Soil movement at work sites 'difficult to predict'
DAMAGE to the houses in the Bukit Timah Watten Estate due to underground soil movement from the Downtown Line construction may have been unforeseeable, four independent engineers told The Straits Times yesterday.
This is because the strength and properties of Singapore soil vary so much that it may be difficult to predict how it will shift or move in response to construction.
Site surveyors may also not be able to obtain enough soil samples in the country's built-up urban environment.
Still, one engineer has suggested that special sensors placed closer to the Downtown Line construction site nearest to the damaged houses could have provided an early warning that something was amiss.
At least 40 homes in the upscale neighbourhood in central Singapore sustained damage from shifting walls and floors that have cracked as a result of construction works on the upcoming Tan Kah Kee MRT station of the new Downtown Line.
Following complaints, the Land Transport Authority (LTA) checked the houses to ensure that they are structurally safe and started interim works to prop up car-porch roofs and help move gates that could no longer open.
Yesterday, the LTA said that it had investigated the soil in and around the train station and the tunnel construction zone, both before and during construction.
Soil samples were collected at 20m intervals along the perimeter of the station and tunnel.
These samples enabled the LTA to analyse the types of soil beneath the ground and use computer models to estimate the way the area would be affected by the construction work.
From this and other data, such as the types of buildings in the area, the LTA had concluded that only buildings within a 250m radius of the construction site were likely to be affected.
This included part of Hwa Chong Institution, but Watten Estate fell just outside this zone.
The LTA attributed the damage caused to the houses to water seepage after excavation works. The ground then adjusted, causing cracks in the buildings.
Asked for their views on the issue, four independent engineers said that there were no fixed guidelines for taking soil samples outside a construction zone.
But they added that, in their view, the LTA's pre-construction survey was "reasonable" and in accordance with industry practices.
"In every construction project, there is the possibility of unforeseen soil conditions. You can only take so many soil samples for each project, and the soil profile is seldom uniform," said Professor Leung Chun Fai, from the National University of Singapore's civil and environmental engineering department.
Mr Chong Kee Sen, vice-president of the Institution of Engineers, said: "The soil formation in Singapore is notorious for very high variation in terms of rock levels, strengths and properties."
Other engineers said the built-up nature of the neighbourhood may have limited the number of samples that could be taken.
Associate Professor Leong Eng Choon, from the Nanyang Technological University's School of Civil and Environmental Engineering, said: "In some cases, it's impossible to take a sample. You can't have a sample site in someone's property."
But he also noted that the monitoring instruments installed by the LTA could have raised a red flag for the agency.
The LTA said at a briefing on Tuesday that it had installed about 2,400 instruments within the 250m zone to monitor the effects of construction.
These included ground settlement markers - which measure soil movement - as well as vibration sensors.
Prof Leong said: "If the impact of the works spread that far away (to Watten Estate), then the impact nearer to the construction site would have been much higher.
"If the sensors near the site were correctly placed, working properly, and checked regularly, they would have provided early warning of the impact of the construction that would be felt farther away."
The LTA said on Tuesday that 28 "recharge wells" would be built by next month to prevent further damage to the houses.
These wells will pump water back into the underlying soil to replace the seepage caused by the construction works.
In the meantime, excavation work for the Tan Kah Kee station, at areas closest to the estate, has been stopped.
It will resume only when water has been pumped back into the area and the water level in the ground has been stabilised, the LTA said.
It added that this will not delay construction of this part of the Downtown Line, which is due to open in 2015.
Taken verbatim from:
Post a Comment