1 INTRODUCTION

The project (Photo 1 and Photo 2) is to provide a public library and an indoor recreation centre to meet the

cultural and recreational demand of a population of 271 000 in Tin Shui Wai, Hong Kong. The completed

building also signifies the idea of a town hall with a piazza as a place for gathering. The whole building is

divided into two parts: a public library and an indoor recreation centre. The site (Figure 1), which is of a

trapezoidal shape on plan with a length varying from about 89m to 105m and with a width of about 55m, is

located at the junction of Tin Fuk Road and Ping Ha Road. The site is sloping gently downwards from

+7.9mPD at the northwestern tip to +6.1 mPD at the southeastern end. The proposed development (Figure 2)

is a reinforced concrete structure, with steel trusses on 2/F of clear span 25m×25m, on 3/F of clear span

25m×25m and on roof of clear span 44m×42m, and with a basement of maximum excavated depth of 7m. The

construction works commenced in April 2009, and the indoor recreation centre has been in use since end-2011

and the new public library has been opened to the public in end-2012. The engineering and architectural

designs were awarded respectively Structural Excellence Grand Award 2012 of the Hong Kong Institution of

Engineers and Medal of the Year 2012 of the Hong Kong Institution of Architects.

Photo 1. Outlook of Completed building

Photo 2. Front Elevation of Completed building

ABSTRACT

This paper will present a trail that demonstrates the process of the planning, design and

construction of the foundation and basement in a very difficult ground with cavernous karst in

Scheduled Area No. 2 in Tin Shui Wai, Hong Kong with thorough considerations of technical

issues and alternative solutions. In this project, an appraisal was carried out to determine the

effect of geological constraints on the foundation design and to assess the suitability of different

types of foundation systems for the project with a basement of 7m depth. Sinkhole hazard is

another major challenge to engineers when founding a structure and/or carrying out excavation in

cavernous karst area. Formation of a sinkhole usually leads to a sudden depression on the ground

surface caused by the collapse of cavities. This paper will further describe how the sinkhole

hazard was investigated and/or dealt with at the planning, design and construction stages of this

project.

Foundation Design and Construction in Cavernous Karst : a Local Experience

K.P. YIM

Greg Wong & Associates Ltd

C.Y. KAN, C.T. WONG & M.K. LEUNG Architectural Services Department, Hong Kong

Figure 1 Site location plan Figure 2 Internal arrangement of proposed development

Figure 3 Geological Section across the Site

Figure 4

(Source: Adapted from GEO 2007)

2 SITE GEOLOGY

2.1 Fundamental soil mechanics theories

The development lies on an area designated by Geotechnical Engineering Office (“GEO”) of Hong Kong SAR

Government as “Scheduled Area No. 2”, in which geology is very complex with cavernous karst. Scheduled

Area No. 2 is an elongate curved valley (Figure 4) with a northwesterly trend (Darigo 1989; Frost 1989), with

granitic and volcanic rocks forming the high relief topography on either side of the valley, weathered

metasedimentary rocks forming low hills adjacent to the granitic rock and thrust and reverse faults running in

northeast direction at about 60o, which are interrupted by a series of northwest to southeast cross-faults. The

bedrock underlying the superficial deposit includes marble, metasiltstone, sandstone, and conglomerate of the

Carboniferous Yuen Long Formation and Lok Ma Chau Formation (where large cavities may be found), and

volcanic rocks of Jurassic Repulse Bay Volcanic Group. A large portion of Tin Shui Wai area is underlain by

volcaniclastic breccias, and only a few areas are underlain by the Yuen Long Formation (Darigo 1989; Frost

1989; and Lai and Tang 2006). However, the site in this project falls squarely within one of such a few areas,

and the underlying rock is pure white marble belonging to Ma Tin Member of the Yuen Long Formation. Ma

Tin Member consists of pure marble bedrock of virtually pure calcium carbonate, which renders it susceptible

to dissolution (Frost 1989). Cavities of maximum height of 24.5m have been recorded (Lai 2004).

