HongKong-FOUNDATION_DESIGN__CONSTRUCTION_IN_HONGKONG_PRESENT_BEYOND

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					                    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,

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

The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                154
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

The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                155
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.

The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                 156
    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


                             Figure 1 - Pile construction in marble rock


(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
The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                              157
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
                                                       Figure 3 - Imperfection at toe interface
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                     Figure 4 - Segregated concrete
        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.

The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                     158
(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                                     Weak Seam Layer
adequately assessed from a single borehole.                                  Interface Core Test
Due to random nature of rock joints and
limitation of information from one single                              Predrill hole
predrill hole, rock condition revealed in
post construction core test may well differ
from the rock condition encountered in
predrill (see Figure 5).                    Figure 5 - Typical defects associated with
                                                    geological condition
The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                      159
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.



The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong               160
(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:
                                    kh E
            R           =                                                                 (1)
                            S + 0.5 (Cc+Cp+Cq)
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:

                               XE
             R         = S + 0.5 (C +C )                                                     (2)
                                   p  q

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



The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                161
                       1.0



                       0.9
     Blow Efficiency




                       0.8



                       0.7



                       0.6



                                 e = 0.32   e = 0.50        e = 0.70
                       0.5
                             0       20        40            60            80     100
                                                Pile Length (m)
             Figure 6 - Effect of coefficient of restitution and pile length on
                                      blow efficiency

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.




The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong             162
                      1.14


                      1.12


                      1.10
    Ratio of K h /X




                      1.08


                      1.06


                      1.04

                                    Cc=5.0mm,   Cp+Cq=40mm
                      1.02          Cc=5.0mm,   Cp+Cq=50mm
                                    Cc=2.5mm,   Cp+Cq=40mm
                                    Cc=2.5mm,   Cp+Cq=50mm
                      1.00
                             0         10           20            30            40        50
                                             Set Per Last 10 Blows S10 (mm)


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



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

The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                  163
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:

                         QL
             es    =     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:

                         (Qt+αsQs)L
             es    =         AE                                                                  (4)

 where       Q     =     Qt + Qs
             Qt    =     end bearing
             Qs    =     shaft resistance
             αs    =     distribution factor for shaft resistance (FHWA 1992)




The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                   164
      Figure 8 – Pile under axial
      compression load




                                               Figure 9 – Load distribution in pile (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 10 – Bending of piles in weak soils (Tsuen Wan)


The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                     165
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 11 – Pile damaged by 12 tonnes drop hammer from 1.5m height (Kowloon Tong)


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 12 – Proof load test (Tsuen Wan)

The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                166
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          Figure 13 – Damaging effect of pile driven into
carried down by the pile and              heavily jointed rock (Tomlinson 2000)
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

The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                   167
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                  Figure 14 – Stress relaxation of pile driven into
always required to drive
to     very     small     set        heavily jointed rock (Tung Chung)
(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               Figure 15 – Statistical analysis of 4320 piles of
such circumstances, piles
will be driven to refusal            35-80m long (Southeast Kowloon)
(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




The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong       168
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              Figure 16 – Hypothetical allowable sets of piles
the first one to use
one-dimensional                         using different hammer types and driving formulae
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

The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                169
pile test results. These methods were improved and finally became the Case Method.




                     Figure 17 – Basic principle of dynamic pile testing
(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:


The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong              170
(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

The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                 171
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.




The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                 172
                                        30



                                        25
Mobilized Bearing Pressure, qa (MPa)




                  All                                     Bearing pressure that
                                                          can induce settlement
                  ow
                                        20                of about 1% of the
                  abl                                     pile diameter at the
                                                          pile base.
                  e

                  Be 15                                                                                                              14.5

                                                                                                                             12.5
                  ari

                  ng                                                                                                   10
                                        10
                  Pre
                                                                                                         7.5

                                                                                               5                Recommended
                                         5                                                                      allowable bearing
                                             3                                       3
                                                                                                                pressure

                                                                                                                                    88
                                         0
                                             0       10          20        30       40         50        60            70    80             90     100
                                                                                    Rock Mass Rating (RMR)


                                       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)



                                       (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

                                       The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                             173
    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.

                                              10000
Mobilized Shaft Resistance in Rock, τ (kPa)




                                               1000




                                                                                    τs = 0.2 σc 0.5




                                                100
                                                      1                        10                                100                1000

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

                          Legend:
                               ● = Shaft resistance substantially mobilised
                               ∆ = Degree of mobilisation of shaft resistance unknown


                                                          Figure 19 – Mobilized Shaft Resistance in Rock Sockets

    (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

    The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                                             174
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 state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                175
 Interface Soft    Investigation     Remedial            Further        Remedial           Bored pile
Layer Thickness                   Works/ Proposal     Investigation   Works/Proposal
    S ≤ 100            N/A         Flush clean +          N/A             N/A
                                    normal grout
 100 < S ≤ 150         N/A         Flush clean +                           N/A
                                                                                         drillhole
                                   pressure grout
 150 < S ≤ 200    Sonic test (Fan Flush clean +            N/A             N/A
                   shape) with     pressure grout
                   satisfactory
                      results
                  Unsatisfactory                        coring for         N/A
                      results                          second hole
                                                         S ≤ 100       Flush clean +       Soft
                                                                        normal grout       materials
                                                      100 < S ≤ 150    Flush clean +
                                                                       pressure grout
                                                      150 < S ≤ 200      Pressure jet
                                                                      clean + pressure
                                                                            grout
   S > 200        Further investigation + submit remedial proposal

                        Figure 20 - Example of Defect & Remedial Measures


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

The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                   176
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

The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong                177
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.


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                       —


      of Structural Engineers, United Kingdom, Vol. 83, No. 20, pp 25-29.


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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