[Forth04] Groundwater and Geotechnical Aspect of Excavation in Hong Kong

8/8/2019 [Forth04] Groundwater and Geotechnical Aspect of Excavation in Hong Kong

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Groundwater and geotechnical aspects of deep

excavations in Hong Kong

R.A. Forth*

School of Civil Engineering and Geosciences, University of Newcastle upon Tyne, Drummond Building, Newcastle upon Tyne NE1 7RU, UK

Received 6 March 2002; accepted 18 September 2003

Abstract

Consideration of groundwater is a key element in almost every construction project. The design of deep excavations for

basements or underground railway station concourses below the water table require that the water pressures are taken into

account. Whilst these can be considered to be hydrostatic in soil, the decreasing permeability of rock with depth and the fact that

groundwater flow is invariably along discrete fractures means that the water pressure is unlikely to be hydrostatic at depth.

Groundwater control for deep excavations can be achieved by a number of methods such as grouting, pumping or structural

walls or a combination of these. For tunnelling projects grouting is extensively used, but the development of sophisticated

tunnelling machines has led in many cases to the demise of compressed air as a means of groundwater control.

D 2003 Published by Elsevier B.V.

Keywords: Permeability; Rock; Groundwater; Dewatering; Settlement

1. Introduction

The measurement of groundwater pressure in soil

and rock is carried out by installing piezometers and

measuring the water pressure whether continuously

or, more commonly, at intervals. Hydraulic piezom-

eters are usually employed in soil but in rock, where

flow is often along discrete discontinuities, thegroundwater pressure is more accurately measured

using pneumatic piezometers.

In Hong Kong, the distinction between soil and

rock in engineering terms is that rock is cored by

rotary drilling using diamond bits and excavated by

percussive methods or blasting. Thus weathered rock

(e.g. completely weathered granite) which is obtained

by Mazier or Triefus sampling tubes with tungsten

carbide bits is considered to be soil for the purposes of

this paper. A typical Hong Kong ground profile

consists of made ground overlying alluvial/colluvium

which in turn overlies weathered rock before fresh

rock is encountered.

For a relatively homogeneous rock type, the dis-continuities tend to become less frequent and tighter

with depth very often leading to extremely low per-

meabilities. It is in fact possible that even at relatively

shallow depths the water pressure reduces to zero. In

many civil engineering projects, water in the rock

need not be a problem in comparison with water in the

overlying soil, unless artesian conditions are encoun-

tered, and measurements are not often undertaken.

Hoek and Bray (1981) addressed the problem

proposing that hydrostatic pressure would buildup to

0013-7952/$ - see front matterD 2003 Published by Elsevier B.V.doi:10.1016/j.enggeo.2003.09.003

* Fax: +44-191-222-6613.

E-mail address: [email protected] (R.A. Forth).

www.elsevier.com/locate/enggeo

Engineering Geology 72 (2004) 253260


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the mid-point of the excavation followed by a decline

to zero at the base of the excavation (Fig. 1). Matson

et al. (1986), working at the North Point, Hong Kong,

Metro concourse, recorded fracture frequency with

depth and showed that the number of fractures per

meter reduced from about five at t he rockhead to

about two at 3035 m depth (Fig. 2). From this, they

considered that the Hoek and Bray approach wasconservative and suggested an alternative pressure

distribution (Fig. 3).

However, in order to check fracture flow pneu-

matic piezometers were installed at specific loca-

tions in the rock, where discontinuities were

present. The head of water measured at these loca-

tions was recorded and found to be in the range

0.2 0.4 of hydrostatic (Morton et al., 1984), thus

confirming Matson et al.s (1986) assumptions. In

fact, the water pressure reduced almost to zero at

about 30 m below ground level. The use of mea-

sured water pressures reduced the required rock

anchor capacity up to 30%, with considerable cost

savings.

The assumption that permeability decreases at

depth is also confirmed by studies in Sweden (Ahl-

Fig. 1. Possible groundwater pressure models for excavated rock

face.

Fig. 2. Fracture frequency versus depth. Fig. 3. Comparison of possible water pressure versus hydrostatic.

R.A. Forth / Engineering Geology 72 (2004) 253260254


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bom et al., 1991) where permeability of the order of

10 10 m/s were measured in crystalline rock (Fig. 4),

similar to the values obtained in the studies of the

proposed nuclear waste repository site at Sellafield

(Chaplow, 1996).

This paper draws on the authors experience of

dewatering projects in Hong Kong, notably on the

construction of the Island Line of the Mass Transit

Railway constructed between 1982 and 1986.

2. Groundwater control

For relatively near surface excavations for typical

civil engineering projects in urban areas, groundwater

control can be achieved by a number of methods such

as grouting, pumping or structural walls, or a combi-

nation of these. Typical ground treatment methods

used in the construction of the Island Line (ISL) of the

Hong Kong Mass Transit Railway (MTR) are illus-

trated in Fig. 5.

