This was an answer from the Minister for Transport to a question I filed for the 8 April 2013 Parliament sitting. The question was not reached by the end of Question Time, so a written answer was provided to me.
Mr Gerald Giam Yean Song: To ask the Minister for Transport (a) how many sinkholes have occurred since the start of excavation works for the MRT Downtown Line; (b) how many of these sinkholes caved in again after being filled; (c) how many motorists or pedestrians suffered injury or had their vehicles damaged as a result of these sinkholes; (d) whether this is a reflection of the quality of planning conducted by LTA prior to tunnelling works or such works not being carried out according to plan; and (e) what is the Ministry doing to eliminate the occurrences of such sinkholes which can potentially cause injury or death to motorists or pedestrians.
Mr Lui Tuck Yew:
Sinkholes can occur naturally when earth beneath the surface is eroded, typically by water, resulting in subterranean cavities. In the excavation of underground structures, cavities can form when water or fluvial sand leaks into the excavated area. If these cavities are not detected and filled up in time, they will eventually cause a sinkhole.
For the Downtown Line, there has been only one incident of sinkhole to date. This happened on 16 March 2013 at Woodlands Road. Investigations show that due to the varied nature of the ground conditions, the excavation works of Downtown Line Stage 2 (DTL2) had led to differential ground settlement, causing an underground water pipe to rupture. The soil movement, coupled with water from the ruptured pipe, weakened the ground under the road and resulted in the sinkhole.
On 23 July 2012, there was also an incident of a localised road depression along Bukit Timah Road resulting from tunnelling works for DTL2. The depression was detected promptly and action was taken to remedy it, before it deteriorated to become a sinkhole.
For both incidents, there was no injury to motorists or pedestrians, nor damage to vehicles.
There had been a number of other sinkhole incidents, but these were not related to any MRT construction activity. In the Keppel Road incident in January this year, investigations found that the sinkhole was caused by an old water pipe bursting. A car was trapped but no one was hurt in the incident. In the two cases on Clementi Road in March 2013, preliminary investigations show that the erosion of soil into an old abandoned utility manhole may have caused the sinkhole and led to the road caving in again after the first sinkhole had been filled. A motorcyclist was reported to be slightly injured trying to avoid the sinkhole.
To minimise the occurrence of sinkholes, prior to any major underground construction, LTA carries out comprehensive investigations of ground conditions and ensures that appropriate construction methods are used. Extensive instrumentation is installed to monitor the ground and building settlement at all times. In addition, LTA closely monitors the condition of the roads, especially those near to excavation sites or where tunnelling works are ongoing, to look out for signs of ground settlement or movement. Tunnelling control parameters are also strictly followed to ensure safety. In addition, LTA checks for potholes, cracks and other physical defects on all roads in Singapore regularly.
I assure Members that the safety of motorists is of utmost importance. My Ministry takes a serious view of these incidents. LTA will step up its monitoring efforts. This includes installing more extensometers, which are geotechnical instruments that locate and measure settlement, displacement and deformation in soil and rock, as well as opening up small sections of the pavement or road surface adjacent to excavation sites to physically check for the presence of any voids underneath. To minimise the risk of sinkholes forming due to soil erosion into abandoned utility structures underneath public streets, as in the case of the sinkhole that re-collapsed at Clementi Road, LTA will work with the utility agencies and companies to check their underground structures and records, and to carry out the necessary removal or rectification works. LTA will also work with the utility agencies to tighten the monitoring and maintenance regime to prevent rupture of underground water pipes which will weaken the ground and lead to formation of sinkholes.