Deep Shaft Design and Construction

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Design and Construction of Deep Shafts in Hong Kong

Special Administrative Region (SAR), China

L. J. Pakianathan

Mott MacDonald Pte Ltd, Singapore

A. K. L. KwongUniversity of Hong Kong, Hong Kong

D. D. McLearie Montgomery Watson Harza, Hong Kong

W. K. Ng Drainage Services Department, Government of the Hong Kong SAR, Hong Kong

ABSTRACT: Shafts play an essential part in the construction, operation and maintenance of tunnelsand deep underground structures but are rarely given exclusive prominence in technical publications.The aim of this paper is to summarise the experiences gained in Hong Kong SAR during theconstruction of the Harbour Area Treatment Scheme Stage 1 where seventeen shafts were constructed.Their excavated diameters range between 2.5 m to 50 m and at a maximum depth over 150 m these arethe deepest shafts below sea level in Hong Kong. All shafts were located in reclaimed land and inclose proximity to the sea. The upper shafts in soils and weak rock were constructed by diaphragmwalling method and the lower shafts in rock by mainly drilling and blasting. Raise boring and blindshaft drilling methods were also employed. The upper shafts and permanent shaft linings weredesigned using conventional methods and the primary support selection for the lower shaft was basedon Barton’s (1974) ‘Q’ system. Settlement monitoring and inclinometer measurements wereundertaken during excavation to confirm the design assumptions. During construction severaldifficulties were met that had to be overcome. All shafts with the exception of one were successfullyexcavated and completed. This paper addresses the key design and construction issues and thedifficulties that were encountered which may be common for deep shafts constructed in an urbansetting near a coastline.

1 INTRODUCTION

The Harbour Area Treatment Scheme (formerly known as Strategic Sewage Disposal Scheme) is anenvironmental improvement project aimed at cleaning up the waters in the Victoria Harbour. The firststage consists of transfer tunnels linking the primary treatment works located at the southern part of Kowloon and eastern part of Hong Kong Island to a centrally located chemically enhanced treatmentfacility at the Stonecutters Island. A network of 25 km long transfer tunnels were constructed in

bedrock at depths varying between 75 m and 145 m below sea level making these the deepest tunnelsto date below sea level in Hong Kong SAR.

In order to construct the tunnels and to transfer the sewage from the coastal treatment works, 17 shaftswere constructed. The excavated diameter of the shafts varies from 2.5 m to 50 m and they reach downto a maximum depth of over 150 m. The decision to locate the tunnels at a deep level in the rock well

below toe levels of pile foundations, made it possible to construct the tunnels along a most direct aswell as shortest route. It became necessary however to sink deep shafts to link the tunnels to the

ground surface. The functions of the different types of shafts are summarised in Table 1.



Table 1 Function of different types of shafts

Figure 1 shows the location of the shafts and Table 2 shows their particulars. All deep shafts wereexcavated in two parts as upper and lower shaft to suit the operation and the differing groundconditions. The upper shafts were constructed by diaphragm walling or open cut methods through soiland weak rock and the lower shafts were excavated by drilling and blasting, raise boring or blind holedrilling methods in hard rock. Of these, diaphragm walling and drilling & blasting methods were

predominantly used.

The upper section the production shafts were typically 10 m in diameter and reduce to 8.0 m at thelower section by the installation of a 1 m thick toe level ring beam at the rock/soil interface, Figure 2.

The drop shafts are of a larger diameter in the upper section to function as a chamber to remove air from the sewage and to accommodate a bell mouth and vortex drop pipe. These reduce in size toapproximately 2.5 m excavated diameter in the lower section. The drop shafts incorporate a 4.0 m deepsump below the tunnel invert level to accommodate submersible pumps for emergency dewatering.

The land based riser shafts were excavated at the same size as the production shafts to enable theremoval of the tunnel boring machines. The permanent linings for the riser shafts are made of steel

pipes or in-situ concrete. Their internal diameters are identical to those of the tunnels to maintain thesame flow velocity so as to prevent any sedimentation at the shaft bottom.

The pumping station shafts were sized on the basis of the required holding capacity and pumping

arrangement. They are up to 38m deep and are founded in soil. The Stonecutters Island Main PumpingStation (SCIMPS) shaft at 50 m diameter is among the largest in Asia.

