I showed a safety advisor with a main contractor for the Singapore Mass Rapid Transit (SMRT) extension project my blog about the cracking of houses on Lorong 14/15A due to work on the Petaling Jaya North Sewer.
His job is to make sure that his company's sub-contractors abide by the safety regulations, abide by best practices, take the necessary precautions and so forth.
He said that the damage to properties on Lorong 14/15A is inevitable with any major underground construction, even in Singapore which has solid granite rock underground, and in some cases, the costs of restoration has eaten into the contractor's revenue and in some cases has made the contractor or sub-contractor bankrupt.
Thus, it's imperative that contractors must make every effort to minimise the occurrence of such damage, which is what part of his job is to ensure.
He also said that especially where the water table below is close to the surface of the ground, pumping out the water will cause soil subsidence and cracking due to sinking of the ground above.
Looking at the picture of the vertical, circular, concrete lined shaft, he estimated that the diameter of that concrete rings is not much smaller than the diameter of SMRT tunnels, which are around six metres, and that in the case of the SMRT tunnels, if studies of soil conditions, soil stability and the water table are not properly conducted, the water will try and flow through any crack or crevice and typically can damage buildings within 100 metres on either side of the tunnel, and result to loss of value of property, such as in the case of Wharton Estate in Singapore.
It's important to have a thorough analysis of soil conditions, soil stability and so forth before any underground work such as tunnelling is undertaken. The work most preferably be approved by a relevant authority, which in Singapore is the Ministry of Manpower, and managed by competent supervisors. He also doubted the soil stability so close to a river, as in the case of Lorong 14/15A.
He also explained the pipejacking is normally done in soft soil, where the pipe is just pushed through the the soil without the need to bore a hole through the soil first.
He advised that affected residents on Lorong 14/15A should get permission of the relevant authorities to allow them or a qualified person to go down the vertical shaft to inspect what's at the bottom.
9 August, 2014: Singapore