November_1967 — Page 32

CHUNG PUI

TUNNEL

PLOYER COVE

STAGE

MUD DUMPING

AREA

STAGE I TUNNELS

SHUEN WAN

TOLO

HARBOUR

TAI PO KAU

WHITE HEAD

MA LIU SHUI

LOK WO SHA

CHEUNG KANG

LOWER SHING MUN DAM

TAI SHUI HANG

TURRET HILL QUARRY

SHA TIN TREATMENT WORKS

QUARRY

AREA SHOWN TO ENLARGED SCALE ON FIG. 2

3 FATHOMS'

COVE

N

DECOMPOSED ROCK

SAND

2

3

Fig. 1.2: Location of Plover Cove reservoir and sources of materials

up the areas of Cheung Kang and Lok Wo Sha which were both ac- cessible by works roads to the load- ing installation at the White Head borrow area.

Excavation in borrow areas had, as far as practicable, to be carried out to conform with terrace layouts suitable for future building develop- ment; in particular, the terraces at Ma Liu Shui were required to pro- vide much of the site formation for the new Chinese University of Hong Kong. Material unsuitable for use in the dam could, however, be left un- excavated.

A considerable number of rock boulders and seams of moderately de- composed rock were met during ex- cavation; full development of the terrace layouts planned for future development was not possible with- out excavating and transporting to spoil tips large quantities of unsuit- able material. Thus, excavation had in general to be confined to a large number of small faces with conse- quent difficulty of access and restric- tion in the size of excavating plant that could be employed.

Material for the dam was trans- ported by sea in barges loaded by hopper and belt conveyor installations

58

4 MILES

at Ma Liu Shui, White Head and Shuen Wan; at the latter area road access was available for transporting fill for work above water level. Ex- cavation in the larger terraces was done by Bucyrus Erie 71B or 51B excavators supplemented by Caterpil- lar 988s loading 18 cu. yd. capacity trucks. On the smaller terraces Cater- pillar 977 or 955 traxcavators loaded 3 cu. yd. trucks.

Considerable difficulties were met in supplying decomposed rock to the dam due to the necessity of selection on a fines criterion and stone content for the three zones of the dam, and the variability of the material sup- plied from the faces being excavated. This resulted in delays in loading barges with the ype of material re- quired by the varying demands of the dam. These difficulties were over- come by the creation near loading in- stallations of stockpiles where ma- terial from different sources and of varying fines content could be mixed by tipping from lorries and dozing to create material all suitable for the

core.

The stockpile was re-excavated at Ma Liu Shui by a Demag wheel ex- cavator and belt conveyor and, at White Head, with an excavator and

lorries. Separate stockpiles were also formed of stony low fines material to provide a reserve of general fill so that core material could be conserv- ed. The formation of stockpiles great- ly assisted the supply of material dur- ing and immediately after wet wea- ther when access roads in the borrow areas were unsuitable for traffic.

Decomposed rock for the bottom blanket and for general fill was dumped from 1,200 cu. yd. bottom opening barges in specified zones. These barges had vertical sides to as- sist rapid discharge and were fitted with inboard-outboard propulsion units at bow and stern to assist in manoeuvring and accurate location.

The majority of the core was plac- ed by the grab dredger "Biarritz" which is illustrated in fig. 1.5. This unique vessel was designed principally for the main dam operations of dredging to depths beyond the reach of conventional dredgers and for placing the large volume of fill re- quired for the core. A high capacity was needed in view of the compara- tively short time available for con- struction of the dam and to avoid the use of a large number of conventional grab dredgers which would cause congestion on the site. Some details of this machine are given in the ap- pendix. In areas where small quan- tities of decomposed rock were re- quired to be placed the grab dredgers Lima 2400 and Bucyrus Erie 71B and 51B were used.

A typical sequence of construction operations used in forming the core of the dam is illustrated in fig. 1.6. The core was raised in a series of mounds running longitudinally along the dam commencing at the Tai Mei Tuk abutment. Considerable over- placing of material was required to ensure the required minimum thick- ness of the core; the mounds were raised in a series of thin layers, each about 3 ft. thick, to reduce the lateral spread of the material to a minimum which, if allowed to occur, resulted in a flattening of the side slopes and the formation, by segregation, of per- vious lenses within the core.

When operations were carefully controlled side slopes of 1 in 4 were achieved but, if the thickness of the layers was increased and placing con- centrated in localised areas, slopes as flat as in 10 resulted.

In spite of close control of dump- ing of decomposed rock and sand, considerable redredging of the core was necessary to remove potentially pervious material which had drifted on to the core during these opera- tions. In addition, trimming was necessary of the surfaces of dumped decomposed rock underlying sand drains and of the upper surfaces of these drains to ensure that the speci- fied minimum thicknesses and vertic- al tolerances on the surfaces of these drains were achieved. All available bucket and grab dredgers shared in this work.

During dumping of the large vol-

Far East Architect & Builder November, 1967