Contractors Skanska-Shui On-Balfour Beatty Joint Venture excavated all the production shafts andKwun Tong pumping station shaft under an advance works contract DC/93/10. The value of thiscontract was HK$226 million and the works commenced in August 1994. In parallel another advancedworks contract for the construction of diaphragm walls and soft ground excavation of the SCIMPS andriser shaft was awarded to Leighton Contractors at HK$116 million. The remaining drop and riser shafts were excavated later on as part of the tunnelling works contracts as shown in Table 2.

2 GROUND CONDITIONS

The shafts were constructed through recent Fill, Marine Deposits, Alluvium, Completely to HighlyDecomposed Rock and Bed Rock. The marine deposits are generally soft, greenish grey clays withvariable amounts of silt, sand and shell fragments. The alluvium deposits are generally characterized

by variable firm to stiff silts and silty clays. The completely decomposed rock is generally firm,clayey, sandy Silt with some angular to sub-angular fine to occasional coarse gravel sized rock andquartz fragments. The bedrock is made up of either Granite or Volcanic Tuffs.

Three out of the seventeen shafts were excavated in volcanic tuffs and remainders were in granite. Theground water table was at sea level and the water met in the shafts was saline.

Shaft Type Function

Production shaftsDrop shaftsRiser shafts

Pumping station shafts

to excavate the tunnels and to construct the permanent liningto transfer the sewage from the terminal manholes to the tunnelsto convey the sewage from the tunnels back to the surface installations

to raise the hydraulic head of sewage using submersible pumps



Table 2. The main features of the shaftsUpper Shaft Excavation Lower Shaft Excavation Shaft

No.

Location Function

Ø

(m)

Depth

(m)

Constructio

n method

Contract

No.

Ø

(m)

Depth

(m)

Construction

method

Contract No. Geo

1 Kwai ChungPTW

Drop shaft 13.5 33 Diaphragmwall

DC/93/14 DC/96/20

2.5 107 Raise boring DC/96/20 Gran

2 Tsing Yi PTW Production and

drop shaft

8 3 Open cut DC/93/10 8 137 Drill and blast DC/93/10 Gran

3 StonecuttersIsland STW

Riser shaft 10 63 Diaphragmwall

DC/93/11 8 68 Hydraulichammer;Drill and blast

DC/93/14DC/96/20

Gran

4 StonecuttersIsland STW

Pumping stationshaft

50 38 Diaphragmwall

DC/93/11 - - - -

5 StonecuttersIsland

Outfall productionand drop shaft

10 10 Diaphragmwall

DC/93/10 8 97 Drill and blast DC/93/10 Gran

6A &6B

UnderseaOutfall

Outfall riser shafts - - - - Blind holedrilling

DC/93/18 Gran

7 To Kwa Wan Production shaft 10 60 Diaphragmwall


8 To Kwa WanPTW

Drop shaft 12 32 Diaphragmwall

DC/93/14 2.5 109 Raise boring DC/96/18 Gran

9 Kwun TongPTW

Drop shaft 13 32 Diaphragmwall

DC/93/14 2.5 116 Raise boring DC/96/18 Gran

10 Kwun TongPumpingStation

Production anddrop shaft

13 37 Diaphragmwall



Pumping stationshaft

15 25 Diaphragmwall

DC/93/10 - - - -


Production andriser shaft

10 33 Diaphragmwall


13 Tseung KwanO PTW

Production anddrop shaft

10 32 Diaphragmwall

DC/93/10 8 63 Drill and blast DC/93/10 VolcTuff

14 Shau Kei WanPTW

Drop and riser shaft

9 25 Diaphragmwall

DC/93/13 4.5 105 Drill and blast DC/96/17 Gran

15 Shau Kei WanPTW

Diversion chamber shaft

7.5 26 Diaphragmwall

DC/96/17 - - - -

16 Chai Wan PTW Production anddrop shaft 9 26 Diaphragmwall DC/93/13 5 107 Drill and blast DC/93/13 VolcTuff

17 Chai Wan Production shaft 10 21 Diaphragmwall

DC/93/10 8 75#

(113)

