No_1_1956 — Page 70

con-

con-

The central portion of the dam acts as a spillway and the road across the top of the dam is carried over this on a bridge consisting of 19 spans each 21 feet long, formed of pre-cast reinforced crete sections supported on masonry piers. The draw-off arrangements consist of a wet well and valve house on the front face of the dam with 4 inlets at dif- ferent levels, the water flowing to the tunnel supply system through a 48-inch pipe passing through the dam.

ernment also provided two 7-ton derricks with 125 feet jibs, one of which was placed at the quarry and one at the dam, with a view to placing a larger propor- tion of displacers and thus saving cement.

As the excavation proceeded on the flanks of the valley, poor quality rock and a number of soft seams were expos- ed; these were not revealed in the preli- minary borings with the result that the foundation depths were deeper than was originally anticipated, and the volume of work to be done for Stage I of the dam was increased by about 25%.

water-retaining face of the dam consists contract was to construct the main dam tinued with the Contractors' plant. of granite masonry blocks the joints of to a height of about 150 ft. After con- The plant provided by the Government which are caulked, and a backing of struction had been started, Government included a crushing plant for production water-tight concrete, the thickness of decided to carry out Stage II of the of coarse and fine aggregates; a batching which varies with the depth. All the Scheme concurrently, which involved plant with two 1-cubic yard mixers; and exposed parts of the dam are faced with raising the dam to 200 feet and build a to-ton capacity cableway for transport- granite masonry. The maximum width ing the earth dams referred to above. ing the concrete to the dam, The Gov- at the base of the dam is 143 feet. When completed the total volume of the

Work on the main dam was started crete will be about 240,000 cubic yards. at the beginning of 1953 and during that year was mainly confined to the large amount of preliminary work which was necessary on work of this nature, and on rock excavation for the dam foundations. The Tai Lam Chung River flows throughout the year, but the dry weather flow is such that it can be contained in a timber flume which was constructed оп the left flank of the valley to enable work to proceed in the dry on the right flank. The first concrete was placed in the dam in August, 1953 and work was con- tinued for the rest of the year on the and such good progress was made,

The plant lay-out proved very efficient par- The earth embankments have cut-off right flank. In this part of the dam was trenches sunk to

watertight rock and constructed the diversion Tunnel, 10 feet ticularly during the last winter, that the above ground level the concrete core diameter, into which the river was divert- time lost earlier was recovered and the wall is supported by rolled fill embank- ed at the end of the year thus enabling son's rains at a level of 170 ft. above dam was ready to hold the present sea- ments. The core wall is cast in panels the flume to be removed and the remain- datum, which is in fact 20 with rubber water-stops at the vertical der of the dam foundations in the bottom than originally envisaged for Stage I, the ft. higher joints and bitumen filled horizontal of the valley to be opened up in the

capacity at the higher level being 1750 joints allowing a measure of flexibility in dry.

million gallons compared with 850 mil- this wall without loss of watertightness. The upstream face of the embankments, the construction of the dam were provid-

The major items of plant required for lion gallons for Stage I.

which has a slope of 1 in 3, is covered

In March this year the river was ed by Government for the use of the diverted into the 30-inch diameter scour with concrete slabs to avoid erosion due Contractors, in order to give an to wave action. The downstream slopes, tunity

pipe laid through the diversion tunnel, to contractors who would not which are at I in 2 with flat berns at otherwise be in a position to tender for and the tunnel itself plugged with con- intervals, are turfed. There is a masonry this work.

There was some delay in the crete at the upstream face. wave wall at the top of the embank-

arrival of this plant from the U.K. and it was not erected and ready for opera- tion until October 1954. Meanwhile con- crete work on a limited scale was con-

ments.

It was originally intended to carry out the Scheme in 3 stages and the original

oppor

By May 26th the dam was ready for impounding and it is expected to start this about the middle of June by closing the 30-inch Valve on the Scour Pipe.

THE CONSTRUCTION PLANT FOR TAI LAM CHUNG Excavation and Plum Handling Plant (Extracted from the talk given to the Hong Kong Engineering Society by W. Phillips, B.Sc. (Eng.), D.I.C., A.MICE. AM.ÏW.E., A.M.I.Struc.E. Senior Asst. Engineer, Headworks)

A

Before the tender for this work was let,

Government had arranged to provide the tenderer, viz., following plant for use by the successful

At the commencement of the contract, the contractor had in his possession two 3-ton electric derrick-cranes, and with the

months, it was thought that these would meet the requirements of handling suffi- cient soft material and

to enable

rock

concreting operations, on a large scale, to start

The dam, now under construction, as placing of 180,000 cubic yards of concrete purchase of a 5-ton derrick within a few has already been mentioned, was original- inclusive of plums. ly to be built in two stages, the first stage being straight

a

concrete mass gravity dam 150 feet high and 825 feet long, with

volume of approximately 180,000 cubic yards of concrete, together with a small roll-filled embankment; and the second stage, the heightening of the main dam to 200 feet, thereby increasing its length to 1,200 feet, the construction of a 1,100 feet long roll-filled embankment as an extension to the dam, and three small roll-filled embankments from 30 feet to 50 feet high.

The following notes give a brief descrip- tion of the principal plant used for the construction of the main dam to Stage I and for the roll-filled embankment tension to the main dam Stage II. General

(1) One quarry

(2) (3)

(4)

stone crushing and screening plant with a capacity of 60 tons per hour of stone aggre- gate in four sizes. One concrete weigh-batching plant One 10 ton radial travelling cableway.

The

upon the arrival of Government plant, which was scheduled to be deliver- ed and ready for work in the latter half of 1953. Although the contractors had not planned to use plums in the construc- tion of the dam, on the Engineers' per- suasion, they purchased bogies so that one travelling derrick could be installed in the quarry. This was to Government advant- One grouting machine for pres- age as every 1% plums saves Government sures up to 200 lb. per square inch. about HK$30,000 in cement costs.

aim was 30% plums, which would require In addition to the above. the China more and larger derricks than the con- Light & Power Co., Ltd. agreed to pro- tractors possessed. The purchase by vide, free of charge to the contractor, a Government of two 7-ton derricks from central distribution point for electric the Colombo Municipality, at an all-in power, during the construction period, at cost of HK$ 200.000 has resulted in а 346 volts 3 phase 50 cycles.

large increase in the quantity of plums The provision of plant, other than that handled. Although the total plums to enumerated above, was to be the respon- date has averaged 22%, these two derricks sibility of the contractor, and this plant have handled about 70% of that figure. would include such items as air compres or say 15%. Thus the total saving would sors for supplying air to quarry drilling be $450,000 less $200,000, or $250,000, equipment, derrick-cranes, etc. The der and Government still retains the derricks The original contract involved the ex- rick-cranes would serve a dual purpose: at the end of the contract. cavation of about 25,000 cubic yards soft that of handling excavated material and material

Concrete Weigh-Batching Plant (decomposed granite), 40,000 that of handling plums both at the quar- cubic yards of rock (granite) and the ry and at the dam.

ex-

The work involved in the construction of this dam falls into two main categories the excavation of soft material and rock to dam foundation level and the production and handling of concrete and plums to be placed in the dam.

As already stated, the quantity of

68

Page 70Page 71