No_7_July_1968 — Page 37

CIVIL ENGINEERING AND PUBLIC WORKS

Panoramic view of the works taken late last year. In the foreground are the aeration tanks and in the background are the compressor house and sludge handling structures

to serve a local population of 21,000 from the branch sewer.

was

Both the pumping stations are fair- ly deep (Braddell Road 35 ft; Bartley Road 21 ft.) and construction made difficult by very soft ground. In both cases, sheet piles and steel fram- ing were used for the cofferdam.

At Braddell Road, the pumping station was founded on 140 mass con- crete in situ piles made up of 42 in. concrete caissons from 5 ft. to 10 ft. deep. In the case of Bartley Road, no foundation piles were provided but the sheet piles were left in and the periphery walls of the pumping sta- tions were concreted against the sheet piles.

Rising mains

The difficulty of routing a direct rising main to the treatment works and the length (19,000 ft.) with its attendant high velocity head was a problem in the initial planning of the scheme. This problem is aggrevated by the fact that the possible rising main route rises to a peak at Upper Serangoon Road (1 mile from the pumping station) and falls to cross the low ground along Bartley Road be- fore rising again to the treatment works site. The peak would give rise to problems of surge and air release and admission which though not in- soluble would be troublesome and ex- pensive to overcome.

The arrangement adopted, there- fore, was to pump to the high ground near Serangoon Road (in 3,420 ft. of rising mains), gravitate in 5,530 ft. of gravity sewer (3,460 ft. of 66 in. and 2,070 ft. of 54 in, diameter) to а second pumping station at Bartley Road and pump again to a high point at Kim Chuan Avenue (3,260 ft. of rising main) from which the fluid gravitates in a 5,160 ft. length of 72 diameter gravity sewer to the

in.

works.

Far East BUILDER, July 1968.

The dual rising mains were laid at 4 ft. centres in the same trench with a maximum depth of up to 15 ft. No special beddings for the pipes were used except that the pipes were laid on 6 in. of sand bed in poor ground conditions.

The pipes used were mild steel pipes of 25 ft. length with spigot and socket and welded joints manufactur- ed in accordance with B.S. 534:1934. Each pipe was hydraulically tested to a pressure of 650 ft. head of water. The pipes were lined internally with cement fondu concrete to 1⁄2 in, thick- ness for 24 in. dia, and 34 in. thick- ness for 39 in. diameter.

As an added protection against sewage gas, the top third was painted with coats of tar epoxy paint to a thickness of 15 mills. Externally, the pipes were wrapped with bitumen im- pregnated fibre-glass tissue.

A sacrificial anode system of catho- dic protection using 6 No. 22 lb. mag- nesium anodes was applied to the Braddell Road rising main over half its length extending from the pump- ing station. For the remainder of the route of the rising main, the external coating of the pipe afforded sufficient protection as the soil, according to a corrosion survey, was classified as "mildly aggressive" only.

Gravity sewers

The gravity sewers were designed for velocity (full or half full) of not less than 4 ft./sec. to reduce the risk of septicity.

The first section of the gravity sewer from the discharge chamber near Serangoon Road to Bartley Road pumping station is 5,530 ft. long of which 3,460 ft. is 66 in, diameter and 2,070 ft. is 54 in. diameter, part of which was constructed in a tunnel 7 ft. 6 in. x 7 ft. 6 in.

The second section of the gravity sewer from the second discharge

chamber at Kim Chuan Avenue to the treatment works is 72 in. in diameter and is 5,160 ft: long with the final 700 ft. section constructed in tunnel.

In the tunnel sections, the heading was excavated to an arch shaped sec- tion and supported by steel ribs at 1 ft. 6 in. interval with close timber

piling boards. All the timbering and

steel ribs were left in.

The pipes used in both gravity sewers were high alumina cement lin- ed r.c. spigot and socket pipes with rubber rings. These pipes were manu- factured locally: they were hydrau- lically tested to B.S. 556:1955 and load tested to A.S. A35:1957.

Treatment works

(a) Treatment

From considerations of space as well as local conditions, treatment by activated sludge was selected and puri- fication to at least Royal Commission Standard was designed for; i.e. the effluent will be less than 20 parts per million of biochemical oxygen de- mand and 30 parts per million of sus- pended solids.

(b) Staging

The work was arranged so that half of the required capacity would be built initially; i.e. for a D.W.F. of 12 m.g.d. and additional units added as required in future in increments of 6 m.g.d. of D.W.F.

Certain installations, however, are built to the full ultimate requirements to serve a population of 500,000. Al- lowance is also made in the layout for a further expansion to serve an- other 200,000 people if the need arises.

(c) Primary treatment:

(i)

Pretreatment

As grit removal and comminu- tor facilities are provided in

the pumping stations, there is

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