FOUNDATION DESIGN & CONSTRUCTION IN HONG KONG – PRESENT & BEYOND, Daman Lee HK-Taiwan 2005

The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong 153

FOUNDATION DESIGN & CONSTRUCTION

IN HONG KONG – PRESENT & BEYOND?

Daman D.M. Lee*, W.K. Pun

§, Arthur K.O. So

# & C.C. Wai

¤

* Ove Arup & Partners Hong Kong Limited

§ Geotechnical Engineering Office, Civil Engineering & Development Department

# China State Construction Engineering (Hong Kong) Ltd.

¤ Gammon Construction Limited

Abstract: This paper gives a state-of-the-practice account of key issues pertaining to the

design and construction of two of the major types of deep foundation systems in Hong Kong,

namely large diameter bored piles (including barrettes) and driven piles. For bored piles,

key issues such as negative skin friction, rational design approach with consideration of

settlement, use of shaft friction, quality control and mitigation on common defects are

discussed. For driven piles, the limitations of the commonly adopted driving formulae are

further deliberated and the criteria for the pile loading tests are re-visited. Finally,

suggestions are put forward on the discussed issues with an intent to attract more fruitful

discussions and debates within the industry for further advancement.

INTRODUCTION

In recent years, Hong Kong has been through a turmoil time with regards to pile foundation

design & construction. There were cases of deep foundations found to have been

constructed short of the design length, occurrence of soft materials at the toe of large diameter

bored piles. These had brought about some changes in the industry. Recently, the

Buildings Department has published the first version of the Code of Practice for Foundations

(CoPF), as part of a whole new series of codes for the private development industry.

Furthermore, the Geotechnical Engineering Office (GEO) of the Civil Engineering &

Development Department has prepared a new version of the GEO Publication No. 1/96 - Pile

Design and Construction, which takes into account of the latest pile testing data and other

advances of pile design and practices into consideration. It is therefore considered timely for

this paper to discuss some of the latest practices and look ahead to other possible areas for

improvement.

This paper focuses on large diameter bored piles and driven steel H-piles only. They

together exhausted a majority of the piling resources in Hong Kong and both have had their

fair share of events in recent years. For example, it was baffling to observe that segregation

or grout loss at the toe of bored piles has suddenly known to be a wide-spread problem.

There was neither any apparent change in the casting technique nor significant change in the

design mix of the tremie concrete. This might have become apparent due to the introduction

of more stringent quality control, eg the systematic coring of the interface between the

concrete and the founding rock for every single pile. The situation with driven H-pile is

equally captivating. For example, there has been a flourish of publications in recent years on

the use of wave equations to replace the various forms of driving formulae, with an aim to

tackle the shortcomings of the driving formulae. While this is happening, some contractors

are confused; they continue to use drop hammers to achieve final set but at the same time,


they are experiencing problems of achieving the required set in cases of very long piles.

The development of the design and construction of foundation in Hong Kong has been largely

influenced by the economic climate through time. The use of simple, albeit conservative

empirical approaches, have served its purpose till recent years. However, with the increasing

budget constraint these days, cost-effectiveness must deserve greater consideration. Out of

all different components within a structure, foundation has always been singled out as the one

that is least optimised and hence there is much room for improvement.

CURRENT PRACTICES

General

The governing of foundation design in Hong Kong can be broadly divided into two families.

First, for designs related to private developments, they are under the control of the Building

Ordinance (BO), supplemented by the Building (Construction) Regulations and administered

under the Building (Administration) Regulations. A number of Practice Notes (PNAP) were

also issued that provide guidance to the practitioners; design and construction following these

notes are deemed to satisfy the BO. The Buildings Department (BD) also recently issued the

Code of Practice for Foundations (BD, 2004). Foundation designs are submitted to the BD

for vetting and approval. Second, for public works, foundation design and construction in

general follows the General Specification for Civil Engineering Works (Hong Kong

Government, 1992). Practitioners often make reference to the GEO Publication No. 1/96:

Pile Design and Construction (GEO, 1996). The designs are submitted to the client

government departments for vetting prior to the invitation to tender. In some special area, eg

areas underlain by marble, the designs are also submitted to the GEO for vetting. For major

construction project, independent checking may also be specified.

Traditionally, foundation design in Hong Kong emphasizes on preventing failure of structures.

Much effort in the design is given to determining the pile capacity. Designers do not

specifically consider foundation settlement. This is deemed-to-be satisfied by the use of

large global safety factors on the foundation capacity or by prescribing a conservative

presumed bearing pressures of the founding material.

Bored Pile

(i) Presumptive rock bearing stress

Many of Hong Kong’s private developments often have columns transferring loadings of

more than 100MN down to the foundations. To match with the intense economical and

housing situations in the late eighties, the adoption of presumptive rock bearing stress

approach has played an important role. It recognizes the fact that Hong Kong’s solid

geology is predominately granite and volcanic tuff and hence the recommendation in BS8004

was further simplified to the selection of presumptive bearing stress based only on the total

core recovery (TCR) of core samples from the predrilling core run and an index strength on

the core samples. This simplified approach needs to be conservative as it takes no account

on the likely settlement and assumed none of the load is resisted by the soil above the

founding. Typically, designers only need to specify the piles to be constructed to a rock

stratum with 5m core run of rock with TCR of greater than 85% and rock strength greater than

25 MPa, giving a rock bearing stress of 5MPa. The use of the next higher grade at 7.5MPa,

requiring TCR greater than 95% and rock strength better than 50MPa is very much less often

because of the high risk of not being able to achieve the requirements when individual

predrilling is carried out for each pile and then having to carry out the re-design using a lower


stress value. In those days, the speed in completing the structures to put on sale tends to

outright the investment to optimise the design and quantities of the construction materials.

This is no longer the case now.

In 1999, a series of preliminary pile loading tests was carried out in an attempt to provide cost

effective solutions for the foundation of the Kowloon Canton Railway Corporation (KCRC)

West Rail. The test series included test to explore: (i) the use of higher end bearing stresses

on rock greater than those commonly used, (ii) possible use of shaft-grouted barrettes, (iii)

combined use of end bearing with rock socket friction and (iv) lateral design stiffnesses of

pile groups. On (i), the test eventually lead to the adoption of a bearing stress of 11.25MPa

on Grade II rock with a TCR of 95% and UCS of 50MPa in one of the stations.

The CoPF (BD, 2004) introduced a new category of presumptive bearing with an allowable

bearing stress of 10MPa. The requirements of the new Category 1(a) bearing stress are

given as: “Fresh strong to very strong rock of material weathering grade I, with 100% total

core recovery and no weathered joints, and minimum uniaxial compressive strength of rock

material (UCS) not less than 75 MPa (equivalent point load index strength PLI50 not less than

3 MPa)”. When dealing with fresh granite and tuff, the required strength should generally

be attainable. For designers and clients, the finding of rock joints at the post-tender

predrilling stage, or worst still at the pile founding level verification stage, would put the

entire piling contract into disarray. The project would experience delays due to the need for

re-design, which may not even be possible if the original design was too tight. For

contractors, there is little incentive for them to put forward alternative design using this new

presumptive bearing value category even after the predrilling for individual piles because of

the high contractual risk of finding rock joints, however minor they might be, during the post

construction pile verification stage.