Drill and blast DC/93/10 VolcTuff

# - Shaft excavation was discontinued before completion



3 DESIGN

3.1 General

The design of the shafts was based on their function which was initially to provide temporary accessfor tunnel construction and then to transfer the sewage from the treatment works to the deep level

tunnels followed by conveying to the central treatments works or outfall. The availability of suitableland space and the location and orientation of the terminal manholes at the treatment works were mainfactors in deciding the location of the shafts. The upper shaft situated within the soft ground wasdesigned as an octagon suitable for construction by the diaphragm walling method. The lower part of the shaft was designed to take advantage of the inherent strength of the rock during the temporarystage and to with stand hydrostatic pressures during the permanent stage.

3.2 Upper Shaft

The upper shaft was designed to withstand the loading from the ground and ground water pressurewith an allowance made for surcharge and flooding of the surrounding area. The permissible deviationof the diaphragm wall panels from true verticality was 1:75. The thickness of the walls was chosen to

maintain at least 300 mm contact between adjacent panels for the worst case scenario where their verticality is offset in the opposing directions. For a 30 m deep shaft this works out as 1100 mm. Thetypical thickness adopted for the diaphragm walls was either 1000 mm or 1200mm. A 150 mmconstruction tolerance was added to the required internal radius and the contractor proposed to trim

back any excess concrete encroaching beyond this. Where the diaphragm walls were very deep and theresulting thickness is excessive specialist equipment was used to control verticality.

The quasi-circular shafts were designed to carry the loads in hoop compression without any internal propping or strutting. At toe level a nominal 1 m x 1 m ring beam was designed to tie the individual panels together. Where the rock head variation was more than 1 m then deeper ring beams weredesigned and installed. Where it is not possible to install a toe level ring beam as in the case of theShau Kei Wan diversion chamber, shear pins were drilled and grouted into the rock.

The reinforcement for the panels was selected not only to carry the forces but also to make the cagessufficiently rigid for handlings purposes and to minimize the entrapment of bentonite mud during theconcrete placing. Steel pipes and inclinometer tubes were incorporated into the rebar cages to facilitatethe drilling of contact grouting holes and for monitoring respectively.

3.3 Lower Shaft

Four types of primary support as shown in Figure 3 were specified. The primary support design for thelower shaft was based on the Barton’s rock mass quality ‘Q’ system, Barton et al, (1974). Using theinformation from the initial site investigation (boreholes drilled at the centre of the shafts) it was

possible to estimate the corresponding ‘Q’ numbers and select the appropriate support type at differentdepths. The Bills of Quantities were prepared using this method to quantify the extent of the differentsupport type. As the work proceeded the exposed rock face was geologically mapped after each roundof excavation and the ‘Q’ value was re-calculated and agreed with the Engineer’s Representatives onsite prior to installation of the appropriate support type. The extent of estimated support type under contract DC/93/10 is compared with the actual in Table 4.

Table 4. Comparison of estimated support type with actual

Support type Estimate (m) Actual (m)

Type AType BType C

Type D

13936737

25

9735399

19



The permanent lining was designed to withstand full external hydrostatic pressure under floodingconditions assuming that the shaft was empty. The inner surfaces of the permanent linings that areexposed to condensation were protected by a High Density Polyethylene (HDPE) fully welded

protective membrane. This is resistant to hydrogen sulphide attack from the sewage. The surface areasthat are always fully submerged did require such protection.

4 CONSTRUCTION OF UPPER SHAFT

4.1 Guide Walls

The diaphragm wall construction began with the construction of guide walls. These were temporarystructures constructed along both faces of the diaphragm wall. The top of the guide walls was locatedapproximately 0.5 m to 1.0 m above the surrounding ground level so that a positive head of bentoniteslurry can be maintained in the excavation to control ground settlement. A sheet pile cofferdam wasfirst erected before the excavation of the typically 1.0 m deep guide walls in view of the high groundwater table. The guide walls were constructed of nominally reinforced concrete.