3 GROUND INVESTIGATION AND GEOPHYSICAL SURVEY

The site is located at a karstic formation with extensive cavities, which imposes geological constraints on the

choice and design of foundation. Firstly, excessive stresses on the roof of the cavities due to the foundation

load may cause the collapse of cavities, especially when the rock thickness above the roof of the cavities is

thin. Secondly, an increase in the overburden pressure during dewatering above the cavities may trigger

sinkhole collapse. Extensive ground investigation (including the sinking of 50 nos. of drillholes with full

geological logging and installation of 9 nos. of standpipe piezometers) have also been carried out to avoid

overstress of underneath cavities and to tackle the sinkhole hazard. To locate the spatial extent of cavities in a

karst site, conventional method by sinking drillholes would be very time consuming and expensive (Morris et

al 2005). In particular, sinking drillholes can only provide information on the “vertical height” of the cavities

but not the “spatial extent” of the cavities. Cross-hole seismic geophysical method was employed to delineate

the spatial extent of the cavities. 13 cased drillholes with depths between approximately 25m and 43m and

eight sections were selected to conduct the tomographic survey. A vertical spacing of the sparker source

(which serves as the emitters) at 1m c/c was chosen, and a 24-channel hydrophone spaced at 1m c/c was used

at the receiver side. About 16,000 travel times were determined and used for tomographic inversion. However,

the method of tomographic survey is limited in spatial resolution depending on the drillhole distances and the

geometrical density of ray paths. Cavities with smaller dimensions (e.g. less than 1m or 1.5m) or thin rock

covering may not have clear identification. Figure 5 shows one of the seismic tomograms superimposed with

the results of drillholes obtained in this site.

With extensive ground and geophysical investigation, the underground conditions underlying the

development have been defined. The site was reclaimed from old low-lying agricultural land in the 1980s.

The ground level varied between +7.9mPD and +6.1mPD. Geologically, there are 10m to 25m thick

superficial deposits of fill (about 5m to 7m thick) and alluvium overlying the decomposed sedimentary,

metamorphosed and granitic rocks (Figure 3). Three fault lines run across the site. Rockhead at most parts of

the site could be found at 20m to 25m below ground. However, in the eastern part of the site, cavities were

found in marble. A zone of deeper rockhead was found at the north-eastern corner of the site and the rockhead

may be as deep as 75m below the existing ground level. Infilled cavities of “height” up to 11m were found

mainly in the marble on the north-eastern part of the site, and the bottom of these cavities is at about 55m

below the existing ground level. Besides, there are three nos. of faults running across the site.

Figure 5 Tomographic image of the site superimposed with results of drillholes

3 PROJECT PLANNING AND FOUNDATION CONSIDERATION

In constructing an annex to TWG Hospitals Kwok Yat Wai College adjacent to this site, many of the driven

H-piles were damaged from hard-driving through the karstic layer and pile slippage along sloping bedrock

(Sze 2006). With a clear geological and hydrogeological model showing especially the spatial extent and

distribution of the cavities underneath the proposed development, the layout of the building was re-planned at

the early stage of the project, such that the proposed structure was shifted away from the northeastern portion

of the site as far as possible (Figure 3), and hence avoiding potential difficulties and reducing cost and time in

constructing the foundation works. This approach (termed by Lee and Ng (2004) as the “first rule” in the

foundation design in cavernous karst area) has been employed by in a project in Tin Shui Wai and a project in

Ma On Shan (Fletcher et al 2004; Wightman and Lai 2006).

Commonly adopted foundation systems in cavernous karst areas in Hong Kong include: shallow

foundation; floating piled-foundation (e.g. Pakt-in-Place (PIP) piles); driven steel H-piles; and large diameter

bored piles or rock-socketted steel H-piles. For this project, shallow foundation in the form of pad or raft

footing was ruled out, as founding the building on either the fill or alluvium layers will result in excessive

settlement in the building. For a floating piled-foundation using PIP piles, the karst surface should lie at deep

level so that the stress induced at the pile tips will not overstress the roof of cavities (Wong 2003). In this

project, the founding level of the pile tips is just at a few metres above the karst surface at some locations, and

the thin layer of intact rock above the cavities may cause the roof over cavities to collapse. Driven steel H-pile

with or without pre-boring is one of the most popular options for piles in cavernous karst area with due

allowance on pile redundancy for uncertainties. However, in this project, driving steel H piles without pre-

boring was not technically feasible as founding H-piles on top of karst surface might again possibility cause

the collapse of the roof over cavities within the karst. Driving steel H-piles with pre-boring was also

undesirable because of the environmental sensitivity of the site, with three secondary schools, a West Rail

Station and a Light Rail adjacent to the site.

