Anti-flood tank design tried and tested

Upcoming tank at Botanic Gardens similar to smaller version near Siglap
Feng Zengkun Straits Times 26 Jul 12;

A NEW water detention tank to be built near the Botanic Gardens by 2015 to help prevent floods in the area is not the first in Singapore.

It is likely to be similar to a smaller version built in Opera Estate near Siglap in 2001 to reduce the risk of floods there, according to plans unveiled by national water agency PUB last week.

The tank was built under a school field and cost $31 million when it was completed in 2001. The new tank, which will be under a new carpark for coach buses at the Botanic Gardens, is expected to be 11/2 times larger.

PUB declined to say how much it will cost.

Independent engineers told The Straits Times that the process of building the underground tank, if done properly, will not affect the safety of the ground and buildings nearby.

The rainwater from the new tank could also be used to irrigate the greenery in the Botanic Gardens and nearby Dempsey area, they said.

The Gardens tank is expected to be 15m - or about four housing block storeys - deep, and have a 38,000 cubic metre capacity, about the volume of 15 Olympic-sized swimming pools.

It will hold rainwater temporarily during storms and pump the water back into the drainage system after the storm ends.

It is one of PUB's two new infrastructure projects to keep the Orchard Road and surrounding areas dry.

The other major project is a canal to be completed by 2017, which will divert rainwater from the over-stressed Stamford Canal to the Singapore River.

PUB will call for tenders for both projects by the year end.

Mr Kevin Kho, 51, an engineer with more than 20 years' experience, said the larger size of the new tank is not likely to make its construction more difficult.

He said that there are several tried and tested ways to make sure construction of the tank does not affect the surrounding area.

One is to pipe in a mix of cement and sand to harden the soil before excavating it. This prevents cave-ins of soil which may weaken the ground nearby.

Another method is to drill lines of deep holes 1m to 2m-wide and fill them with concrete. These are called caisson piles and are used to create walls which can be braced to prevent soil slips.

After the ground is excavated, a frame of pillars is used to hold up the new roof, said Mr Kho.

'This is very old technology used in basements and underground carparks throughout Singapore. It's nothing very risky,' he said.

A PUB spokesman said last week that contractors for the new detention tank will have to show that its roof can withstand the weight of the coach buses in the new carpark.

Since the Opera Estate underground detention tank was built, floods in the area have been reduced from four to six times a year to almost none.

The construction company which built the tank has since folded.