4.2 Diaphragm walls

The diaphragm walls were excavated as eight separate panels generally using clamshell grabssuspended from a 50 Tonne crawler crane. The storage silos for the bentonite and plants for slurryseparation and desanding were installed on site prior to the commencement of excavation. The panelswere excavated in one to three bites. The operation of the grabs was stopped when a hard stratum wasreached and it was no longer practical to use this method. Circular and rectangular chisels wereemployed to excavate through the hard stratum until the predefined toe level of the panel which is atleast 500mm below the top of Grade III rock was reached. Following completion of excavation of a

panel stop ends were installed and recirculation and pumping out of bentonite from the toe level wascarried out for long periods of time (usually overnight) to remove all sediment deposits from thefounding level which were mainly sand and rock chippings.

When the trench is sufficiently clean the reinforcement cages were lowered in sections up to 12 m longand coupled up vertically using bulldog clips. A tremie pipe was positioned with its end at the bottomof the excavation to enable underwater concrete placing. A high slump Grade 35 concrete mix wasdelivered to the site and was discharged directly from the truck mixers to the hoppers fitted on top of atremie pipe. During concrete placing the tremie pipe was carefully lifted up with the free end securely

buried at least 1 to 2 m inside the fresh concrete to avoid contamination from bentonite. The displaced bentonite was returned to the storage silos after being cleaned in the separation plant.

4.3 Contact Grouting

In general practice excavation inside the diaphragm walls rarely continues deep to expose the toe of the wall panels. In the case of HATS, shafts were sunk below the founding levels of the diaphragmwalls and therefore some form of cut off against possible water ingress through the uneven joint at thewall/rock interface became necessary. This was achieved by drilling at least 5 m below the toe (of thedeepest panel) through pre-installed pipes cast in the wall panels and injecting a stable cement groutvia a single stage packer. This method proved to be effective in stemming any ingress at the wall/rock interface but despite this two shafts required additional treatment described in Section 9.7.

4.4 Excavation

The excavation of the soft ground inside the diaphragm walls was carried out by a 0.25 to 0.3 m 3

capacity backhoe type excavator and loaded into 4 m3

capacity muck skips. The filled skips wereremoved to the surface by a crawler crane. Any water that was trapped inside was removed by

pumping into the muck skips as the excavation proceeded. The shaft walls were surveyed for each 1.0



m depth and any projection inside the required internal perimeter was removed by a hydraulic hammer mounted on the excavator. The reinforcement bars that became exposed during this operation werecoated with anti corrosive paint and protected further by a layer of sprayed concrete.

4.5 Instrumentation and Monitoring

Inclinometer readings to detect any horizontal movement of the wall panels were taken daily when theupper shaft excavation was in progress. This was necessary to verify that the design assumptions andconfirm the stability. The readings were generally satisfactory overall but occasionally unreliablereadings were detected. In the latter case extensometer pins were installed and additional convergencereadings were taken.

4.6 Water Ingress

The specifications stipulated that the upper shaft shall be watertight. However during excavationseepage was observed in a few locations, mainly through the wall panel joints and the isolated

bentonite pockets. The leaks through the joints were repaired by drilling and injecting with a chemicalgrout. The trapped bentonite pockets were repaired by first removing the loose materials followed by

scabbling back to sound concrete and then backfilling with a repair concrete mix. Any seepage water was first diverted using pipes during this operation and then grouted after the repair concrete hasreached sufficient strength.

4.7 Progress rates

The diaphragm wall construction and excavation have taken approximately four to six months. Thedelay at the Shau Kei Wan D/S can be attributed to the large variation in the rock head level requiringextensive chiselling. The durations of upper shaft construction activities are summarised in Table 5.

Table 5. Duration of upper shaft construction in calendar days

Shaft Guide walls Diaphragmwalls Toegrouting Softexcavation Ring beam Totalduration

SCO D/S 20 41 12 26 24 123TKW P/S 20 66 16 47 13 162KTPS D/S 18 46 17 31 22 134KTPS R/S 23 58 14 48 21 164TKO P/S 20 56 17 48 27 168KTPS 21 69 16 60 - 166CW P/S 22 51 13 24 12 122SKW D/S 21 110 11 35 12 189

5 CONSTRUCTION OF LOWER SHAFT

The lower shafts were constructed by drilling and blasting and by raise boring methods. A typicalcycle of advance for drilling and blasting consisted of cleaning the face, marking out and drilling shotholes, charging, blasting, fume clearance, mucking out and support installation. Probing and groutingwas done at a certain frequency as described below.