Large diameter bored pile socketted into rock was therefore adopted, especially due to relatively shallow

rockhead (about 20m to 25m below ground) at over three-quarters of the site. The base of the socket was

located at a depth where there are no cavities existing within the zone below the pile base to a depth equal to

the diameter of the pile base. Karst area also contains steeply inclined bedrock surfaces inducing additional

stress from one pile onto the adjacent piles and/or the adjacent cavities. In designing the length of rock socket

of pile, the founding level of rock socket was required to pass through the adverse joints, and to follow the

commonly adopted rule of thumb in Hong Kong with an “angle of stress dispersion” between 30o and 45o to

the vertical (Wai 1991). Similar to large diameter bored pile socketted into rock, rock-socketted steel H-piles

could also be a solution. However, for cavernous karst area, such foundation system may have difficulties in

pre-boring and forming rock sockets, as it is difficult for the casing shoe to bring the casing through the

cavities and to form rock socket on the sound bedrock, except that a secondary permanent casing is left for

pile installation; but this would lead to a high construction cost of foundation. Indeed, there is no published

case on the successful employment of rock-socketted steel H-piles in cavernous karst area in Hong Kong.

Hence, a compromise scheme was to adopt rock-socketted steel H-piles only at areas with cavity-free marble

and/or granitic bearing rock underneath, whereas large diameter bored piles are used at areas with cavernous

marble bedrock. The final adopted foundation design employed a total of 50 nos. of large diameter bored piles

with either 1.5m or 2m diameter, and 99 nos. of rock-socketted steel H-piles. With the adoption of two

different piling systems, the design of the pile caps and superstructure was also required to cater for the

possibility of differential settlement due to the difference in the elastic shortening of reinforced concrete bored

piles and structural steel rock-socketted steel H-pilesIn order to develop a simple and workable design

equation for friction piles in soil especially including the effect of the pile construction method on the K value

and the variation of tanδ with depth for each construction method, ArchSD has been carrying out loading tests

on a number of meticulously instrumented friction piles (nominal diameter 300-600 mm or more) with

different construction methods since the mid 1990s. Among these piles, at least four different construction

methods have been adopted. The typical details of these piles are as summarised in Table 1, and the geology

and soil strata of various sites (together with the SPT-N values versus depths) are summarised in Table 2.

4 PILING AND BASEMENT CONSTRUCTION WORKS

In constructing the rock-socketed piles, the presence of karst cavities may result in a large amount of

overbreak, leading to concrete loss from pile and hence affecting the pile integrity. Different construction

methods (e.g. filling with lean concrete for cavities up to 1.5m and installing permanent secondary corrugated

liner for cavities over 1.5m in Figure 6) were therefore adopted in this project, such that no excessive ground

settlement or sinkhole collapse would be induced. Moreover, constructing a 7m-deep basement and removing

a total of 18,000m3 spoil soil together with dewatering of up to a maximum 3m drawdown in this project is a

challenging task for every engineer, particularly when the site lies adjacent to the West Rail, the Light Rail

and a school. A cofferdam of sheet piles around the perimeter of the basement was driven to a maximum

depth of about 13m below the existing ground level down to the -8.0mPD, which provides an effective cut-off

for the drawdown. Three rows of waling were installed, and raked shoring was employed (Photo 3).

Moreover, the excavation was divided into two zones, and each zone was carried out in two stages, so that

when dewatering commenced on site, the increase in the effective stress at the karst surface would have

partially been compensated by the removal of overburden soil. During the construction works, all

measurements during the construction of accumulative distortion, total movement, vertical or horizontal

pressure on the structures and peak particle velocity induced on the structures were closely monitored with 15

nos. of settlement markers, 5 nos. of tilting check points, 5 nos. of check points and 15 nos. of standpipe, such

that the effects of construction works on the nearby railway structures were found to be within the acceptable

limits. The piling works was completed in May 2009, and the superstructure works commenced in June 2009.

No sinkhole subsidence was noted throughout the construction period.

