No_3_March_1969 — Page 40

celerate the velocity of flow from tun- nel to the open channel where the flow velocity is only around 8 ft. per second.

In order to convey the design flood flow of about 3,000 cusecs, the twin tunnels take a circular form with an internal diameter of 12 ft. 6 in. to 13 ft. 6 in. The maximum flow depth is estimated to be around 10 ft. all through.

The top width of the trapezoidal open channel is about 50 ft., nearly as wide as the existing Bukit Tima Timah canal at the downstream end. This, in some way illustrates the inadequacy of the existing Bukit Timah canal which serves more than twice the catchment area of the diversion channel.

Design of twin-tunnels

In order to ascertain the sub-soil conditions in the tunnels area, about 25 bores were made. From these in- vestigations it appears that one tunnel will encounter uncertain geological con- ditions where the formation cannot be fully ascertained. The materials en- countered include sandstone, shale and coarse sandy clay. For the other two tunnels, the material to be encountered is simply stiff, coarse, sandy clay.

The design allows for the twin tun- nels to be separated at 40 ft. 6 in. cen- tres. In soft ground tunnelling, primary lining in the form of 20 in. wide liner plates and one complete ring RSJ rib per unit of 20 in. c/c will be erected as soon as sufficient excavation has been done to allow for erection. This will ensure that at no time will an unsup- ported cut exist for a length of more than about 20 in.

The primary linings, made of cor- rugated steel plates, have been designed to carry the full bending and compres-

Section of trapezoidal open channel drain

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sive load anticipated fol- lowing excavation due to the possible difference between the vertical and lateral earth pressure. The primary lining will be left in the tunnel to form part of its per- manent structure. Con- crete lining, 15 in. thick, will then be cast and this lining is designed as a ring to carry the full burden of the earth load.

Construction and cost

Construction of the first Bukit Timah diver- sion canal was divided into seven parts, each part being given out under one contract. The idea of dividing the whole project into parts was to encourage maximum par- ticipation of local contractors.

The first contract was awarded to wards the end of 1967 and by Decem- ber, 1968, six contracts had been awarded leaving a small contract to be let out in 1969.

The initial estimated cost of the first Bukit Timah diversion canal, in- clusive of land acquisition and com- pensations, was about S$6.84 million. From the contract values obtained so far it is expected that the original esti- mate will be exceeded by about 10 per cent. The increase in cost is attributed mainly to the higher cost attached to tunnelling work.

According to the contract price the unit cost per tunnel per foot run is about $$950, which is about 25 per cent higher than the original estimate.

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Outlet transition from twin tunnel

The total length of the twin tunnels is 1,505 feet with a total cost of S$2,900,000, inclusive of the cost of the transitions and head-wall structures. Tunnelling will take up nearly 40 per cent of the expected total expenditure.

The excavation at the approaches to one of the tunnels has now been completed and work on the head-walls commenced in January, 1969. The 131⁄2 ft. diameter is the largest tunnel- ling works ever attempted in Singapore and the contractor has to procure the services of experienced tunnelling per- sonnel from Malaysia where extensive tunnelling works have been carried out.

The corrugated liner-plates used as primary lining were manufactured lo- cally. Originally it was intended to purchase imported liner-plates, but the local product, costing less, was tested and found acceptable.

So far three contracts have virtually been completed. These include 540 ft. of 36 ft. x 12 ft. three cells deep culvert, 730 ft. of 43 ft. x 12 ft. U shaped open channel and 1,500 ft. of trapezoidal channel with a top-width of 59 ft.

Construction of these structures has been straight forward, the only pro- blem being that of dewatering during the season of heavy rain. Excavation work for the three contracts was not very extensive, involving a volume of 200,000 cu. yd. Total volume of exca- vation for the first diversion canal is 675,000 cu. yd., part of which is being transported to the west coast for the reclamation of an area of tidal swamp.

The target completion date for the first diversion canal is December, 1970. So far progress on the work is accord- ing to schedule.

Far East BUILDER, March 1969

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