The tricky business of tunnelling

The tricky business of tunnelling


LTA says it does rigorous soil and building checks before tunnelling, and underground cables are protected


By Nur Dianah Suhaimi


Rest assured, no stone is left unturned – safety-wise – when tunnels are dug here.


Given the intensive tunnelling activity islandwide, some Singaporeans have expressed concern in the wake of the road cave-in two weeks ago at Cornwall Gardens, in the Holland area.


There is tunnelling work there for the Circle Line MRT. The cave-in disrupted water supply and phone lines to several houses.


Addressing the issue, the Land Transport Authority (LTA) told The Sunday Times that the Cornwall Gardens cave-in had happened because of the loose ground there.


Vibrations from the digging caused some of the soil to shake, leaving a cavity on the surface.


Follow-up action was swift because soil samples had previously been taken and there had been constant monitoring.


The LTA said that extensive investigations are carried out before tunnelling work begins anywhere in Singapore. Tests will determine the kind of soil being dug.


This is important because Singapore’s geology varies. In Bukit Timah, the ground is tough granite, which is difficult to bore through.


In Kallang, however, the marine clay ground can be as soft as toothpaste.


The soil samples indicate what precautions to take and which machine to use.


But it is mixed ground – the kind that is neither here nor there – that poses a big challenge, since it is a combination of soft soil and hard rock, said Mr Ow Chun Nam, director of Circle Line 4.


The area around Cornwall Gardens has this mixed soil.


Apart from soil checks, the LTA will check buildings located 500m from the tunnelling work. The buildings’ structures and their foundations are analysed.


The LTA also digs trenches in the ground to locate underground utility pipes and telecommunications cables. These need to be protected, especially to avert accidental ruptures.


Tunnelling itself is done using tunnel boring machines.


In the process, extremely strong ring-like structures are put in to secure the ground. These rings form the underground tunnels that MRT trains now pass through.


Mr Paul Fok, group director for engineering at the LTA, gave an idea of their strength: ‘They are almost impossible to destroy, even if an earthquake were to happen.’


He expects more tunnelling works in future. ‘Singapore can be like Swiss cheese. Right now, our underground is nothing compared to Tokyo, Hong Kong or New York, which have many railway tunnels running underground. ‘


Swiss cheese is a type of cheese that is riddled with holes.


Given land scarcity, future roads may have to be partially or wholly underground, the LTA’s masterplan has said.


Professor Yong Kwet Yew, a civil engineering academic at the National University of Singapore, said this prospect left more space above ground for residential and commercial areas.


‘It makes sense to put transportation underground. It also makes our ground surface aesthetically pleasing,’ he said.



Source: Straits Times

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