For the deep excavations, the diaphragm walls

were very effective when founded in bedrock.

However, where bedrock was deeper than the base

of the excavation grout was injected beneath thewalls. In the context of the ISL project, the grout

was only required to be fully effective for about a

year or two as there were no significant delays in

the programme of construction. However, research

is needed into the performance of the grout in the

longer term where, for example, building pro-

grammes are delayed for technical or, more likely,

financial reasons.

For tunnelling, groundwater control was achieved

by grouting and compressed air. The use of com-

pressed air meant that all site investigation boreholes

and piezometers along the route had to be backfilled

and thoroughly sealed which, of course, prevented the

collection of groundwater level data during the tun-

nelling phase.

Ground treatment designs were based on the soil

conditions for each particular site and were generally

specified as a grout percentage of the soil volume,

and an injection pressure. In most cases, grouting for

groundwater control was two stage: cementbenton-

ite followed by chemical, with grout volumes spec-

ified for each type. Typical volumes for cement

bentonite grout were 5% to 30%, depending on soiltype (fill through to completely weathered granite

as, for example, illustrated in Fig. 5) and for chem-

ical grouts 20% to 40%. Jet Grout piling or Jet

Special Grouting was also employed and may use

greater than 50% grout by volume. The chemical

grouts most commonly used were sodium silicates

with hardener volumes depending on setting times

required.

Grouting pressures were dependent on depth of

grouting and desired permeation rates, and were

usually in excess of overburden or water pressure.Injection methods used varied from site to site and

include, in soil, Tube-a-Manchette, Lag and Jet Spe-

cial grouting (a replacement method). Rock grouting

was usually done using a staged method.

Generally, the soil grouting was effective both as

groundwater control and ground consolidation. The

chemical grouts appear to be able to permeate soils

with up to 10% to 20% passing the 75 Am size,

although this was by no means a general rule, as

the cement bentonite grouts can penetrate the

Fig. 4. Hydraulic conductivity plotted against depth for rock mass

and fracture zones within crystalline rock in southeast Sweden.

Polynomial regression lines have been fitted to the data (redrawn

from Ahlbom et al., 1991).

R.A. Forth / Engineering Geology 72 (2004) 253260 255


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coarser sands and gravel sizes. In most cases, the

soils encountered were able to be effectively

grouted using these two stage mixes. The major

problems, which were encountered, tended to be in

the loose alluvial sands (Cater et al., 1984). These

coarser sands provided resistance to grouting. It is

not known whether this was due to groundwater

flows or excessive grout travel or a chemical effect.

Consequently, they often provided an opening in

the grout curtain allowing water ingress during

excavation or tunnelling. Fig. 6 shows the grout-

penetration ranges in the Hong Kong soils as

determined from experience gained by the MTRC

on the ISL.

3. Case histories

A number of buildings were carefully monitored

before, during and after construction of tunnels and/or

Fig. 5. Ground treatment methods.



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deep excavations. As previously stated the tunnelling

operations carried out under compressed air precluded

the collection of hydrogeological data during this

important phase. For the station concourses, the

dewatering took place simultaneously with the exca-

vation of spoil from within diaphragm walls. Hence, itis not possible to separate ground movements caused

by excavation and those caused by the dewatering

process. Some typical time-settlement plots are in-

cluded for three buildings (Buildings A, B and C as

described below) adjacent to construction activities in

Central Hong Kong (Fig. 7). From this plot, it can be

seen that the total amount of settlement due to

excavation (including dewatering) is significant but

not substantial in comparison with the settlement due

to preliminary works and diaphragm walling.

4. Building descriptions

Building A is a 15-storey reinforced concrete

building constructed in 1956. It measures 29 m by

49 m in plan, is 54 m high (15 storeys), and has a

small basement and lift-pit along the centre of thewestern side (see Forth and Thorley, 1994).

The foundations are 432 mm diameter vibro cast

in situ reinforced concrete piles. The piles are in

groups of 310 with isolated pile caps of 1.31.8

m thickness. Pile spacings are at two pile diame-

ters within the group and groups are generally

spaced 4 7 m apart. The founding level of the

piles is not recorded but is thought to be at the top

of the completely weathered granite strata. The

piles therefore would be founded at 1517 m

Fig. 6. Grout penetration ranges.



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below ground level. The design capacity of the

piles is 61 t.

Building B was constructed in the mid-to-late

1970s (Building Plan approval was given in 1973).

The building is a reinforced concrete framed structure,

31 storeys (110 m) high and measures 74 by 45 m in

plan. It has one basement below ground level (see

Forth and Thorley, 1996).

The foundations consist of 2 m diameter bored

reinforced concrete piles varying in length from 41 to

64 m. The central piles support a 3.2- to 4.1-m-thickraft with the perimeter piles, in groups of 1 to 4,

supporting 2.4- to 4.6-m-thick caps. Founding levels

of the piles vary from 48 mPD on the south side to

60 mPD on the north side. All piles are founded on

what was logged as completely to moderately weath-

ered granite. A trial pile on the site was founded at

an SPT N value reported to be 400.