5.1 Probing and grouting

The ground conditions were explored by two methods during excavation. Advance probing whereholes up to 30 m long were drilled at the four corners of the shaft was the preferred method. The other

method was to drill an approximately 100 mm diameter central hole all the way down to the shaft bottom using a down the hole hammer.



Where significant water inflow was met in the probe holes further holes were drilled to inject cementgrout. It was common practice to maintain approximately 5 m overlap between fans of probe holes.

5.2 Drilling and blasting

The lower shaft excavation was carried out generally by drilling and blasting. Immediately below thetoe of the diaphragm walls 1.5 m long blasting holes were drilled. At each shaft a trial blast wasconducted to confirm the blast design, to demonstrate compliance with the Mines Departmentregulations and to prove that blasting induced vibrations were below the permissible limits. As theground conditions improved with the depth of excavation the shot hole length was increased to 2.4 m.Two types of full face blasting patterns namely ‘wedge cut’ and ‘parallel hole cut’ were used. Wherethe water inflow was high the shaft blasting was done in two halves so that the lower half was used asa temporary sump while drilling was carried out in the upper half.

A typical cycle began with the cleaning the rock face after mucking out and marking the centre of theshaft by lowering a plumb bob from a steel beam temporarily placed over the shaft top. The outer

perimeter of the excavation was then marked out by spray paint taking account of the primary support

thickness. The locations of individual blast holes were marked out as dots of spray paint. The holes for the wedge cut were drilled at an inclination dipping towards the shaft centre. The ring of holesimmediately in front of the perimeter holes were drilled vertically down and the perimeter holes weredrilled at a slight angle dipping away from the shaft center. With the parallel hole cut, relief holesapproximately 100 mm in diameter were drilled near the shaft center and all blast holes were drilledvertically downward. It was important to drill the wedge cut holes accurately to maintain an evenspacing of the rings at the toe of the holes. This became particularly important in massive granite withfew joints. On occasions blast hole numbers were increased where such conditions were encountered.

The blasting vibration can be estimated using the equation given in Geoguide 4, GEO Hong Kong(1992):

A = KQdR -b (1)

where A= predicted particle velocity in mm/s; Q = maximum charge weight per delay in kilograms;R= distance between the blast and the measuring point in metres; K= rock constant; d= chargeexponent; and b= attenuation exponent. However the Mine’s Department equation (2) for calculatingthe peak particle velocity (PPV) was more widely used:

PPV = K(R/Q0.5)B (2)

The site specific constants K=644 and B=-1.22 were derived from a regression curve representing alarge number of measurements taken at various locations in Hong Kong.

5.3 Spoil Removal

The spoil removal commenced soon after blasting and smoke clearance. The equipment used was thesame as that used for the upper shaft with the exception of a 15 tonne Hagglund gantry crane replacingthe crawler crane. The skips were only 75% loaded to avoid the risk of falling rock.

5.4 Primary Support

The contractor proposed certain changes to the typical primary support types stated in the contractdocuments and these were accepted by the Engineer. The main changes are as follows:

Type A – replacement of chain link mesh with 20 mm sprayed concrete since there was a risk of flyrock from blasting being temporarily caugt in the mesh.



Type D – replacement of the steel arch ribs with a mesh reinforced sprayed concrete beam.

5.5 Progress Rates

The excavation was carried out in two 12 hour shifts. The planned and actual rates of progress for rock

classification/ primary support types are compared in Table 6.

Table 6. Average excavation progress per week

Rock-MassClassification “Q”

SupportType

Planned ProgressRate (m/week)

Actual ProgressRate (m/week)

>40.4 – 40.1 – 0.4<0.1

ABCD

10.39.34.73.9

8.16.33.82.4

6 UNUSUAL DIFFICULTIES

A number of difficulties were encountered during the construction of the shafts. These ranged fromcoordinating the construction activities with the 24 hour operation of the treatment works, dealing withlandfill materials present in the soft ground, substantial variations in the rock head levels, ground water leakage and ground loss, constraints imposed by explosives delivery and limitations on blasting

periods, environmental restrictions and the necessity to complete the works in a timely manner so as toavoid delays to the following on tunnelling works. Some such difficulties were not anticipated by thecontractors and therefore different mitigation measures were tried starting with the simple and

progressing to the more complex based on the success. As a result significant delays were encounteredeven though almost all the problems were resolved successfully.

