umes of material required in the bot- tom blanket near the central section of the dam, some of the fine material segregated and tended to drift into adjacent areas of the dredged trench and to accumulate to a thickness of as much as 6 ft. The drift of this ma- terial was attributed partly to tidal currents and also to the tendency for the material to flow into deeper sec- tions of the dam; the layer so formed had a density greater than sea water but virtually no shear strength.
The permeability of these accumu- lations was very low and they were a potential danger to stability due to the low rate at which construction pore pressures in them were able to dissipate. After investigations into the properties of the material had been carried out, it was decided that thicknesses up to 21⁄2 ft. could be allowed to remain. Thicker accumula- tions had to be removed by dredging which was very time consuming due to their semi-fluid nature.
Special measures had to be adopted near the Harbour Island abutment where the dredged trench was at its deepest and about 65,000 cu. yd. of decomposed rock fines had accumul- ated to a thickness approaching 30 ft. The dredger "Biarritz” made a num- ber of passes over the area removing as much of the material as it con- veniently could. This reduced the thickness of the deposit by as much as 12 ft. It then concentrated its efforts on small sections of the area removing as much as possible of the accumulation in a particular section and immediately afterwards placing decomposed rock filling through the disturbed fines layer on to the foun- dation to form a layer of placed ma- terial about 12 ft. thick. Subsequent investigations showed that the placed fill had successfully displaced the fines and was resting on the dredged foun- dation.
The layer of placed material was overlain by the residue of the original fines deposit which formed a layer less than 7 ft. thick on totp of the fill. Decomposed rock filling with a low fines content was then dumped into the area and most of the remaining fines were absorbed.
Sand Filling
A total of nearly 2,300,000 cu. yd. of sand was required for the con- struction of sand drains to accelerate the dissipation of pore pressures in the surrounding filling and as bedding to the rock protection on the external slopes of the dam. Sand piles were also originally envisaged to reduce excess pore pressures in the embank- ment but their use was unnecessary.
Several grades of sand were specifi- ed (by percentage passing a no. 200 B.S. sieve) depending on the degree of permeability required. The sand was obtained by dredging in a num- ber of shallow inshore deposits around Tolo Harbour (fig. 1.2.). Cut- ter suction dredgers were normally
action being very effective in reducing the percentage of fines in the sand to acceptable limits. During periods of maximum demand, a bucket dredger was used to supplement their output.
Working the small inshore sources available proved a difficult task. Wastage was high due to the small thickness and limited extent of the deposits. Other losses amounting to nearly 30% of the gross volume of sand excavated from the dredging
areas
were caused by the washing effect of the dredgers and by tidal currents at the dam-site.
In the Shuen Wan and Three Fathoms' Cove areas, the presence of coral within the sand deposits resulted in accelerated wear of the dredger cutter heads and reduced their rate
of output. Sand containing large proportions of coral fragments was considered unsuitable for use in cer- tain portions of the dam due to the risk of segregation during deposition.
Although the estimated content of the designated sand deposits was sub- stantially in excess of the net require- ments, due to the above factors. additional areas, one at the north-east end of Plover Cove, another in the Three Fathoms' Cove area and pos- sibly a further deposit near Sha Tin had to be worked to avoid a shortfall.
Sand was transported to the dam in bottom opening barges of 400 to 600 cu. yd. capacity towed by tugs and dumped directly in areas de- signated for sand filling. As stated before, considerable trimming of the surfaces of sand drains was required to conform with the required vertical tolerances. Increased accuracy of trimming was required for the sand bedding to rock protection as permis- sible tolerances were less.
Rock
About 1,280,000 cu. yd. of rock fill of various gradings was required to protect the exposed surfaces of the completed dam against wave action, to construct the rock mounds and to provide protection for general filling. at sea-bed level and at the level of the closure sill, against erosion by in-
PORTAL OF MAIN TUNNEL.. OF STAGE 1 SYSTEM
PUMPING MAIN
INTAKE CULVERT
PERMANENT PUMPING STATION
PLOVER
COVE
BEACH CUT-OFF
creased tidal currents generated during construction of the closure mound (estimated maximum velocity 9 ft./ sec.).
The finer grades of rock fill have been designed as filters to retain the sand or decomposed rock bedding be- neath them and to be themselves re- tained by the overlying riprap. The sizes of the different grades of riprap were chosen to ensure stability under the action of the waves to which they would be exposed at varying tide levels, heights of impounded water and storm surge.
The most severe conditions are likely to occur with typhoon winds blowing from the north-east and the reservoir full: under these conditions waves with a significant height of 8 ft, may be expected, thus riprap with a maximum size of 10,000 lb. and an effective size of about 2,250 lb. was specified for the affected areas, lighter grades being used on the lower por- tions of the slopes of the dam.
To provide the required quantity of rock, a large quarry was developed at Turret Hill with a comprehensive crushing plant to produce the finer elements of the required gradings. This quarry is equipped with a belt conveyor for loading barges and a crane for handling rock skips; an extensive reclamation has been form- ed with overburden removed from the quarry face to provide space for the crushing plant and the large stock- piles of different gradings of rock re- quired to maintain continuous supply to the dam.
Subsidiary quarries were opened up at White Head and Ma Liu Shui to augment the supply of rock which does not require processing.
Rock spoil arising from the driving of the Stage II tunnels is being used for sea- bed and temporary sill protection.
Rock was transported and deposit- ed in the dam by a variety of vessels. Filter rock, which was required to be desposited accurately in position and to close tolerances on level, was transported from Turret Hill in trains of floating rock skips, each of 22 cu. yd. capacity, towed by a tug. These
TEMPORARY FLOATING DUMBAG STATION FOR PUMPING OUT THE RESERVOIR FOLLOWING CLOSURE
TAI MEI TUK PENINSULA
MAIN DAM
BEACH CUT-OFF
GAUGE HOUSE
INSTRUMENTED SECTIONS
TOLO
HARBOUR
AREA OF SILL OF CLOSURE GAP
used for this work, their washing Fig. 1.3: Site of main dam and associated works
Far East Architect & Builder November, 1967
TUNNEL FOR PUMPING OUT
NORTHERN SUBSIDIARY JAM
SPILLWAY
TUNG TÀU CHÂU
SOUTHERN SUBSIDIARY DAM
HARBOUR ISLAND
6:00
3000 PLET
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