(ii) Combined rock socket & end bearing

The CoPF (BD, 2004) allows the use of combined rock socket and end bearing, without the

need of further load testing, by simply summing their capacities arithmetically, provided that

the socket length is less than 2 times the diameter of the pile or 6m, whichever is lesser. The

introduction of this design approach is not intended to push for even higher total capacity of

bored piling because the capacity is dictated by the stresses in the pile shaft. Instead, its

introduction gives an alternative to the use of bell-out. Despite the overwhelming use of

bell-out in Hong Kong, many practitioners remain sceptical about its effectiveness. One of

the main reasons for such scepticism is that cleaning of the base in the extended area cannot

be ascertained prior to casting.

(iii) 45° Load Spread

This has been one of the most intriguing requirements of recent years regarding the founding

level of large diameter bored piles. It comes into play when the difference in the intended toe

levels of adjacent piles is greater than the clear horizontal distance between them. When this

happens, it follows that by assuming a 45° spread of end bearing load of the pile at a higher

founding level, additional load would be experienced by the pile at a lower founding level.

This rule had created havoc in the industry, generating frequent contractual friction between

the site supervision personnel and the contractor. At the beginning, the designer would have

made allowance for such load spread based on possible founding levels deduced from a

rockhead contour map. The contractor’s tender would have based on such information.

During construction, each proposed bored pile’s founding level is tentatively determined

based on a borehole at each pile location (i.e. the pre-drilling). In order to avoid last minute


revisions to the pile founding levels, the contractor is forced to complete a whole cluster of

pre-drilling holes before piling commences. Even then, construction problems frequently

arise because despite its intended purpose, the founding levels determined by the pre-drilling

are seldom final and adjustments are often needed. The situation is often difficult to rectify

when the original design of the piles are close to the limiting bearing values.

From a technical point of view, the introduction of this check is puzzling. The most onerous

load spreading situation for two adjacent piles occurs when the piles’ founding levels are the

same and the two piles are directly next to each other. According to the 45° load spread rule,

however, such a situation does not require further checking. In the CoPF (BD, 2004), there

is no such 45° load spread checking requirement but its application is still quite wide-spread

amongst practitioners.

Where the difference in pile founding levels is large, indicating a possible steeply sloping

rockhead either locally or globally, it is more appropriate to run a check on the stability of the

rock mass in cases where there are unfavourably orientated jointing. This possible rock

sliding mechanism is obviously constrained by the overburden pressure and hence it is a

straight-forward exercise to determine at what depth such instability would render impossible,

however adverse the joint sets might be.

(iv) Bored Pile Construction Process and Method

Bored pile construction in Hong Kong is highly mechanized and plant intensive. Commonly

adopted method to facilitate pile excavation is to fully case the excavation with temporary

steel casing to rockhead or to use bentonite / polymer as supporting fluid. Excavation in soil

is normally carried out by grabbing within a temporary steel casing driven into ground. In

certain places of Hong Kong such as Tin Shui Wai and other reclaimed areas, auger and rotary

boring rig provide an alternative method to grabbing. Underground obstruction such as

corestones and boulders are usually removed by chisel. Excavation in hard stratum and

formation of rock socket are normally achieved by reverse circulation drilling (RCD). Where

required, bell-out is also formed by RCD to even out the higher shaft stress then the rock

bearing stress. Upon completion of the excavation, a steel reinforcement cage is installed

into the pile bore. Self-compacting concrete is placed by under-water tremie method to

complete the bored pile.

A consequence of the simplistic design approach to found on rock described in earlier sections

is that piles may eventually need to be constructed to depths greater than 70m, which is

widely regarded as the practical limit of casing installation (even shorter at 50m for the very

large diameter piles). The telescoping method is sometimes used for the construction of

deep piles (depth > 70m), in layered rock or rock with solution features (Figure 1). This

method is intending to ensure the entire bored hole in soft layers to be fully cased. However,

installing more than one layer of casing is time consuming and requires a very experienced

crew for its operation and successful retrieval of all the casing. For public sector works,

casing may be allowed to stop short of rockhead provided that either some form of slurry is

used for the lateral support or the soil is judged to be competent without any major risk of

collapsing. Such practices remain rare in the private sector works. The reluctance in

allowing the use of slurry support for bored piling is perhaps difficult to justify, considering

the fact that excavated bore for circular piles ought to be inherently more stable that that for

the rectangular barrettes. Foundation contractors in Hong Kong are introducing more

innovative and advanced tools such as hydraulic under-reamer and combined bit to facilitate

pile excavation.


(v) Quality Control of Bored Pile Construction

(a) Post Construction Testing

In Hong Kong, two methods are normally used to check the quality and workmanship of the

bored pile, as part of the quality control measures. Direct coring method may focus on the

quality of the pile-rock interface (i.e. interface coring) or the entire concrete shaft (i.e. full

coring). It provides a visual inspection of the interface or quality of concrete at the location

of the core hole. The major drawback of this method is that the core covers only a small

percentage of the concrete shaft or pile-rock interface. Sonic logging method is used to

investigate the homogeneity and the integrity of the concrete of deep foundation. The method

is relatively simple to conduct and can detect multiple defects. However the test can only

provide information on the concrete bounded within the reservation tubes.

The frequency and requirements of

post-construction tests are specified by the

Engineer and are usually spelled out in the

construction drawings and contract

specifications. It is normal practice in

Hong Kong that in a foundation project all

piles will be subject to interface coring

and sonic logging test. To satisfy

statutory requirement, 1 % of the bored

piles will be subjected to full coring test.

To facilitate post construction testing,

reservation pipes are fixed inside the steel

reinforcement cage (see Figure 2) prior to Figure 2 - Reservation pipe fixed in cage

Figure 1 - Pile construction in marble rock

1. Use Grab & Rotator to go through 1st Cavity

2. RCD drill to 2nd cavity

3. Install & drive 2nd layer of casing and RCD drill to lower cavity

4. Remove RCD and tremie concrete to plug the cavity

5. Re-drill the hardened concrete to pile founding level


lowering the completed cage into the pile bore for concreting. For large diameter bored piles,

the usual arrangement would be to install one 220mm diameter and three 50mm to 75mm

diameter mild steel pipes. The larger pipe would also be used for interface coring. Upon

completion of the interface coring, sonic logging test would be carried out using all four

reservation pipes.

(b) Defects in Bored Piles

Defects in bored piles are attributed to

several factors, including unforeseen

ground condition, construction process &

method and workmanship. As

foundation work always falls on the

critical path of the construction, fixing

defects in deep foundation can be an

expensive exercise in terms of cost and

time.

Defects commonly seen in bored piles are

segregated concrete and pile toe

imperfection (see Figures 3 & 4).

Segregated concrete could be caused by a

variety of factors including:

• Concrete material being too dry or

contaminated.

• Washout of fines in concrete by

fast-moving underground water in

permeable water-bearing soil after

extraction of temporary casing.

• Excessive lateral movement of the

tremie pipe, thereby creating

internal channel for water flow.

• Rapid extraction of the tremie pipe,

leaving water passages in the

thickening concrete.