Building C was also constructed in the mid-

1970s, the Building Plan approval being given in

1972. The building is a reinforced concrete framed

structure, 16 storeys (53 m) in height, and is L-

shaped to fit the site geometry. In plan, the building

is 30.5 m long and from 5 to 9 m wide (see Forth

and Thorley, 1993).

This building, unlike the others considered, has

foundations consisting of 305305 high yield steel

H-piles, varying in length from 31 to 33 m. Pile

founding levels range from 27 to 29 mPD,

which is some 3 to 8 m below the upper surface

of the completely weathered granite strata. The

piles support four separate pile caps from 1.5 to1.9 m thick. A test pile on the site, loaded with

twice the design load of 150 t is reported to have

settled 21 mm. This pile was founded in CWG at

28.7 mPD.

5. Construction activities

In the vicinity of the building, the construction

activities associated with the MTR Island Line con-

Fig. 7. Buildings A, B and C: time-settlement plots.



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sisted of diaphragm walling, deep excavations and

soft-ground tunnelling.

Excavation of the boxes within the 1.2-m-thick

diaphragm walls was done in all instances by topdown methods using temporary strutting in addition to

progressive installation of the permanent floor slabs.

Dewatering for these excavations was by deep wells

within the box, the soil being dewatered to 12 m

below excavation levels.

Readings from an inclinometer in one of the

diaphragm walls adjacent to Building A showed

considerable horizontal movement of the base of

the wall towards the excavation. The bottom of the

inclinometer in this case was founded below the

base of the diaphragm wall in completely weathered

granite. An inward movement of up to 80 mm was

measured (Fig. 8) at the base of the inclinometer.

Based on a theoretical dewatering settlement calcu-

lation a value of over 20 mm settlement is sug-

gested due to drawdown. This would indicate 510

mm settlement due to wall deflection, which repre-sents 0.07 0.15 deflection/settlement ratio. This

compares with 0.17 0.25 reported by Morton et

al. (1980, 1981).

An interesting groundwater effect on this and

other buildings was that of settlement rebound after

construction was completed and the compressed

air employed during adjacent tunnelling was swit-

ched off. This is observed in particular in the time-

settlement record for Buildings A and C (Fig.

7) and was of the order of 10% of the total set-

tlement. Minor architectural damage occurred to

Building A.

Building B settled a total of 20 mm during

excavation and dewatering and, as was the case

for A above, the building tilted towards the exca-

vation by a maximum of 1:4900. This settlement is

surprising given that the building is founded some

10 to 18 m below the diaphragm walls and 20 to 30

m below excavation level. It is considered the most

l ikely reason for the 20 mm of settlement is

dewatering effects causing down drag on the piles

during consolidation of the CWG and superficial

deposits, in conjunction with probably high pileloads. As extensive piezometric data is not available

around the building, a theoretical estimate cannot be

undertaken. It is unlikely, however, to be conclusive

as the pre-construction estimates based on flow-net

analysis showed negligible movement. It is also

possible that inward movement of the diaphragm

walls may have had a similar down drag effect on

the foundations resulting in the 10 to 20 mm of

settlement. In either case, it should be noted that

this degree of movement had no effect on a

building of that size, rigidity and condition. Nodamage occurred to this building.

Building C settled some 22 to 45 mm during

excavation although the direction of tilt was away

from the adjacent site. The reasons for this apparently

unusual behaviour are:

(a) the effect of piling on a nearby site;

(b) ground treatment drilling near the northwest corner

which is not able to be completely separated from

the excavation settlement; andFig. 8. Building A: adjacent diaphragm walling inclinometer

readings.



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(c) the complexity and rigidity of the diaphragm walls

adjacent to the building which may have the effect

of minimising settlement due to wall deflection in

that area.

If the above factors are taken into consideration, it

would seem that the building settled evenly by some

20 to 40 mm due to the excavation. Based on reported

maximum drawdown in the area, the likely dewater-

ing settlement is theoretically of the order of 25 mm

compared to the original estimate of 7 to 10 mm. This

would suggest that the settlement due to wall deflec-

tion would be 0 mm in the northeast corner and up to

10 to 20 mm elsewhere. Inclinometer data is not

available to determine the percentage of lateral move-

ment this represents, although it agrees with the

original estimate of 12 mm based on the structural

design. No damage occurred to this building.

6. Summary

Control of groundwater in urban environments is a

key element in successful construction. For deep

excavations in rock, a realistic estimate of permeabil-

ity has to be made to avoid over-design. In weathered

rock or soft ground conditions, sound constructiontechniques and careful monitoring of ground and

building movement gives confidence to designers in

estimating the effects of groundwater drawdown nec-

essary to construct large excavations.

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[Forth04] Groundwater and Geotechnical Aspect of Excavation in Hong Kong