(a) Filling with lean concrete (b) Installing corrugated liner

Figure 6 Methods of constructing large diameter bored piles through cavities

Photo 3 Excavation and lateral support system during basement construction

5 EPILOGUE

From the planning, design and construction of this project in cavernous karst area, the following concluding

remarks could be made:

(a) Appropriate ground investigation is crucial for the success of sitting of building, design and construction of

foundation for this type of project with very difficult ground condition. Early recognition of the geological

constraints at cavernous karst area can permit the subsequent detailed ground investigations to be targeted

to obtain specific information for the design. Mapping the particular site in the geological maps is also vital

for the planning of ground investigation, and in the choice and design of the foundation systems. If large

cavities are likely to be present (e.g. in Ma Tin Member of the Yuen Long Formation), geophysical survey

method using cross-hole seismic tomography, wherever affordable, may be used, which can provide more

information on the spatial extent of cavities and hence removing some uncertainty or risk of foundation

design and construction. This will obviously not only improve the design of foundation, but also benefit the

project on cost and time.

(b) With more comprehensive information on the complicated geology underneath the site, the building layout

can be redesigned to minimize costs and construction time. Similarly, alternative foundation systems are

available. Floating piles or shallow foundation in general are most economical if the loadings are not high.

If, however, cavities are present at high level near ground surface, rock-socketted large diameter bored

piles passing through cavities are recommended.

(c) Roofs over cavities within the karst may collapse due to the increase in the stress and construction

activities, and it is vital for engineer to study carefully the increase in the stress induced by the foundation

and the dewatering process.

ACKNOWLEDGEMENT

The authors would like to record their thanks to the Director of Architectural Services, Hong Kong SAR

Government for his kind permission in publishing this paper.

REFERENCES

Darigo, N J (1989), “A Volcanic Breccia in the Buried Karst of Hong Kong”, Proceedings of the Third Multidisciplinary

Conference on Sinkholes and the Engineering and Environmental Impacts of Karst, St Petersberg, Florida, 2-4

October 1989, pp. 225-32.

Fletcher, C J N, Pang, V P Y and Yim, K P (2004), “Geological Modeling and Foundation Appraisal for Project Planning

of Development Sites in Tin Shui Wai, Northwestern New Territories”, Proceedings of the Conference on Foundation

Practice in Hong Kong, Hong Kong, 4 September 2004, pp. F1-14.

Frost, D V (1989), “Palaeokarst of Yuen Long, North West New Territories, Hong Kong”, Proceedings of the Third

Multidisciplinary Conference on Sinkholes and the Engineering and Environmental Impacts of Karst, St Petersberg,

Florida, 2-4 October 1989, pp 239-46.

GEO (2007), GEO Publication No. 1/2007: Engineering Geological Practice in Hong Kong (Hong Kong: Geotechnical

Engineering Office).

Lai, K W (2004), “The Influence Factors of Buried Karst upon the Foundation Design of Hong Kong”, Proceedings of

the Conference on Foundation Practice in Hong Kong, Hong Kong, 4 September 2004, pp. I1-10.

Lai, K W and Tang, D L K (2006), “The Characteristics of Marble and Marble-Bearing Rocks in Hong Kong and Their

Influence on Foundation Engineer”, Proceedings of the Seminar on Geotechnical Works in Karst in South-East Asia,

Hong Kong, 26 August 2006, pp. 79-92.

Lee, D M and Ng, M (2004), “Design Strategies for Deep Foundation in Areas of Marble Formation in Hong Kong”,

Proceedings of the Conference on Foundation Practice in Hong Kong, Hong Kong, 4 September 2004, pp. C1-17.

Morris, V, Gehrig, D and Bryant, T (2005), “Detection of Three-Dimensional Voids in Karstic Ground”, Proceedings of

the Tenth Multidisciplinary Conference, 24-28 September 2005, San Antonio, Texas, pp. 562-71.

Sze, W C (2006). “Case Study for a Driven Steel H-Pile Foundation in the Tin Shui Wai Marble Area”, Proceedings of

the Seminar on Geotechnical Works in Karst in South-East Asia, Hong Kong, 26 August 2006, pp. 155-78.

Wai, M (1991), Technical Note TN 2/91: Checking Procedures and General Requirements for Foundations in Areas

Underlain by Marble (Hong Kong: Geotechnical Control Office).

Wong, H Y (2003), “Design and Construction of Soil Friction Bored Piles in Hong Kong, with Particular Reference to

Marble Areas”, Proceedings of the 23rd

Annual Seminar, Geotechnical Division, The Hong Kong Institution of

Engineers, 9 May 2003, pp. 265-82.