To minimize the occurrence of toe defects, many contractors have their own methods in

pouring the first load of concrete and a slightly different concrete mix etc. The occurrence

of toe defects has been significantly reduced both in terms of number and extent but more

effort is still needed in improving the technique before this problem can be completely

eliminated.

Toe imperfection could be related to workmanship and unforeseen ground. In most cases,

the need to pour concrete to meet tight construction programme often leads to improper or

insufficient cleaning by airlifting, resulting in the presence of compressible silt layer in the

pile base. Thick weathered joints or silt trapped in crevices in the pile base may also be the

cause of toe imperfection. Furthermore complete cleaning of the pile base is particularly

difficult for bell-out base as the reinforcement cage restricts the movement of the airlifting

pipe to within the confined area of the cage. Soft materials could sometimes migrate to the

central area of the pile from the bell-out after air-lifting and prior to concreting.

Figure 3 - Imperfection at toe interface

Figure 4 - Segregated concrete


(c) Remedial Measures

Current practice in foundation construction in Hong Kong is that once a defect is identified in

the pile, remedial measure is required to bring the pile up to the statutory requirements and

standards. Before such remedial work is carried out, a rigorous investigation which could

include sinking additional core holes through the pile shaft, further sonic testing, or other

investigative tests such as the “fan shaped” sonic test, would need to be carried out. The

purpose of these additional tests is to provide further information to aid in the identification of

the possible extent of the defect so that appropriate remedial measures could be adopted for

rectification.

When a pile is suspected of having segregated concrete regardless of the thickness of the

imperfection and the pile depth, the common approach is to sink additional full cores in the

pile, terminating typically 1 m below the suspected defect zone. Once the extent and volume

of segregated concrete are identified, high pressure water jet would be used to remove loose

aggregates in the defect zone. Voids left behind would be backfilled with cement grout

injected under high pressure. Usually a proof core hole would be required to penetrate

through the grouted zone to confirm the effectiveness of the remedial work.

Similar remedial measures would be adopted for piles with imperfect toe. Investigation to

verify the extent of toe imperfection would be carried out by coring through the remaining

reservation pipes for sonic logging test. Sometimes additional full cores may be required to

supplement the information from interface cores and to facilitate cleaning and grouting of the

pile base.

Toe defects in large diameter bored pile became a significant issue since the year 2000. It is

believed that this discovery was made as a result of the introduction of the systematic coring

of the interface via a reservation tube attached to the side of the reinforcement cage. The

logic being that when carrying out full depth coring, it is always aimed towards the centre of

the pile to avoid clashing into the pile reinforcements. At the pile centre, the interface is

likely to be at its best where the pouring of the concrete takes place via the tremie pipe.

During the concrete pouring process, soft materials that exist would be tend to be pushed to

the outer circumference of the pile, giving poor results when being cored. The

reinforcement cage could also act as a trap to these soft materials, thus giving a poor rock-pile

interface when being cored.

(vi) Defects associated with Geological

Condition

In Hong Kong, a predrill hole is required

at the proposed bored pile location to

identify the rockhead and to facilitate the

founding level determination. Often joint

regime underneath a bored pile cannot be

adequately assessed from a single borehole.

Due to random nature of rock joints and

limitation of information from one single

predrill hole, rock condition revealed in

post construction core test may well differ

from the rock condition encountered in

predrill (see Figure 5).

Interface Core Test

Predrill hole

Weak Seam Layer

Figure 5 - Typical defects associated with

geological condition


Weathered joints and seams identified in the recovered rock core from post-construction core

test are often treated as defects. The normal, albeit conservative, approach to deal with such

irregularities is to carry out engineering assessment in terms of stress and settlement

calculation to justify the rock strength. Where the joint is highly weathered and extensive in

thickness, often the piling contractor is required to remove the weathered material using

high-pressure water jet cleaning followed by pressure grouting.

The approach taken to deal with imperfection in bedrock has been an interesting subject. As

previously described, end-bearing bored piles in Hong Kong are normally designed to found

on moderately weathered rock with a minimum of 85% total core recovery. This means that

15% of the core could have been highly to completely weathered materials, which are soils in

engineering terms. It is interesting to observe that in the case of up to 15% of core loss, no

remedial actions are required but when up to 15% of jointed materials were recovered that do

not meet the required grade for founding, remedial actions were often imposed..

Driven Piles

In Hong Kong, driven piles are generally hammered to a depth where penetration resistance

reaches a pre-determined value. The Hiley Formula is widely used as a field control in

determining the penetration resistance required for a given pile capacity. Design of driven

piles based on soil mechanic principles alone to determine the required length of the piles in

soils is not common, unless they are designed to resist uplift forces or where only soft soils

are encountered. In the past, driving formulae based on the Newton’s laws of impact were

normally used. They are simple and provide acceptable prediction as long as their basic

assumptions as particles and spontaneous energy transfer upon impact are not extensively

violated. According to Smith (1960), there were 450 formulae of this kind in the file of the

Engineering News Record and more were proposed since then. However, it is well known that

modern techniques based on one-dimensional wave propagation theory can better represent

the driving process particularly when the pile is long. They are more complicated and require

pile-soil modelling and numerical analysis. In the last two decades, there were also energy

approaches based on wave mechanics and energy conservation. They determine the impact

energy delivered to pile head by dynamic measurement and relate it to work done by pile-soil

system to predict pile capacity.

Experience in driving piles indicates that small-displacement piles, e.g. steel H-pile, will be

terminated at depths where the standard penetration test (SPT) N-values in the soils are in the

range of 150 to 200, or at bedrock level. For large-displacement piles, such as prestressed

precast concrete piles, they are usually terminated at depths where SPT N-values exceed 100.

These SPT N-values provide a basis of estimating the likely depths where piles will meet the

required driving resistance. These are not design requirements.

The pile capacities are generally governed by the compressive stresses on the nominal

cross-section of the prefabricated piles. For example, the capacities of steel H-piles are based

on limiting the compressive stress to 30% of the characteristic yield strength of the steel at

working load (BD, 2004).

Given the many inherent uncertainties in the design of driven piles, a certain number of piles

are always selected for loading testing to confirm the pile capacity.


(i) Pile driving formula

In Hong Kong, Hiley (1925) formula is traditionally used to determine the allowable set for

driven piles. The formula is known to suffer from several fundamental deficiencies. The pile

is assumed to be a rigid mass in the formulation. This assumption ignores the flexibility of the

piles and the stresses that develop in soils as the compressive stress waves travel along the

pile shaft. Only those compressive stress waves that reach the pile toe are responsible for

advancing the pile. This deficiency particularly affects the accuracy of predicting capacities of

long piles. Although it is termed as dynamic formula, it neglects the dynamic resistance of the

soil, which depends on soil viscosity and rate of penetration of the pile.

