450 ft. was done with a soil sampler developed by the Swedish Geotech- nical Institute for continuous sam- pling of soft strata; the longest sample retrieved was about 45 ft. in length.
As complimentary works to the above, over 1.000 field tests compris- ing standard penetration, vane shear, salinity and permeability tests (includ- ing a large-scale well pumping test on the neck of the Tai Mei Tuk penin- sula) were carried out. In addition to normal classification tests, 1,200 lab- oratory triaxial, consolidation and permeability tests were undertaken.
In view of the compressible nature of the sea-bed in Plover Cove, it soon became very clear that a flexible type of dam using naturally occurring ma- terials would be needed. To show that such a scheme would work, an 80 ft. high fully instrumented test mound was constructed in the sea- using decomposed granite as the prin- ciple banking material. Successful completion of the mound proved, without doubt, that it was possible to form under water an adequately stable and impermeable embankment with- out any unduly complicated tech- niques of selection or placing.
Two further important results em- erged from the construction, namely that a base formed by dumping large- sized rock into the sea bed mud would be unlikely to form a satisfactory foundation and that a 35 ft. high 1 to 1 dredged slope in mud was unstable.
Stage I
Stage I of the project, which was commenced in 1961, has now been completed. This stage provides for
the conveyance of raw water, diverted from the Tai Po river and collected from the mountain streams between Tai Po Tau and the Shing Mun river, through about 8 miles of tunnels, via the Lower Shing Mun balancing re- servoir, to the 80 m.g.d. treatment works at Sha Tin. Abstractions from the Tai Po river include water divert- ed from Tau Pass which, in turn is augmented with supplies pumped from the Colony's northern streams and from China's East River Scheme.
An inflatable, neoprene-coated nylon dam at Tau Pass directs stream flow into a culvert and a similar inflatable dam is used at Tai Po Tau to divert river flow into the nearby pumping station (and thence into the stage I tunnels) or into the main stage II tunnel (and on to the Plover Cove reservoir). At Lower Shing Mun, a subsidiary pumping station can pump Plover Cove water to areas fed by the existing Jubilee reservoir.
Stage II
is
After purification at the Sha Tin treatment works, treated water pumped via pipelines, which have been laid through the Lion Rock tunnel, to service reservoirs in Kowloon be- fore distribution to urban areas in Kowloon and Hong Kong island. Works south of the Sha Tin treatment works have, in general, been designed and supervised by Water Authority, Public Works Department, whilst those north of the treatment works were entrusted to the consulting en- gineers.
Stage II was started early in 1964 and is now nearing completion. It
involved the conversion of a large coastal inlet of Plover Cove in Tolo Harbour into an impounding reservoir of 30,000 m.g. useful capacity by the construction of a 14-mile long main dam, two subsidiary dams and a spill- way, and the driving of a 6-mile long large diameter tunnel from Tai Po Tau to Plover Cove.
Tunnels branching off this main tunnel, together with a separate sec- ondary tunnel, will tap indirect catch- ments to supplement the natural run- off into the Plover Cover reservoir. The main tunnel will also be used to convey water, from the stage I catch- ments and sources to the north of Tai Po Tau, into storage in the Plover Cove reservoir.
The weir at the reservoir portal has a crest level of 19 ft. P.D. which is sufficiently high to promote gravity flow of stored water through the main stage II tunnel to Tai Po Tau: when the level in the reservoir is below + 19 ft. P.D., water is raised through a low-head pumping station at Tai Mei Tuk, into the tunnel portal.
In addition, two irrigation dams are being built at Hok Tau and Lau Shui Heung; water in excess of irrigation requirements will be fed into the tun- nel system and thence into storage at Plover Cove. The River Indus pump- ing station and an adjacent inflatable dam are, in essence, an extension to the stage II works.
An important aspect of the stage II works has been an extensive study of the effects of changing Plover Cove from the natural marine state to an artificially created freshwater regime.
The main problem in commissioning the reservoir is replacing the trapped
58
+640
SHING MUN RIVER ↓ JUBILEE
OVERFLOW
TO KOWLOON (VIA LION ROCK TUNNEL)
TO JUBILEE RESERVOIR
SHA TIN
PUMPING STATION
LOWER SHING MUN BALANCING RESERVOIR
+331
$328
• 320
296
SUPPLY BASIN
PUMPING STATION
155
SHA
TREATMENT
WORKS
TIN
+88
SUPPLY BASIN
TO SHING MUN RIVER
+438
+420
IRRIGATION
LAU SHUI HEUNG
+390
4345
+409
VIL
325
AK
+66
STAGE L
TUNNELS AND
INTAKES
+62
+59
+45
+15
!
TAI PO
TAU PUMPING
STATION
↓
FABRIDAM
STREAM INTAKE
JET DISPERSER
X VALVE OR GATE
=
PIPE LINE
MAIN TUNNEL
MINOR TUNNEL
== OPEN CHANNEL
VERTICAL SHAFT
Far East Architect & Builder October, 1967