6.1 Proximity to the sea

All shafts were located close to the sea. Three were located within 10 m of the sea wall and two of theoutfall risers were in fact constructed below the sea bed. During the excavation of the upper shaft of Chai Wan and Shau Kei Wan shafts which are located very close to the sea wall several large size (upto 1.5 m long) rock backfill that was placed previously to construct the sea wall were encountered.These could not be plucked out by the clam shell grab and therefore a cactus type grab was usedinstead. In addition substantial losses of bentonite slurry also occurred through this medium. This

problem was overcome by backfilling the excavated trenches with completely decomposed granite andre-excavating through it. Furthermore only one panel was excavated at any one time.

The surcharge loading within a 10 m zone behind the sea wall was restricted to 10 kN/m2

. Thislimitation made it difficult to install bentonite slurry silos and gantry crane foundations near the shaftand special dispensation was sought for the Shau kei Wan diversion chamber shaft to install the slurrytanks immediately behind the sea wall because of limited land space. There was sufficient redundancyavailable in the sea wall designs to accommodate additional temporary loading but systematicsettlement monitoring of the sea wall coping was undertaken to demonstrate that the excess surchargeloading did not have any adverse effect.

6.2 Working on Reclaimed Land

The shafts were constructed on land that was reclaimed from sea. The fill material and placingmethods in the case of old reclamations were not as strictly controlled as those new. At Kwun Tong

shaft site there was evidence of household refuse in the land fill. In addition that site was previouslyused as a fuel farm for unloading fuel from ships and storing there temporarily. Upon close



investigation contamination from the fuel was found to be limited since additional fill material had been placed over the site and the surface runoff has washed away and diluted the fuel concentration inthe ground. However during the excavation of the upper shaft through the land fill material bubbling of gases was observed. Gas monitoring was carried out to detect hydrogen sulphide and explosive gasesand forced ventilation was set up. The natural ventilation was found to be sufficient to disperse thehazardous gases since they occurred at a relatively high elevation.

At Tseung Kwan O the shaft was located in an old land fill and at the foot hills of a recent wastedumping site. During the excavation of the upper shaft leachate was encountered. This was carefullyremoved and disposed off site. Frequent gas monitoring readings were taken to check for explosive gascontent. The gas concentration measured was sufficiently low after dispersal by forced ventilation.

Since the diaphragm walls were watertight in the case of both shafts the hazardous gases problem wasresolved once the layer containing decaying waste was removed.

6.3 Variations in rock head level and deep weathering

One borehole was drilled at the centre of each shaft during pre-tender site investigation. Since the foot

print of a shaft is relatively small significant variations in the rock head levels were not anticipated anda variation of less than 1m was expected. However during the detailed design of the upper shaftdiaphragm walls three or more boreholes were drilled and substantial variation in the rock head levels

between different panels was discovered for Stonecutters Island riser shaft, To Kwa Wan productionshaft and Shau Kei Wan shafts. The largest variation of 7.5 m between the highest at and lowest toelevel was met at the Shau Kei Wan Diversion chamber shaft. In the case of the Stonecutters IslandRiser Shaft and To Kwa Wan production shaft deep weathering compounded the difficulties taking thediaphragm walls to some 50 to 60 m deep. A hydro-fraise type diaphragm wall trench cutter was usedto ensure the verticality of the deep wall panels. In addition deep toe level ring beams were constructedto compensate for the variations in the rock head level.

There was not sufficient space inside the Shau Kei Wan diversion chamber to install a ring beam andin any case this shaft was not required to be deep. The panels were dowelled into rock with a bundle of 4 x 50 mm cement grouted reinforcement bars. Inclinometer and convergence measurements weretaken during the shaft excavation to verify the stability and no adverse trends were observed.

