No_1_1956 — Page 69

THE TAI LAM

CHUNG DAM

Progress to

May 1956

Brief notes by Mr. J.C.A. Roseveare, D.S.O., B. Sc., M.I.C.E., M.I.W.E. Resident Engineer, Headworks.

A view of the completed dam photographed from the model.

THESE TWO PHOTOGRAPHS MAKE AN INTERESTING COMPARISON

Photograph taken at the site of the dam during May 1956.

The Reservoir is situated one mile from the sca. at 16 miles on the Castle Peak Road, New Territories.

When full, it will have a capacity of 4.500 million gallons, when the water sur- face area will be about 500 acres. The direct catchment area draining into it is 4,040 acres, but the total area will be in- creased in the final stage of the Scheme when catchwaters and a tunnel from other catchments are completed.

ACTOSS

The Reservoir is formed by a main dam the Tai Lam Chung valley, and three subsidiary dams at low cols at the heads of adjacent valleys. The main dam is in two parts and consists of a straight gravity section concrete dam 1,200 feet long with a maximum height above the foundation, of 200 feet. The cut-off trench at the West end of the dam con- tinues through a low hill, and the second part or extension of the main dam, con- sists of an earth embankment with con- crete core wall, the maximum height be- ing about 80 feet, the total length of the two parts being 2.300 feet. The three subsidiary dams are earth embankments of similar design to the extension of the main dam.

The reason for the two different types of dam construction on the same site is the differing foundation conditions en- countered. For a gravity section dam sound rock able to carry heavy foundation pressures is necessary, and where these conditions are not satisfied some other form of construction is usually necessary.

The main concrete dam is founded on solid rock throughout and to ensure the watertightness of the foundation which is essential to the dam's stability, a con- crete cut-off wall is built into the rock on the upstream side. The trench for this is carried down in all cases to watertight, or nearly watertight ruck, and to test its degree of watertightness, holes up to 30 feet in depth are drilled in the bottom and tested under water pressure. If necessary, the holes are grouted until tight at 100 lb. per square inch.

The main body of the dam consists of 12 to I concrete with 20% of large stones known as displacers or plums. The

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