In 1991, hydraulic hammers were introduced to local market as a step to minimise the

environmental impact of percussive piling. The piling industry proposed to adopt an

improved driving formula, the HKCA Formula (HKCA, 1994), which allows the use of

hydraulic hammer in the process of taking final set for driven piles. In principle, the HKCA

Formula is based on the energy approach (Broms & Lim, 1988). It lumps together the

various efficiency terms in the Hiley Formula into a single hammer factor, Kh. The HKCA

Formula is expressed as follow:

R = khE

S + 0.5 (Cc+Cp+Cq) (1)

where R = pile resistance

Kh = 0.7 for pile driving system without a cushion

= 0.6 when an additional pile cushion is used

E = hammer energy

S = pile set

Cc, Cp, Cq = temporary helmet, pile and soil compression

This formula is different from Hiley formula as a constant Kh value is used in lieu of the blow

efficiency η which is equal to (W+e2P) / (W+P) and is length dependent as shown in Figure 6.

However, the Kh value adopted is later found to be too conservative to use. Contractors are

therefore forced to pitch piles with hydraulic hammers and final set them using drop

hammers.

In 2004, a revised HKCA formula was proposed (HKCA, 2004) such that:

R = XE

S + 0.5 (Cp+Cq) (2)

where R = resistance of pile

E = rated energy of hammer

X = energy transfer ratio (ETR)

= 0.8-0.9 for a particular hammer of different drop heights

= 82% according to GRLWEAP manual (GRL 1995)

S = final set of pile

Cp, Cq = elastic compression of pile and soil


Figure 6 - Effect of coefficient of restitution and pile length on

blow efficiency

0.5

0.6

0.7

0.8

0.9

1.0

0 20 40 60 80 100

Pile Length (m)

Blow Efficiency

e = 0.32 e = 0.50 e = 0.70

The advantage of this formula is that it is independent of hammer type and Cc value. Despite

this difference, the two versions of HKCA formula are basically similar because Kh is equal to

kX where k = [S+1/2(Cc+Cp+Cq)] / [S+

1/2(Cp+Cq)]. Figure 7 shows that k tends to be very

constant as both S and Cc are generally smaller than Cp+Cq particularly when the pile is long

and for the driving condition and type of cushion used.

Further improvements have been made to reduce the uncertainty of energy transferred from

the hammer to the pile (HKCA, 2004). This involves taking measurements by Pile Driving

Analyzer (PDA) during trial piling stage to establish site-specific data on the efficiency of

driving hammers. The selection of the mean energy transfer ratio is best taken based on a

consistent and statistical approach. Selection based on the lowest measured value does not

necessarily reflect the hammer efficiency and could result in overly conservative final set

table. Measurements should also be taken at different stages of pile driving to ensure

consistency of the selected hammer efficiency.


Figure 7 - Variation of Kh/X with final set parameters

1.00

1.02

1.04

1.06

1.08

1.10

1.12

1.14

0 10 20 30 40 50

Set Per Last 10 Blows S10 (mm)

Ratio of Kh/X

Cc=5.0mm, Cp+Cq=40mmCc=5.0mm, Cp+Cq=50mmCc=2.5mm, Cp+Cq=40mmCc=2.5mm, Cp+Cq=50mm

Hydraulic hammer is now the standard equipment in driving piles. However, piling

contractors continue to use drop hammers in taking the final sets of piles in many cases. This

may partly attribute to the ease of varying the energy output of a drop hammer in order to suit

the prescribed range of penetration in the final set. More measurements and analysis will be

required for using hydraulic hammers in computing final set of the piles.

In recent years, high grade (e.g. Grade 55C) and heavy steel sections are commonly used, as

they can carry larger foundation load. As a result, the weight of the drop hammer and its

drop height has to be increased, so as to satisfy the maximum and minimum penetration

resistance at final set (Woo & Ng, 2005). This increases the chance of damaging a pile and

poses a safety risk to the personnel taking records of the final set. From safety and cost

considerations, it is more sensible to use hydraulic hammers for setting piles.

The use of hydraulic hammers in taking final sets has been successful in some projects

(ArchSD, 2003). Fung et al (2004) described the procedures of ascertaining the pile

capacities by using the Hiley Formula with modified parameters. Despite the final set table is

still based on the Hiley Formula, the parameters are selected such that the pile capacity

predicted by the Hiley Formula is equal to 85% of that predicted by the CAPWAP analysis.

This is equivalent to using CAPWAP analysis to ascertain the pile capacities. The use of

hydraulic hammers will require more verifications to calibrate the hammer efficiency

Nevertheless, the data and experience gained will be important to establish hydraulic

hammers as a standard driving equipment for setting of piles. Practitioners are encouraged to

adopt such an approach.

Driven piles will end up seating on bedrock if adequate penetration resistance is not provided


by soils, In this case, the penetration resistance determined by the Hiley Formula is no longer

applicable. A hard-driving criterion, which limits the penetration of driven piles to be less

than 10 mm in 10 blows, is usually adopted instead. Such a hard-driving criterion may lead to

the development of high driving stresses in the pile section and is sometimes monitored, e.g.

by PDA, to ensure the integrity of the installed piles.

The interaction of driving piles is a complex process. Soil resistance developed during pile

driving consists of static and dynamic resistance components. The static resistance

component is more important as they will usually be the only component to carry the

foundation loads at working stage. However, it only represents a portion of pile capacity

during driving or setting of piles. On the other hand, the process of driving piles inevitably

changes the soil properties, e.g. densification of soils, shearing and displacing of soils around

the driven piles and the development of excess pore water pressures.

(ii) Practical Final Set Problems

(a) Whipping of Piles Embedded in Loose Soils

Figure 8 shows that when an impact load is applied, the pile head settles due to a combined

effect of: a) elastic shortening of the pile shaft, b) load-settlement of the soil surrounding pile

shaft and c) load-settlement of the soil at pile base. If the shaft resistance is negligible, such as

in piles embedding in very weak soils or piles with short lengths, the pile head settlement is

simply the combination of elastic shortening of the pile shaft and the deformation of soil at

pile base whereby the theoretical elastic shortening of the piles is given by:

es = QL

AE (3)

where es = theoretical elastic shortening of pile shaft

Q = applied load

L = pile length

A = cross sectional area of pile

E = elastic modulus of pile

However, due to existence of shaft resistance, the load in pile will reduce in magnitude as it

travels down the shaft and becomes significantly small at base leading to very little end

bearing resistance. Figure 9 shows that if the load reduction in pile is taken into consideration,

the elastic shortening of the shaft will be:

es = (Qt+αsQs)L

AE (4)

where Q = Qt + Qs

Qt = end bearing

Qs = shaft resistance

αs = distribution factor for shaft resistance (FHWA 1992)


In principle, the Cp+Cq recorded will be similar to es for a purely end-bearing pile. However,

in a typical reclaimed area of fill, marine deposits, alluvium and saprolites, the temporary pile

head settlement (Cp+Cq) for piles founded on the stiff saprolites can be significantly larger

than the es of a purely end-bearing pile at final set as shown in Figure 10.

Figure 8 – Pile under axial

compression load

Figure 9 – Load distribution in pile (FHWA 1992)

Figure 10 – Bending of piles in weak soils (Tsuen Wan)


This effect is caused by pile whipping along its weak axis (Figure 11) because of insufficient

lateral restraint provided by the upper weak soil layers and reduction in contact pressure at

web faces when soil plug is formed. Because of larger Cp+Cq recorded, the dynamic pile

capacity will not only be under-estimated, but also out of range in the final set table

sometimes. Piles are therefore required to drive deeper which in turn leads to further bending

of the pile shaft and finally damaging of the pile. In order to avoid this happening, some

contractors are tempted to lower the drop height of the ram and hence reduce the momentum

for driving, or to final set the pile a few days later so that Cp+Cq becomes smaller due to set

up effect.