6.4 Marine mud ingress into Diaphragm wall excavation

While the upper section of the Stonecutters Riser shaft was being excavated a sudden inrush of marinemud from the toe of a diaphragm wall panel occurred. This loss of ground through gaps in betweenundetected corestones caused a depression in the ground surface. Since the location of the ground losswas situated close to a 60 m diameter shaft and in an area planned for the construction of an aditlinking the two shafts, serious concerns arose when this incident occurred. The contractor proposed to

fill the shaft with water immediately to contain the mud flow by equalising the hydrostatic pressure,install another diaphragm wall panel behind that affected and then undertake jet grouting from thesurface to strengthen the collapsed ground. These remedial works were successful but caused a major delay to the completion of the shaft excavation. In addition the method of excavation of the adit waschanged to open cut from bored tunnelling by installing additional diaphragm wall panels between the

jet grouted area and the 60 m shaft.

When the shaft was re-excavated after the remedial works frequent inclinometer and extensometer readings were taken to monitor the convergence and verify the stability.

6.5 Flooding incidents in the upper shaft

At the Stonecutters Island outfall shaft an unexpected inrush of water was met at the initial stages of rock excavation works immediately under the toe of the diaphragm walls. During the time of theincident an excavator was cleaning the shaft bottom making it ready for the next round of blasting.



There was sufficient time to rescue the excavator operator before the shaft completely flooded.According to his account the water entered through a clay seam in the rock. It was decided toundertake tube-a-manchette grouting from outside the shaft over the zone of the leak. After completionof grouting the shaft was pumped out dry and no further leakage was met. It was later discovered thatthe water leak occurred through the clay matrix in between two large corestones. This area wasstrengthened by applying sprayed concrete before undertaking further excavation.

A similar incident occurred as well at the foot of the diaphragm wall of the Tseung Kwan O shaft. The pumping system was able to cope with the inflow and therefore it was decided to grout the leaks frominside the shaft. First deep holes were drilled to intercept the leaks some distance behind the shaft walland mechanical packers were installed in these holes. After channeling the water to pipes set in rapidhardening cement, a layer of sprayed concrete was applied to the closely jointed shaft wall and the

packers were grouted up under pressure as soon as the sprayed concrete has reached sufficientstrength. Eventhough the remedial method was successful in staunching majority of the inflow therewas residual leakage from this area which persisted until the permanent lining was installed.

6.6 Wide Clay filled sub-vertical joint in rock

At the Stonecutters riser shaft an approximately 1 m wide clay filled hydro-thermically altered jointwas met immediately below the diaphragm wall panels. The rock mass quality was substantiallyreduced by the presence of this weak material and it was decided to adopt Type D primary supportwhich is based on steel arch ribs and sprayed concrete. The sub-vertical joint persisted for up to about30 m deep before disappearing into the side wall and out side the foot print of the shaft. The steel archribs were continued until the influence of this joint on the shaft was no longer significant.

6.7 Plant breakdown

The high humidity, constantly wet and salty environment in the shaft lead to higher than normal wear and tear of mechanical plant and equipment. The excavator, water pumps and shotcrete pumpssuffered from frequent breakdown. On occasions the gantry crane also broke down contributing to theaverage lost time of approximately 20%. Even though the direct loss of time from plant break downcan be averaged out as 5 hours per day its effect in terms of the 10 hour window available for blastingwas very serious since a lost blast meant that the whole day’s production was lost.

6.8 Delivery of explosives

The storage, transportation and use of explosives were strictly controlled to prevent both misuse andmishaps. The proximity of the shaft site in relation to the Mine’s Department magazine generallygoverned the time of arrival of explosives on site. The earliest delivery was received at the shaftsnearest to the magazine at around 9 to 10 am and the furthest shafts where transportation also includeduse of a boat received deliveries by noon. This was satisfactory for undertaking one blast a day but not

for two. As the excavation works were falling behind programme the contractor attempted to do two blasts a day without success. With help from the client special dispensation was obtained for deliveryof explosives and blasting on Sundays and Public Holidays. This proved to be effective and resulted ina 15% improvement in production.

