December_1965 — Page 37

Wo

DAM

SHING MUN

CIVIL ENGINEERING AND PUBLIC WORKS

GATE SHAFT

MAIN TUNNEL

TOWER BELLMOUTH

SPILLWAY

PUMPING STATION

PIPELINE

VALVE CHAMBER

-400

SCOUR TUNNEL

ΤΩ

ITAI PO PUMPING TAU

¡STATION

STILLING BASIN

SCALE: FEET

This paper by Mr. Chung Ho was read before the Engineering Society of Hong Kong on Octo- ber 21, 1965. It is published by permission of the Society, the Director of Public Works, Binnie Deacon and Partners, and Scott and Wilson, Kirkpatrick and Partners, Hong Kong, Joint Consultants for the Plover Cove Water Scheme, and Mr. S. C. M. Cutting, chief resident engineer.

200 300 400

SUPPLY BASIN DAM

Fig. 1. Plover Cove Scheme Stage I. Lower Shing Mun reservoir and associated works

LOWER SHING MUN

This situated in a narrow steep-sid-

HE Lower Shing Mun Main Dam

ed valley on the lower reach of the Shing Mun River, approximately 11⁄2 miles downstream of the Jubilee Dam. The reservoir which is created by the dam has a maximum storage capacity of about 900 million gallons and forms a part of Hong Kong's Plover Cove Water Scheme.

The reservoir receives water from its own small direct catchment area, from overflow ог draw-off from Jubilee Reservoir, and from the main tunnel system which connects this reservoir, via Tai Po Tau Pumping Station, with Plover Cove reservoir. The function of this reservoir is to balance flood flows from these sour- ces with the demands of Sha Tin Treatment Works. Because of the relatively small capacity, the reservoir may be filled and emptied rapidly.

A small concrete dam just down- stream of the main dam forms a supply basin. Water from the re- servoir or from the main tunnel sys- tem reaches the supply basin through large diameter pipes and thence is led to Sha Tin Treatment Works by the draw-off tunnel (Fig. 1).

an

The main dam is a composite rock- and earth-fill embankment with impervious core and a concrete cut- off wall. The rock was obtained both from quarry sites just upstream of the dam and from tunnel excavations, and the earthfill was obtained from resi-

By

Chung Ho, B Sc (Eng),

DIC, AMICE, MASCE

dual soils in the neighbouring hills.

In order to reduce leakage of water under the dam, a grout curtain, con- sisting of three lines of grouthholes, was put down beneath the core into bedrock, using diamond drilling equipment.

A number of piezometers were embedded in the foundation and the embankment to measure pore water pressures both during construction of the dam and operation of the reser- voir. Surface settlement and deflec- tion points have been provided on the downstream slope and two U.S.B.R. type internal settlement devices were installed near the centre of the dam.

Construction of the embankment started in September 1962 when clearance of boulders, gravel and other alluvial deposits in the riverbed was begun. Embanking operations started in January 1963 when the downstream rockfill was placed. The placing of compacted earthfill began in March 1963. The main dam was completed in December 1964.

Foundations

Along the river bed, bedrock of medium to coarse-grained granite was exposed after clearing the

Far East Architect & Builder December, 1965

MAIN DAM

in

alluvial deposits, which varied depth from a few inches to 12 ft. The rock on both abutments has weathered in situ to a depth of over 100 ft., the more completely weather- ed upper zone forming a residual

soil.

The foundations of the upstream rockfill zone were on fresh granite in the riverbed area and moderately to highly weathered granite оп the abutments; and those of the down- stream rockfill zone, lying mostly within the riverbed, were mainly on fresh or slightly weathered granite bedrock.

The core and shoulders of the dam were founded in the abutments on coarse sandy completely decomposed granite. It was necessary to strip the topsoil and to remove the more clayey slopewash which varied from 3 to 4 ft. to well over 10 ft. thick in some places.

Embankment

The rock and earthfill embankment consists of a central clay core with more sandy materials making up the upstream and downstream shoulders. The upstream shoulder is overlain by rockfill, with a filter layer in between (Fig. 2). Intercepting graded filters were placed both between the core and the downstream shoulder, and between the downstream shoulder and the hill slopes and foundation.

73