Figure 12 presents the static load test result of a project in which piles were driven through

very thick layers of loose or weak soils. The maximum pile head settlement minus the

residual settlement was found

to be larger than the

theoretical elastic shortening

of the pile shaft. It was

interpreted, amongst other

information, that the piles

could have bent along the

weak axis by whipping.

(b) Damage of Piles

Founded on Strong Rock

When a pile is driven to a

strong rock stratum, extreme

care has to be taken because

of possible pile damage due

to compressive stress wave

rebounded from the pile toe.

Figure 11 – Pile damaged by 12 tonnes drop hammer from 1.5m height (Kowloon Tong)

Figure 12 – Proof load test (Tsuen Wan)


According to the wave mechanics theory, when a pile is subjected to a suddenly applied axial

force, a stress wave is induced which travels away from the point of application. If no waves

reflect back to this point, the force in the pile is proportional to the velocity of particle motion.

In sign convention, the particle velocity is positive and in the direction of propagation for

compression wave, but becomes negative and in opposite direction to propagation for tension

waves. If the pile is completely free, the stress wave will arrive at the pile toe at time t after

the stress is induced at the pile head. For a free boundary condition (e.g. driving in loose soils),

a stress wave of identical magnitude but opposite sign will be reflected back to the pile head.

The velocities in the two waves are superimposed during reflection, causing the velocity at

the pile toe to be doubled (velocity components of same magnitude but opposite direction).

However, for a pile with a fixed toe, like pile founded on strong rock, the reflected wave is of

the same sign and magnitude as the initial wave but opposite direction. The force will be

doubled during reflection at the fixed end and therefore overstress and damage the pile itself.

Figure 11 is a typical example of a damaged pile driven into rock. This pile is made of a

305x305x180kg/m Grade 55C H-section of capacity 8,850kN (i.e. 90% fy). It is 12.1m long

and is driven to rock by a 12 tonne drop hammer with 2.8m ram drop. The Cp+Cq and S10 are

35mm and 5mm respectively at final set. Ultimate capacity predicted by the Case method and

CAPWAP are 10,410 kN (106% fy) and 8,438kN (91%fy) respectively, and the shaft resistance

is only 760kN.

(c) Stress Relaxation for Piles

Driven into Weak or Heavily

Jointed Rock

However, if the rock stratum is

weak or heavily jointed, Figure

13 shows that the founding

material can be shattered under

prolonged hard driving. This

enables the pile to penetrate to a

considerable depth before the

design resistance is acquired.

Besides, further penetration is

probable during re-strike because

the rock fragments plugging at

pile toe can be loosened resulting

in stress relaxation near it. These

dislodged rock fragments will be

carried down by the pile and

wedged into the space between

the flanges until the anticipated resistance is regained. Furthermore, driving adjacent piles in a

group can cause quake in the rock mass. This can further loosen the rock plug and make the

pile end-bearing resistance to deteriorate, especially when the piles are driven in close spacing

and at varying founding level. Hence, any temptation to continue hard driving of piles to

ensure full refusal conditions is not necessary and should be avoided because brittle rocks

may split up by the pile tip. This splitting may continue as the pile is driven down and require

further penetration to build up sufficient resistance comparable to the original one as shown in

Figure 14.

(d) Unavailability of Hammer Sufficiently Large to Set Long Pile

At present, 305x305x223kg/m grade 55C H-section with pile capacity 7,096kN is the

Figure 13 – Damaging effect of pile driven into

heavily jointed rock (Tomlinson 2000)


predominant pile type of

driven piles commonly

used in Hong Kong.

Founding level of this

section is generally

estimated to be 3-5m into

saprolites with SPT

N-values > 200 based on

contractors’ field

experience. This rule

generally works well in

piles less than 40m long

but encounters final set

problem if the piles are

very long. Figure 15

shows that they are

always required to drive

to very small set

(considered as “refusal"

in the CoPF (BD, 2004) if

the set is less than 10 mm

/ 10 blows) because of the

length effect of Hiley

formula, or the

over-conservatism of the

HKCA formula.

Furthermore, Figure 16

shows that even the

largest hydraulic

hammers available in the

market (24t) or large drop

hammers (20t) fallen

from height up to 4m may

not be able to set these

very long piles. Under

such circumstances, piles

will be driven to refusal

(i.e. less than 10mm per

last 10 blows) and dynamic pile testing methods such as CAPWAP (Rausche et al, 1972) and

Case method (Goble & Likins, 1985) will be used to check the driving stress and pile

integrity.

(iii) Use of Dynamic Pile Testing Methods

(a) History of Development

Figure 14 – Stress relaxation of pile driven into

heavily jointed rock (Tung Chung)

Figure 15 – Statistical analysis of 4320 piles of

35-80m long (Southeast Kowloon)


Hussein & Goble (2004)

gave a comprehensive

history on the development

and application of wave

equation. The earliest

development could be

traced back to Galileo

(1564-1642) for his study

on the dynamics of bodies

in motion. The first wave

equation was derived by

Jean Le Rond d’Alembert

(1717-1783) for a vibrating

string of an organ pipe. But

not until the early 20th

century, Isaacs (1931) was

the first one to use

one-dimensional

stress-wave theory in pile

driving analysis. Glandville et al (1938) first attempted dynamic stress measurements in pile

driving. However, Smith was the first one to produce general solution for practical application

and to use digital computers in civil engineering application. His landmark paper (Smith 1960)

forms the basis of modern wave equation analysis. Concurrent with this is the development of

bonded resistance strain gauges, which permit advancement in dynamic pile measurements. A

new era of pile measurements and analysis then began with the research work at Case

Institute of Technology (now Case Western Reserve University) in USA. Today, the most

commonly used programmes are based on WEAP (Goble & Rausche 1976), TTI (Hirsch et al.

1976) and TNOWAVE (TNO Reports 1985-1996). Figure 17 presents the basic principle of

dynamic pile testing method which applies the one-dimensional stress-wave theory with

electronic measurements and numerical calculations to simulate the pile driving process.

(b) The CAPWAP Analysis and Case Method

By the 1930’s, force measurements were made at pile head during driving. In 1961, Michigan

Highway Department carried out extensive works to measure force and acceleration. In 1964,

the Case Institute of Technology (now Case Western Reserve University) also carried out

large amount of research works to collect force and acceleration measurements at driving or

re-strike, and developed a programme called the Case Pile Wave Analysis Programme

(CAPWAP). In the analysis, records of force and acceleration continuous over time are used

as boundary condition, and soil resistance properties are adjusted until the computed output

force at pile top matches the measured force. The difference between them is the force due to

soil resistance and is called the measured delta curve. This is interpreted using a linear damper

soil model in which the dynamic resistance forces are assumed to be proportional to pile

velocities. The stress waves due to dampers therefore continually change in magnitude with

the velocity at impact, but the shear resistance remains constant at pile top until the stress

reversal is detected when the damper resistance drops off quickly as the velocity decreases,

enabling the shaft resistance and end bearing be separated. In this manner, the damping force

distribution along the pile is iterated until the best possible overall match between the

predicted and measured force plots are obtained. However, CAPWAP analysis requires

substantial numerical computation. Several simplified methods using closed form solutions

for the one-dimensional wave propagation theory were developed and correlated to the static

Figure 16 – Hypothetical allowable sets of piles

using different hammer types and driving formulae


pile test results. These methods were improved and finally became the Case Method.