6.9 Shaft/tunnel junctions

The forming of junctions between shafts and tunnels were undertaken without difficulties in the rock though several of these junctions incorporated a chamfer to permit lowering of the tunnel boringmachine components. However this activity proved to be very difficult in the upper shaft through thesoft ground. During the construction of the Kwun Tong pumping station shaft/adit tunnel junction asudden inrush of sand and ground water occurred. The remedial works necessitated the use of liquid

nitrogen freezing to form an impermeable plug in the soil while the tunnel eye opening was made inthe diaphragm wall, Pakianathan et al (2002). At Shau Kei Wan drop shaft soil grouting followed bythe installation of a circular fan of closely spaced horizontal grouted pipe piles was put in place before



making the tunnel eye opening in the diaphragm wall to construct an adit tunnel. The presence of awater retaining box culvert directly above the this junction made the task even more difficult. This

junction opening was formed without incident or adverse settlement to the structure above.

6.10 Substantial water ingress

The Chai Wan production shaft was located in Volcanic Ash Tuffs within the zone of influence of theChai Wan fault. The water ingress progressively increased from 80 l/min at 35 m depth to 1400 l/minat 96 m despite pregrouting. Majority of the grouting holes turned out to be dry and ineffective inintercepting water making joints which were tight and closely spaced. Initially an OPC cement groutmix at varying water cement ratio (from 5:1 to 1:1) with 2% bentonite was used at a maximum back

pressure of 30 bar. When this was found not very effective Rheocem 650 microcement with 3%Rheobuild 1000 at a water cement ratio of 3:1 was tried at the same pressure. This resulted in higher grout take but the progress became very slow and water inflow continued. As the overall project timetable could not be met at the rate of progress being acheived it was decided to reverse the direction of the TBM drive and abandon this shaft.

Upon completion of the shafts the total inflow rates at the bottom of the shafts were measured before

handing over to the tunnelling contractors. These rates are given in Table 7. Inflow rates from allshafts with the exception of the abandoned Chai Wan production shaft were well below the 300 l/minlimit set for dewatering purposes.

Table 7. Ground water ingress after completion of shaft excavation

Shaft Depth of shaft(m)

##

Totalinflow rate

(l/min)

Inflow rate per metre of shaft

(l/min/m)

Inflow rate per squaremetre of shaft

(l/min/m2)

Tsing YiTseung Kwan OStonecutters Outfall

Kwun Tong Riser KwunTong DropTo Kwa WanChai Wan#

140( 129)95 (63)107 (97)

83 (50)151 (114)138 (83)96 (75)

693618

5460160

1400

0.50.60.2

1.00.51.9

18.7

0.0200.0240.008

0.0400.0200.0760.744

# - Shaft excavation was discontinued before completion##

- Depth of shaft in rock is shown in brackets where nominal water seepage was permitted

7 GROUND SURFACE SETTLEMENT

The ground surface settlement from the excavation was monitored around the shafts to ensure that itdid not exceed the specified maximum value of 25 mm including the measurements taken on seawall

copings adjoining the shafts. In general this limit was not exceeded but at Chai Wan production shaftwhere there was a high ingress of water into the shaft the settlement values were more than 25 mm.This shaft was located on land recently reclaimed from the sea and the long term consolidationsettlement was still in progress at the time of excavation. It was therefore difficult to conclude whether the contribution from shaft excavation alone was responsible for the excess settlement. In any eventthere was no sensitive structure around this shaft at the time apart from the gantry crane foundationswhich were underpinned to compensate for the subsidence.

8 CONCLUSIONS

The aim of this paper is to present the design and construction aspects of the deep shafts in Hong Kong

SAR as a case study with particular emphasis on ground related difficulties and how these were dealtwith in order to complete the works successfully. It is hoped that the problems highlighted and the



solutions described in this paper could provide valuable documented experience to the construction planning of similar shafts in the future.

REFERENCES

Barton N., Lien R. & Lunde J. (1974) Engineering classification of rock masses for the design of tunnel support. Rock Mechanics, Vol. 6, No. 4, pp. 183-236.

GEO, Geoguide 4 (1992), Guide to Cavern Engineering, Geotechnical Engineering Office, CivilEngineering Department , Kong Kong, pp. 76-78.

Pakianathan L. J., Kwong A. K. L., McLearie D.D., Chan W. L. (2002). Pipe Jacking: Case Study onOvercoming Ground Difficulties in the Hong Kong SAR Harbour Area Treatment Scheme. TrenchlessAsia 2002, 12-14 November 2002, Hong Kong.

Deep Shaft Design and Construction

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