixed h value (i.e after a specific number of installation cycles) in a sand with the same operational shear modulus (G) reduces as D increases

 The foregoing, plus the tendency for hammer se-lection to be such that the number of hammer blows is broadly proportional to the pile slender-ness ratio (L/D), suggest that τf may be

tentative-ly considered a function of h/D This approxima-tion is supported by field measurements such as those provided in Lehane et al (2005a), and is

al-so compatible with the occurrence of a ‘critical depth’ at an embedment related to a fixed multi-ple of the pile diameter (such as 20D proposed by Vesic 1970 and a number of workers) The same approximation is employed by the ICP-05 and Fugro-04 design methods

 Based on the former point, the ICP-05 method proposes that τf varies in proportion to (h/D)-c,

where c = 0.38 However, given that this value of

c was estimated on the basis of field tests with

jacked piles (Lehane 1992 and Chow 1997) where the type and number of cycles imposed is less severe than is typical of driven piles, a higher

value of c is considered more appropriate for

off-shore pile Strong indirect evidence in support of this observation is also apparent in Lehane et al (2005a), which shows that the Fugro-04, ICP-05

and NGI-04 progressively under-predict the shaft

capacity of jacked piles as the pile length

increas-es

 The radial effective stress acting on a driven pile

increases during pile axial loading and its

magni-tude (when τf is mobilised and dilation has

ceased) increases as the pile diameter reduces, the

sand shear stiffness around the pile shaft

increas-es and the radial movement during shear

(dila-tion) of the sand at the shaft interface increases

These increases are not significant for offshore

piles (with large D) but need to be considered

when extrapolating from load test data for small

diameter piles in a database The

recommenda-tions of the ICP-05 method are considered

rea-sonable for assessment of the increase in radial

stress (∆σ'rd), but with a modified expression for

the shear stiffness derived from the CPT data

 τf varies in proportion to tan δcv (where δcv is the

constant volume interface friction angle between

the sand and pile); this δcv value, which should be

measured routinely, increases as the roughness

normalized by the mean effective particle size

(D50) increases Verification of the dependence of

τf on tan δcv has been provided by Lehane et al

(1993), Chow (1997), and others In the absence

of specific laboratory measurements of δcv

UWA-05 recommends the trend shown on Figure

1, which is the same as that employed by ICP-05

but with an upper limit on tanδcv value of 0.55

(due to the potential for changes in surface

roughness during pile installation)

 The shaft friction that can develop on a pile in

tension is smaller than that which can be

mobi-lised by a pile loaded in compression for the

rea-sons described by Lehane et al (1993), de Nicola

& Randolph (1993) and Jardine et al (2005)

 Because of the shortage of high quality

measure-ments of τf very close to the tip of a driven pile

and the variable and inconsistent trends shown by

the available measurements, one simplifying

op-tion is to assume τf is constant over the lower two

diameter length of the pile shaft for both closed

and open-ended piles in tension and compression

 Shaft capacity increases with time as shown by

Axelsson (1998), Jardine et al (2005a), and

oth-ers Lehane et al (2005a) show, however, that rate

of increase over the period 3 days to 50 days is

not statistically significant for the UWA database

of load tests A design time of 10 to 20 days is

considered appropriate for shaft friction

calculat-ed using UWA-05

The UWA-05 design equations for shaft capacity

of driven piles arose from the foregoing

considera-tions and are expressed as follows:





 rc rd cv

c cv rf

f

f tan





3 0

* rs c

D

h max A

q 03 0 '













































* rs

D

D IFR 1































2 0 i mean

m 5 1

) m ( D , 1 min

D r G 4 'rd 



where

cv = constant volume interface friction angle

'rf = radial effective stress at failure

'rc = radial effective stress after installation and

equalization

'rd = change in radial stress due to loading stress

path (dilation)

f / fc = 1 for compression and 0.75 for tension G/qc = 185·qc1N-0.75 with qc1N=(qc/pa)/('v0/pa)0.5

pa = a reference stress equal to 100 kPa

'v0 = in situ vertical effective stress

r = dilation (assumed for analyses=0.02mm, as

for ICP-05)

20 22 24 26 28 30 32

Median Grain Size, D50 (mm)

tan  < 0.55

Employed for database evaluation

UWA-05 recommendation

Figure 1 cv variation with D 50 (modified from ICP-05 guide-lines)

3 PREDICTIVE PERFORMANCE OF UWA-05 The UWA database of static loads tests, as discussed

in Lehane et al (2005a & b), was employed to assess the predictive performance of the proposed UWA-05 method The predictions described employed equa-tions (1), (2) and (3) with the following additional considerations:

 Measured interface friction angles, when

availa-ble, were adopted Figure 1 was used in the

ab-sence of measured δcv values

 When the incremental filling ratio (IFR) was

rec-orded, Arb* was assessed using the mean IFR

val-ue measured over the final 3D of pile penetration

while the value of Ars* was assessed from the

mean IFR value recorded over the final 20D of

penetration In the absence of IFR data, Arb* and

Ars* were evaluated using Equations 2d & 3e

The database included 74 load tests at sites where

CPT qc data were measured Pile test data at sites

containing micaceous, calcareous and residual sands

were excluded from consideration – as were sites for

which only Standard Penetration Test data were

available The database included substantially more

pile tests than used for verification of the Fugro-04,

ICP-05 and NGI-04 design methods and was

sub-divided into the following four categories:

(a) Closed-ended piles tested in compression

(b) Closed-ended piles tested in tension

(c) Open-ended piles tested in compression

(d) Open-ended piles tested in tension

A detailed presentation and discussion of this

statis-tical analysis, which was conducted for API-00,

Fugro-04, ICP-05 and NGI-04, as well as for

UWA-05 is presented in Lehane et al (20UWA-05a & b) and may

be briefly summarized as follows:

(i) For the database taken as a whole (i.e including

all pile categories), the UWA-05 method

pre-dicts a mean ratio of calculated to measured

ca-pacity (Qc/Qm) of 0.97 and the lowest overall

coefficient of variation (COV) for this ratio of

0.29; this compares well with the respective

COVs of 0.32, 0.38, 0.43 and 0.6 for ICP-05,

Fugro-04, NGI-04 and API-00

(ii) The UWA-05 method has the lowest COV for

Qc/Qm of all five methods for each of the four

pile test categories (except for closed-ended

piles in compression where UWA-05 and

ICP-05 have the same COV for Qc/Qm)

(iii) The COV of 0.19 for Qc/Qm of the UWA-05

method for open-ended piles in compression is

significantly lower than the corresponding COV

of 0.25 of ICP-05

(iv) UWA-05 shows no apparent bias of Qc/Qm with

pile length (L), pile diameter (D), pile aspect

ra-tio (L/D) and average sand relative density

One of the factors giving rise to the superior

per-formance of the UWA-05 method for pipe piles is

the inclusion of the effective area ratio terms in the

expressions for base and shaft capacities of open

ended piles This is not surprising given the

acknowledged importance of soil displacement on

capacity and the fact that many of the database piles

showed evidence of partial plugging However,

giv-en that the incremgiv-ental filling ratio (IFR) is not commonly measured in practice, the sensitivity of the predictive performance to the IFR parameter employed was re-examined and a summary of this exercise is provided in Table 1

It is clear from Table 1 that the estimation of IFR using the empirical equations 2d & 3e, rather than direct use of the measured IFRs to deduce Ar* val-ues, has only a minimal impact on the COV values for Qc/Qm It may also be inferred that the assump-tion in UWA-05 of a fully coring pile (i.e IFR=1) for the database piles (most of which had diameters less than 800mm) will lead, on average, to a 20%

prediction of capacity Such an under-prediction is in keeping with observed levels of par-tial plugging of (smaller diameter) database piles and suggests that other design methods, such as

ICP-05, which may provide a good fit to the existing da-tabase of load tests, but do not include an appropri-ate soil displacement term (such as Ar*), will over-predict the capacity of full scale offshore piles

Table 1: Sensitivity of pipe pile capacity to A r* (A rb* and A rs*)

4 PREDICTIONS FOR OFFSHORE PILES The UWA-05 method simplifies to the following form for full scale offshore piles, as IFR=1 and the dilation term (∆σ’rd) can be ignored













4 q Q Q

1 0 s b







0.75 D dz

c 1

cv

5 0 3

0 r c

D

h max A

q 03































(4d)















 i22 r

D

D 1

Lehane et al (2005b) examined the implications

of equation (4) and assessed its performance against existing API recommendations and ICP-05 (the best

Method for calculation of Ar* Mean Qc/Qm COV for

Qc/Qm

Open-ended piles in compression

Using Equations 2d & 3e for all tests 0.99 0.23

Open-ended piles in tension

Using Equations 2d & 3e for all tests 0.97 0.26

performing of the three CPT based methods

consid-ered) This examination indicated that equation (4)

provides a more conservative extrapolation than

ICP-05 for shaft capacity from the existing database

(of relatively small diameter piles – with a mean D

of about 0.7m) to typical offshore piles used in

prac-tice Equation (4) also predicts higher base

capaci-ties than ICP-05 because of its assumption that a pile

plug with a length greater than 5 diameters will not

fail under static loading

It is also noteworthy that Equation (4) tends to

provide lower capacities than API-00 in loose sands,

but higher capacities for dense sands in

compres-sion API-00 and UWA-05 predictions for tension

capacity in dense sands are broadly similar for pile

lengths in excess of 20m However, the UWA-05

method, unlike API-00, does not show any

predic-tion bias with L, D, L/D and Dr

5 CONCLUSIONS

This paper has shown that the UWA-05 method:

(i) is a significant improvement on existing API

recommendations;

(ii) provides better predictions for a new extended

database of load tests than the ICP-05,

Fugro-04 and NGI-Fugro-04 CPT based design approaches;

(iii) employs soundly based formulations that draw

on the considerable recent developments in our

understanding of displacement piles in sand;

(iv) provides formulations that enable a rational

ex-trapolation beyond the existing database base

of load tests

6 REFERENCES

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driven piles in sand Proc., 4th Int Con on Case Histories

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driven in sand Proc., ISFOG, Perth

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J of Geotech and Geoenv Engrg., ASCE 128(3): 198-205 Lehane, B M 1992 Experimental investigations of pile be-haviour using instrumented field piles PhD thesis, Univ of

London (Imperial College)

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Report, GEO: 05345

Paik, K., Salgado, R., Lee, J & Kim, B 2003, Behavior of

open- and closed-ended piles driven into sands J of Ge-otech and Geoenv Engrg., ASCE 129(4): 296-306

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