(c) Limited Application of the Methods

In Hong Kong, CAPWAP and Case method are commonly used. However, they are normally

restricted to the detection of pile defects, monitoring of the driving stress and measuring the

hammer efficiency, despite dynamic pile testing methods have been extensively investigated

in the last few decades and are now widely used round the world in evaluating pile capacity.

This is because many practicing engineers consider that dynamic method is “inaccurate” and

involves black box manipulation in the prediction. Others consider that accurate prediction

requires proper selection of damping factors and quake values but correlation studies between

dynamic and static records are limited. Thus, static load test is still the only acceptable

method to verify pile capacity and around 1% of piles have to be selected for this purpose.

(iv) Failure Criteria for Static Load Test

(a) Loading test and acceptance criteria

In engineering application, pile failure is considered to occur long before reaching ultimate

load because settlement has exceeded the tolerable limit of structure above. Hirany and

Kulhawy (1989) mentioned that there were at least 41 methods for its determination and

called them the “interpreted failure load”. They were based on some sort of philosophy or

mathematical rules to generate repeatable value. Examples are Davisson (1972) and FDOT

(1999) based on specified settlement limit, and Brinch Hansen (1963), De Beer (1967), Chin

(1970), Fuller and Hoy (1970) and Butler and Hoy (1977) based on graphical construction.

In Hong Kong, two sets of acceptance criteria for static pile loading tests are in use:

Figure 17 – Basic principle of dynamic pile testing


(1) 90% criterion proposed by Brinch Hansen (1963) adopted in the General Specification

for Civil Engineering Works (Hong Kong Government, 1992), mainly used for public

developments,

(2) the acceptance criteria given in the CoPF (BD, 2004).

While the acceptance criteria given in the CoPF look similar to the off-set’ limit given by

Davisson (1972), there are differences in the acceptance criteria as well as the loading

procedures between the two methods. Davisson developed the ‘off-set’ limit by comparing

the pile capacities derived from wave equation analyses with that by static loading tests. The

‘quick test’ procedure was adopted in the static loading tests, which is different from the

maintained loaded tests commonly used in Hong Kong. Davisson (1972) suggested that the

‘off-set’ limit should not be directly used to interpret the failure load of the pile for any

loading procedures that included load increments held for a period longer than an hour.

The acceptance criteria given in the CoPF (BD, 2004) were introduced in 1990 when the

Building Regulations were revamped. In addition to a ‘off-set’ limit, a residual settlement was

specified in a Practice Note. This residual settlement limit has evolved over the years and it

can now be taken as D/120 + 4 mm or 25 % of the maximum pile head settlement measured

during the tests, whichever is larger. This 25% came from the Buildings Department when a

set of test results in BD’s record was studied when the CoPF was being finalised.

(b) Effect of Residual Settlement after Loading

Test results indicate that residual settlement can indicate some degree of soil yielding at pile

base. However, the measured value can be severely affected by the movement of the reference

beam used for dial gauge measurement. Furthermore, Fellenius (2002) found that downward

force acting on its surface due to side resistance will prevent it from rebounding when a pile is

unloaded, but this lock-in stress is difficult to determine accurately. In local practice,

foundation contractors are tempted to drive piles deeper into the firm stratum in order to

alleviate their risks of not meeting the pile loading test acceptance criteria. As a result, piles

will still tend to be over-driven, leading not only to wastage of material, but also more lock-in

stress and possible damage of the pile because of sustained hard driving. This is certainly an

area where more research would bring tremendous benefit to the local practice.

MOVING FORWARD

In reality, foundations rarely suffer from sudden bearing failure without signs of excessive

settlement. The serviceability of the structure will become questionable long before its

collapse due to bearing failure of the foundation. This is a more critical criterion that ought to

be taken care of in the foundation design. Such a requirement transpires to the need for a

better understanding of soil-structure interaction for single piles and piles in groups. This

design concept has received much attention in overseas practices, e.g. it has been embedded in

design code published by the Federal Highways Administration of the United States.

Unfortunately, there are obvious omissions in the technological advancement to improve our

capability in predicting soil-structure interactions and, hence, the settlement of foundations.

The adoption of simplified design approach common in local practice may partly contribute to

this shortcoming. In recent years, some instrumented pile loading tests were conducted to

support rational design approach adopted in a few infrastructure projects. While these are

positive steps that have resulted in some improvements to the design practice (GEO, 2005),

designs using rational design approach remain a small proportion. Transformation of local


design practice to adopt limit state design approach or even a critical review of current safety

factors cannot be fruitful without the ability of ensuring the serviceability of the foundations.

The industry needs to invest more efforts in these areas by adopting rational design approach

in foundation. High-quality instrumented pile loading tests can improve our understanding on

the load-transfer mechanism between the pile and the ground, as well as the deformation

properties of the ground.

Bored Pile

(i) End bearing capacity

An alternative method for determining allowable bearing pressure is given in GEO (2005).

In this method, the rock mass is characterized by the rock mass rating (RMR) classification

system by Bieniawski (1989). The RMR classification system requires the assessment of the

uniaxial compressive strength of the materials, the rock quality designation (RQD), the

spacing of joints and conditions and orientation of the discontinuities. It is more rational, as it

examines in more details the infilling between the joints and the conditions of the joint

surface. The RMR is also applicable to sedimentary and metamorphic rocks, except for

marble that have been affected by dissolution.

In assessing the data of the West Rail pile loading test, Hill et al (2000) also promoted the use

of a method of deducing rock stiffness based on the RMR. The derivation of the RMR

values from rock cores extruded from the predrilling boreholes is described in detailed by Hill

& Wallace (2001).

In computing the RMR values, Kulhawy & Prakoso (1999) and Littlechild et al (2000)

recognised that two basic parameters in the original RMR (i.e. groundwater and orientation of

discontinuity) are not relevant to foundation problems and they proposed fixed values for

these two parameters. These recommendations are followed in the marking scheme given in

GEO (2005). The individual rating for the joint spacing has been adjusted in Bieniawski

(1989) and the effect of the double counting the joint spacing in RQD has been reduced. On

the other hand, RQD could be very sensitive to joint spacing, particularly when these are

around 100 mm apart.

The correlation between RMR values and deformation modulus of the rock mass is

established based on local pile loading tests conducted in recent years. The allowable

bearing pressure of a rock mass can be assessed by specifying an acceptable settlement using

the rock mass modulus determined from the RMR values. The allowable bearing pressures

recommended in GEO (2005) are established based on a settlement limit generally less than

0.5% of the pile base diameter for RMR > 40 (see Figure 18). Designers can adopt a higher

bearing pressure based on other acceptable settlement consideration. This method offers a

rational basis for assessing the performance of the foundations.

