THE Downtown MRT line has been having a downer of a year.
First, its construction was blamed for cracks in houses in the nearby Watten Estate in Bukit Timah last September.
Next, the Land Transport Authority revealed a month later that its price tag had soared by more than 70 per cent since 2008 to $20.7 billion, partly due to rising construction costs and the need for more stringent safety requirements.
Now, the line has been fingered in a cave-in on Woodlands Road.
Engineers have said that delving underground has risks that cannot be eliminated, but the train line's litany of woes suggest problems tougher than the rocks.
This is worrisome as Singapore will have to develop its underground in other ways, especially in the light of the recent White Paper's population projections, they said. The paper lays out plans for up to 6.9 million people here by 2030. A complementary land use plan by the National Development Ministry added that Singapore will have to expand its uses of underground space.
By putting more industrial utilities such as power plants underground, for example, surface land can be released for people's living and civic uses and for green spaces, said engineering firm Tritech Group's executive director Cai Jun Gang.
So far, Singapore has built an ammunition storage facility, a Common Services Tunnel for utilities and the Jurong Rock Cavern to store petroleum products beneath the ground.
Eight engineers told The Straits Times that Singapore's underbelly could have several tiers:
- Utilities such as water and gas pipes, from near the surface to about 20m deep;
- Train stations and tunnels, offices, malls, carparks, laboratories and other facilities intended for people at 15m to 40m deep; and
- Other uses that involve fewer people, such as cable tunnels, oil storage caverns and reservoirs, from 30m to 130m deep.
A 1999 study commissioned by the Government and Nanyang Technological University (NTU) found that the school could have subterranean lecture theatres and cinemas beneath its campus.
The Ministry of National Development will also complete a study this year to see if facilities such as reservoirs, power plants and landfills can be clustered underground to save surface land.
The engineers said Singapore has several underground rock formations that can be tapped.
Bukit Timah, Bukit Gombak and Woodlands have granite and norite under them. "These are strong crystallised rocks... which are much harder than concrete," said Mr Chong Kee Sen, vice-president of the Institution of Engineers, Singapore (IES).
Limestone deposits stronger than concrete also lie beneath Changi and a 10km stretch of land from Telok Blangah to West Coast Road, including Kent Ridge and Boon Lay.
Dr Zhou Yingxin, a senior principal engineer with the Defence Science and Technology Agency (DSTA), has said that the deposits are enough to "build an underground city", although more studies are needed to determine the amount of usable space.
In the east, material known as old alluvium - mainly cemented sand and clay - can be used for underground construction, added Professor Leung Chun Fai from the National University of Singapore Department of Civil and Environmental Engineering.
The main challenges to digging deep are safety, cost, large variations within the rock formation and Singapore's tropical weather.
Over time, high temperature and rainfall, which seeps into the rocks' cracks and erodes them, have broken down rocks and soil in the Jurong area as deep as 40m below ground, said Dr Chew Soon Hoe, an IES council member who specialises in geotechnical and geological engineering.
Such weathered rocks are weak and soft, making underground construction more difficult as more support systems are needed. Cement grout, for example, may have to be pumped into the ground to strengthen the soil.
Even if there is more suitable rock at greater depth, the shallow, weaker rock has to be reinforced for maintenance and vertical access shafts during construction, adding to costs, added Dr Chew.
The soil and rock formation in Singapore is notorious for having very high variations in rock levels, strengths and properties, said the engineers. The Jurong formation, for example, consists of sandstone, siltstone, mudstone, shale and limestone, while the Kallang formation has loose sand, soft clays and silt overlying other rocks. Rock composition can vary both vertically and laterally.
The latest geology study by DSTA, NTU and the Building and Construction Authority in 2009 mapped out the types of geologic formation expected in various parts of Singapore, but did not specify the depth of each soil and rock type in much detail.
Said Dr Grahame Oliver, an NUS senior lecturer who teaches geoscience: "We know very little about Singapore's geology below about 50m under the surface."
Mapping, studying and determining the rocks' extent and structure may be tedious but is necessary for safe underground construction, IES's Mr Chong said.
This story was first published in The Straits Times on April 16, 2013