Rising to challenge of building MRT tunnel above KPE

Gap of 37cm is all that separates a section of TEL from expressway tunnel beneath it

- Lee Nian Tjoe

Thirty-seven centimetres that is all that separates a section of the Thomson-East Coast Line (TEL) MRT tunnel running parallel to Tanjong Rhu Road from the Kallang-Paya Lebar Expressway (KPE) tunnel beneath it.

The gap is less than half the height of some car tyres.

Routing the tunnel which is about 3m underground - to go across the KPE was necessary for the MRT line to serve residents in the Tanjong Rhu area, a spokesperson for the Land Transport Authority (LTA) told The Straits Times on May 20, ahead of the opening of the fourth stage of the TEL.

Seven new MRT stations Tanjong Rhu, Katong Park, Tanjong Katong, Marine Parade, Marine Terrace, Siglap and Bayshore - will open on June 23.

Avoiding the underground expressway, the spokesperson said, would mean locating the MRT tunnel beyond East Coast Parkway and away from the residential area.

This portion of the tunnel took 24 months to complete. It involved digging up a trench to construct the tunnel before filling up the surface, instead of drilling through the earth without removing the ground above.

To ensure that construction could be done safely, piles pre-cut to the correct length were driven into the ground to form a temporary earth-retaining structure. LTA said this step was "crucial to prevent the risk of over-driving into the KPE structure".

To minimise the risk of damaging the KPE, the excavators used to dig up the earth were fitted with flathead buckets, rather than the typical buckets with teeth.