Although practitioners have limited experience in determining allowable bearing pressure

based on RMR method, its use should be encouraged as it has certain advantages over the

presumed bearing pressures commonly used by local practitioners.


(ii) Shaft resistance of rock socket

The capacities of rock sockets in relation to the rock strength were documented

comprehensively by Hill et al (2000). They reported several full-scale loading tests for the

West Rail of Kowloon-Canton Railway Corporation (KCRC) in which Osterburg cells were

used at the base of the pile. In this form of testing, the stresses developed in the pile base

and the rock socket can be acquired separately. From these tests, the ductile behaviour of the

rock socket was illustrated. Similar strain-hardening behaviour was also reported by Zhan &

Yin (2000) for bored piles socketed in volcanic rocks. Ng et al (2001) reviewed from

various publications the pile loading tests conducted in bored piles socketed in rock and came

to similar conclusions. Such behaviour is important in allowing the mobilisation of shaft

resistance in carrying foundation loads together with the end bearing resistance. The pile

capacity can simply be determined by combining the resistance along the shaft and at the base.

The local experience indicated that shaft resistance could be mobilised in rock sockets longer

than three times the pile diameter (maximum ratio tested so far is 2.92) (Figure 19).

However, it should also be recognised that the pile database available includes rock sockets

formed by RCD only and the movements between the test piles and the rock socket were

generally less than 1% of the pile base diameter. In order to provide an effective alternative

14.5

12.5

10

7.5

5

3 3

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70 80 90 100

Rock Mass Rating (RMR)

All

ow

abl

e

Be

ari

ng

Pre

Bearing pressure that

can induce settlement

of about 1% of the

pile diameter at the pile base.

Recommended

allowable bearing

pressure

Mobilized Bearing Pressure, qa (MPa)

88

Legend:

● = Bearing pressure substantially mobilised

∆ = Degree of mobilisation of bearing pressure unknown

Figure 18 – Determination of Allowable Bearing Pressure based on Rock Mass Rating (RMR)


to practitioners to opt for the use of combined socket and end bearing, instead of the use of

bell-out, a further relaxation of the allowable ratio of socket length to pile diameter is

considered justified, based on local test data.

(iii) Advances in High Performance Bored Pile Concrete

The quality of a bored pile is governed by construction process and properties of concrete.

The two factors are in turn intimately linked. In the past, performance of tremie concrete for

bored piles was controlled mainly by one single attribute, namely, workability or slump. To

meet the ever challenging technical and environmental constraints, high performance PFA

concretes with slump values >175mm are being specified and routinely used in bored pile

construction in Hong Kong. To be suitable for construction of bored pile, high performance

concretes must:

• be self-compacting

• have good workability

• have good slump retention (that is, to remain fluid during the course of concreting)

• be resistant to segregation

• have good mechanical strength

The ability for concrete to retain its slump is particularly important for construction of large

piles. Piles up to 3m diameter and in excess of 70m length are now routinely constructed in

100

1000

10000

1 10 100 1000

Uniaxial Compressive Strength of Rock, q Uniaxial Compressive Strength of Rock, σc (MPa)

Mobilized Shaft Resistance in Rock, τ (kPa)

τs = 0.2 σc 0.5

Legend:

● = Shaft resistance substantially mobilised

∆ = Degree of mobilisation of shaft resistance unknown

Figure 19 – Mobilized Shaft Resistance in Rock Sockets


build-up areas of Hong Kong. Often large quantity (>350m3) of concrete is required to be

poured over a relatively long period of up to 12 hours and more. Pre-mature densification of

concrete may cause blockage of tremie pipe and worse may prevent the extraction of tremie

pipe, leading to cold joint in the pile.

The latest advancement in high performance concrete technology plays an important role in

resolving some of issues concerning defects in bored piles. With the use of third generation

additives and optimization of PFA content, concrete mixes can be designed to suit individual

site condition and other environmental and logistical constraints. For instance, in the Hong

Kong Shenzhen Western Corridor project (Hong Kong section), the whole process of concrete

delivery from the batching plant to the marine piling area followed by pouring into the pile

often took up to 5 hours to complete. Bored pile concrete mixes were specially formulated to

enable the concrete to retain its flow properties to allow concreting to be carried out over an

extended period of time.

(iv) Use of Grout to Prevent Segregated Concrete

Another significant advance in concrete pouring is the use of cement grout prior to charging

of concrete into the pile bore. The precise mechanism of the grout-water-concrete

interaction during charging is not yet fully understood. One school of thought is that the

grout could be acting as a barrier, pushes the water away as the first load of concrete advances

down the tremie pipe, thereby reducing the likelihood of the concrete being “washed-out” by

the water ahead. The use of cement grout has now gained wide acceptance by the piling

industry. Although it is still not possible to eliminate toe imperfection altogether using all

the advance techniques, it is noted that there has been a marked drop in the percentage in

recent years.

(v) Prescriptive Approach to Deal with Defects

Imperfection and defects in bored piles are sometimes unavoidable even with the use of all

the advanced techniques available. The fact that defects exist in a bored pile does not

necessarily compromise the load-carrying capability of the pile in some cases. The key is to

quantify the degree of imperfection and specify appropriate measure to rectify the defect to

bring the pile back to its original design standard.

In year 2001, requested by the Hong Kong Construction Association, Arup carried out a study

of bored pile interface acceptance criteria. In the study, over 200 piling case histories were

reviewed and the toe imperfections were categorized into different thickness of unbound

aggregates and soil inclusions. A series of compression tests was then carried out in the

laboratory to investigate the compressibility of different interface materials. The study can

be used to provide a basis for prescriptive approach in dealing with defects in bored piles.

In fast track piling projects, particularly projects with large number of piles, it is a good idea

to adopt a project specific prescriptive approach to resolve the issue of defects. In this

approach, a checklist summarizes the defects with different severities, their corresponding

investigative regime and remedial measures. An example of a typical checklist is shown in

Figure 20. Items and actions in the checklist would be pre-approved by the Engineer. The

checklist provides a list of actions that are clear to both the Contractor and Engineer. In the

event of a defect occurring, the Contractor would carry out the appropriate remedial action in

accordance with the approved checklist. The prescriptive approach facilitates defects to be

rectified in a timely manner and minimize the impact to the overall construction programme.


The prescriptive approach had been used successfully in several major infrastructure projects

in Hong Kong, such as Deep Bay Link (Northern Section) and Hong Kong-Shenzhen Western

Corridor.

Driven Piles

(i) Residual Settlement on Pile Loading Test

Some questions remain on the imposition of the limit in residual settlement in a static pile

loading tests. The rationale for this requirement has not been clearly laid out to the profession.

If this is included as a check for limiting the differential settlement, it is more appropriate to

specify it at the working load. It has also been suggested that limiting the residual settlement

could limit the creep settlement accumulated from cyclic loadings, e.g. wind load. If this is

the intention, it would be more appropriate to include in the test procedure cyclic loadings at

the appropriate load magnitude.

