October_1970 — Page 37

ind raking bars

old Seletar dam and a subsidary dam and a new subsidiary dam known as the saddle dam, constructed close to the main dam to prevent the water from overflowing into the southern bank of the valley. The saddle dam is 25 ft. high.

A vehicular roadway has been con- structed along the entire length of the two dams, each of which is about a third of a mile long.

The core of each dam is rolled clay, supported by 'shoulder's of the same material but interspersed with drainage layers of sand to release the water squeezed out of the clay during the building of the dam; the weight of the dam causes a pressure at foundation level of over 5 tons per sq. ft.

To ensure that the dam was formed on a firm foundation, it was necessary to remove about 330,000 cu. yd. of soft clays, sands and peats to reach the stronger granitic clays. These exca- vated materials have been used to form a plateau between the old and new dams which both supports the new dam and provides an open space for landscaping and recreation.

Side slopes facing the water are pro- tected from erosion by graduated layers of stone, and the downstream slopes have been turfed over for the same reason. A total of 76 instruments and devices for measuring water pressure, settlements and seepage have been built into the main dam.

The volume of materials forming the two dams is slightly under a million cubic yards. This was placed in ten months at controlled moisture contents and densities. Apart from the sand and stone, all the fill material came from the land surrounding the old reservoir. This excavation in areas which are now under water has helped to increase the capacity of the new reservoir.

The system selects one of four levels for the draw-off of water into a 43 in. steel pipeline which runs under the dam within an 8 ft. square culvert alongside the flood-discharge culvert. Both the culvert structure and the draw-off pipe are flexibly jointed to accommodate settlements and lateral movements caus- ed by the dam above them.

The design of this system was checked and refined by model tests carried out by staff and students of the Singapore Polytechnic.

Steel pipeline

The old Seletar pumping station has been extended to hold three vari- able-speed electrically-driven pumps which draw water from the draw-off tower through the culvert under the

dam and deliver it into a new 43 in. steel pipeline 1.5 miles long, laid to Pierce reservoir alongside the old 39 in. concrete pipeline. At top water level the water can flow through both old and new pipeline by gravity.

Delivery from Pierce reservoir to Woodleigh treatment works is through two existing 48 in. and 30 in. pipelines, assisted by four electrically-powered pumps in a completely new station at the foot of Pierce dam.

Surplus water otherwise wasted over the spillway when Pierce reservoir is full can be passed to MacRitchie reser- voir by two diesel-driven pumps in- stalled in a small station near the head of Pierce reservoir.

The work at Woodleigh treatment works to cope with the increased volume of water from Seletar included modification of the purification plant. Sixteen new filters were installed to replace the old ones. Each of these filters has a sand bed 2 ft. 6 in. thick. Equipment for the dosing of chemicals has also been replaced with larger and more efficient types.

The process of super-chlorination, which is a departure from previous practice in Singapore has been intro- duced. Water is heavily dosed with chlorine in order to effect quick and certain action on bacterial matter. Chlo- rine in excess of the predetermined re- sidual required is then removed by means of sulphur dioxide. A new build- ing to house equipment for treatment with sulphur dioxide has been erected along with a new administrative build- ing containing chemical laboratory, stores, a section for dosing equipment and an auxiliary pump house. The ef fect of these alterations is to increase the capacity of the Woodleigh treat- ment works by over three times from 17 m.g.d. to 54 m.g.d.

Contractors

Principal contractors for the Seletar reservoir project were:

Civil engineering works - Soil Mech- anics, Ltd., Lian Hup Han & Co., Seacon-Lianhuphan Joint Venture, Paul Y Construction Co. Ltd., Chua Ah Bak, Industrial Inspection (FE) Ltd., Evan Lim & Co., Tan Ah Koon, Hume Indus- tries (FE) Ltd., Kwong Lee Engineering Works, Alliance (M) Engineering Co., Nissho Co. Ltd., Pay Ah Heng, Ng Teck Fai, and Sia Eng Hock.

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Electrical and mechanical plant Harlands Engineering Co. Ltd., Siemens Aktiengesellschaft, Harper Gilfillan (S) Ltd., Cycle & Carriage Co. Ltd., George Kent (S) Ltd., and R.E. Morris & Co. Ltd.

Far East BUILDER, October 1970

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