November_1967 — Page 30

COAST OF PEOPLE'S REPUBLIC OF

PUNAND

TUNNEL

THE

SAN

SUBSIDIARY DẠ

Part II

Plover Cove Water Scheme, Stage II

CONSTRUCTION OF THE MAIN DAM

by L. S. Dodd,* BSc., ACGI, MICE

This paper describes some aspects of the construction of the Plover Cove main dam, the materials used and the plant and methods adopted for depositing them in the dam. Also des- cribed are the instrumentation of the dam and certain inves- tigations carried out during construction.

T

"HE works consist of an earth dam

6,800 ft. long situated in the sea at the mouth of Plover Cove in the New Territories (fig. 1.1) which, with the construction of two subsidiary dams each about 650 ft. long and a spillway, creates a reservoir of 30,000 million gallons usable capacity. The location of the reservoir and dams is shown in fig. 1.2 and 1.3 from which it will be seen that the spillway is formed by cutting down most of the island of Tung Tau Chau between the two subsidiary dams. The site of the permanent pumping station at Tai Mei Tuk is also shown.

Top water level of the completed reservoir will be 27 ft. above Prin- cipal Datum compared with a mean sea level of +4 ft. P. D. As the combination of a large quantity of in- coming water and wave action asso- ciated with a severe typhoon can cause a rapid rise of water level, the crest level of the dams has been set at +36 ft. P. D. above which a con- crete wave wall rises a further 4 ft.

The main elements of the dam are shown in fig. 1.4 and consist of the dredging out of sea-bed mud, the de-

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position through water of decompos- ed rock to form the impervious core and shoulders of the dam, the dump- ing of sand layers for drainage of the decomposed rock and the deposition of rock protection.

A critical stage in the construction of the dam was that of closure, that is the final sealing off of Plover Cove from the tidal waters of Tolo Har- bour. Model studies undertaken by the Hydraulics Research Station at Wallingford, England confirmed that closure on a sill of the maximum pos- sible length was desirable in order to limit tidal current velocities through the gap and that the crest of the closure mound being constructed on the sill (fig. 1.4), should be kept as nearly level as possible. The length of the closure gap selected was 3,400 ft. and it is located centrally in the dam (fig. 1.3).

During construction a number of instruments were inserted in the dam to provide information on the be- haviour of the filling materials both before and after pumping out of the trapped sea water in the reservoir.

Tenders for the construction of

the dams, spillway and pumping sta- tion were invited internationally in July, 1963. The contract was award- ed to Societe Francaise d'Entreprises de Dragages et de Travaux Publics of Paris in late December, 1963 for the tendered sum of HK$134.7m, and preliminary works started on the site early in 1964. Closure of the main dam to enable pumping out to start was programmed to take place in mid-October, 1966 and all works were to be completed at the end of 1967.

Some details of the plant employed on the works are given in the ap- pendix.

Work and Materials

Dredging

The first operation required on the main dam-site was the removal of all sea-bed mud which was too weak to support the dam. This exposed the underlying alluvial clays and general- ly clayey sands and, in places in-situ decomposed rock on which they rest. These strata formed an adequate foundation: excessive leakage through them is prevented by the blanket of mud remaining on either side of the dam.

*Mr. L. S. Dodd is deputy chief re- sident engineer for the Plover Cove Project. His paper 'Construction of the Plover Cove Main Dam' and others in this series, were presented to the En- gineering Society of Hong Kong (Session 1966-1967) under the general title 'Some pects of Stage II of the Plover Cove Water Scheme'.

Far East Architect & Builder November, 1967

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