There are few occasions where piles are reported to have failed the criterion on residual

settlement while satisfying the maximum pile head settlement. Practitioners should be

encouraged to investigate the reasons of such non-compliance. The recovery of the pile head

settlement may be restricted by the ‘locked-in’ stress in soils, as a result of reversal of shaft

resistance upon removal of test load (eg Fraser & Ng, 1990; Fellenius, 2002). Other

observation may relate to the hard-driving of the piles which may damage the pile toe. This

could cause plastic deformation of the steel section and larger residual settlement in static

loading test. The finding of these investigations should enable the industry to move forward in

improving the acceptance criteria for local practice.

(ii) “Out of Range” of Final Set Table

In typical final set table in Hong Kong, the allowable set is limited to a range between 25 and

50mm per last 10 blows unless the piles are founded on rock of which it is specified to 10mm

per last 10 blows. The purpose of Philcox (1962) to specify this lower limit of 25mm per 10

blows for piles on soils is to prevent damage of pile due to compressive stress reflected at pile

toe because concrete piles were common at that time. In contrary, the introduction of an upper

limit of 50 mm per 10 blows is to prevent heavy hammer delivering excessive compression

Interface Soft

Layer Thickness

Investigation Remedial

Works/ Proposal

Further

Investigation

Remedial

Works/Proposal

S ≤ 100 N/A Flush clean +

normal grout

N/A N/A

100 < S ≤ 150 N/A Flush clean +

pressure grout

N/A

Sonic test (Fan

shape) with

satisfactory

results

Flush clean +

pressure grout

N/A N/A

coring for

second hole

N/A

S ≤ 100 Flush clean +

normal grout

100 < S ≤ 150 Flush clean +

pressure grout

150 < S ≤ 200

Unsatisfactory

results

150 < S ≤ 200 Pressure jet

clean + pressure

grout

S > 200 Further investigation + submit remedial proposal

Figure 20 - Example of Defect & Remedial Measures

Bored pile

drillhole

Soft

materials


stress to pile head at impact. Others consider this as a mean to minimize settlement which is

also not logical. This rule has always been strictly applied on site and piles set out of this

range will be rejected. However, it is perhaps time to re-visit this requirement because set

values exceeding this range seldom lead to pile damage nowadays because steel piles are

predominantly used in Hong Kong now. Furthermore, the driving stress of a pile can be

estimated using wave equations or measured directly using strain gauges at driving.

(iii) Traditional Pile Driving Formulae or Wave Equations?

There have always been debates between the use between Hiley formula and wave equations.

Back to 1940, Cummings was one of the first to describe the weakness of conventional pile

driving formulae. Since then many engineers have objected the use of them. Terzaghi (1942)

commented that pile driving formulae continued to enjoy great popularity among practising

engineers because these formulae reduced the design of pile foundations to a very simple

procedure. However, the price one pays for this artificial simplification is very high.

Tomlinson (2000) also remarked that many pile driving formulae gave different prediction for

the same conditions. There should not be so many pile driving formulae if their basis are

theoretically sound.

It is unreasonable to continue using drop hammers in final setting of piles after diesel

hammers have been abandoned. However, the use of hydraulic hammers in final setting

requires measurement of energy transfer to pile head, which indeed involves wave equation

analysis. It is therefore logical to seek for further development in the use of the wave equation

approach but at the same time, one should also be mindful in avoiding similar trails to the

original development of the many forms of Pile Driving Formulae.

(iv) Research is the Clue and Application is the Goal

The load transfer process in driven pile is a very complex phenomenon. Shear action to the

particulate soil structure at pile-soil interface leads to spatial and temporal variation in

pile-soil response along pile shaft and at its base during driving, after installation and at

loading. As commented by Randolph (2003), pile capacity is difficult to predict and it is

unable to estimate in many soil types more accurately than 30%. Despite enormous scientific

developments have been achieved in the understanding of the driving process and estimating

of pile capacity, there are aspects still relying on empirical correlation. Furthermore, most

research works are based on model tests or full-scale piles installed in clays or siliceous sands

and the piles installed on land are normally less than 40m long. But in Hong Kong, many

piles installed in reclaimed lands exceed 40m and found on saprolites. Their axial

load-transfer behaviour, shaft resistance and capacity can be different from the current

understanding of pile-soil interaction based on existing database with given limits on pile

geometry, installation method and soil condition. Therefore, focused research and further

fully-instrumented pile loading tests in relation to the local soil and driving conditions should

be invested so as to improve the existing HKCA formulae and determine the appropriate

parameters for wave equations. For the time being, Hiley formula using drop hammers can be

maintained but definitely cease to use when the improved HKCA formulae and/or wave

equations are widely accepted.

CONCLUSIONS

Piling design practices in Hong Kong has been through a time during which international

approaches were further simplified, quite often become empirical, and inevitably trading off

against cost-effectiveness in the process. There have been signs of clients willing to seek for


cost–effective solutions by investing in full-scale testing. Solutions are also emerging for

situations where conventional design and construction are beyond the practical limits, for

example the shaft-grouting solution at Kowloon Station package 7 (Chan et al, 2004). There

are also other techniques that have been in use in other countries that could also be suitable to

use in Hong Kong but have yet been explored (eg base grouting of bored piles in granular

founding materials). All these call for rational design methods and construction

technologies that are not well-covered in existing local guidelines.

This paper illustrates a few salient design and construction issues that are currently in practice

and makes an attempt to illustrate the need to change our approaches and be open to more

rational methods when the opportunities arise. It is the authors’ view that local governing

bodies should establish guidelines to good practices but should also provide room for

innovations.

REFERENCES

BD (2004), Code of Practice for Foundations, Buildings Department, Hong Kong SAR

Government.

Brinch Hansen, J. (1963), “Discussion on Hyperbolic Stress-Strain Response: Cohesive

Soils”, Journal for Soils Mechanics and Foundation Engineering, ASCE, Vol. 89(4),

pp. 241-242.

Broms, B.B. and Lim, P.C. (1988), “A Simple Pile Driving Formula Based on Stress Wave

Measurements”, Proc. of the Third International Conference on Application of

Stress-Wave Theory to Piles, Ottawa, pp. 591-600.

Butler, H.D. and Hoy, H.E. (1977), Users Manual for the Texas Quick-Load Method for

Foundation Load Testing, Federal Highway Administration, Washington, D.C.

Chin, F.K. (1970), “Estimation of the Ultimate Load of Piles Not Carried to Failure”, Proc.

of the Second Southeast Asian Conference on Soil Engineering, Singapore, Vol. 1, pp.

81-90.

Chan, G., Lui, J, Lam, K., Yin, K.K., Law, C.W., Lau, R., Chan, A. & Hasle, R. (2004),

“Shaft Grouted Friction Barrette Piles for a Super High-rise Building”, Proc. of Hong

Kong Institution of Engineer Structural Division Annual Seminar, pp83-98.

Cummings, A.E. (1940), “Dynamic Pile Driving Formulas”, Journal of the Boston Society

of Civil Engineers, Vol. 23.

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ACKNOWLEDGEMENTS

The comments from the Organizing Committee of the Seminar are gratefully acknowledged.

This paper is published with the permission of the Head of the Geotechnical Engineering

Office and the Director of Civil Engineering and Development, Government of the Hong

Kong Special Administrative Region.

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FOUNDATION DESIGN & CONSTRUCTION IN HONG KONG – PRESENT & BEYOND, Daman Lee HK-Taiwan 2005

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