Downtown Line digging a delicate operation

(Above) A temporary viaduct was built to bypass four busy junctions. (Below) Cracks appeared at the Ying Fo Fui Kun clan building in Telok Ayer Street due to tunnelling works.
(Above) A temporary viaduct was built to bypass four busy junctions. (Below) Cracks appeared at the Ying Fo Fui Kun clan building in Telok Ayer Street due to tunnelling works.ST FILE PHOTOS
(Above) A temporary viaduct was built to bypass four busy junctions. (Below) Cracks appeared at the Ying Fo Fui Kun clan building in Telok Ayer Street due to tunnelling works.
(Above) A temporary viaduct was built to bypass four busy junctions. (Below) Cracks appeared at the Ying Fo Fui Kun clan building in Telok Ayer Street due to tunnelling works.ST FILE PHOTOS

Contractors had to consider existing MRT line, road traffic and buildings

Digging a tunnel just 70cm away from a live MRT line on which trains filled with passengers run - that was just one of the many challenges that had to be overcome in constructing Downtown Line 1 (DTL1).

The six-station, 4.3km DTL1, which passes through the Central Business District and the Marina financial district, gives commuters a more convenient route from Chinatown to Bugis when it opens tomorrow.

On Wednesday, Land Transport Authority deputy director for DTL1 Tan Kok Jin told The Straits Times just how much work went into it, especially since it had to be constructed in an area already densely built up, and down.

For one thing, contractors had to build a section of the new line's tunnel on top of another through which trains on the East-West Line run.

Given that the space between tunnels was only 0.7m, engineers had to take special care not to hit the existing shaft when inserting steel pipes that was to form the foundation of the new one.

There was no margin for error, said Mr Tan. "Monitoring instruments were set to see if there was any displacement of the East-West tunnel."

At one point, contractors had to mine under the Bugis MRT station and part of Beach Road. That required them to pump cement into the soft ground first, explained Mr Tan, before inserting large pipes to hold up the land before starting excavations.

Tunnel-boring machines also could not be used in many areas as there was no room to build the entry and exit shafts needed.

Instead, the digging had to be done from the surface down. Once the tunnel was created, the area had to be resurfaced.

Space, or a lack of it, was a common issue. Near Chinatown, construction took place just 1.5m away from pre-war shophouses.

"There was a store selling porcelain, and its shelves would vibrate when we were pounding the ground," said Mr Tan. "Our guys helped carry its products to the floor."

Building the new Telok Ayer station and its tunnels under busy Cross Street took the space problem to a new level. "The traffic volume at Cross Street is very high, so we couldn't close even a single lane," said Mr Tan.

But because of buildings on both sides of the street, there was no room for additional lanes. The solution was to divert the traffic upwards, by building a two-lane, 720m-long viaduct passing over the construction area - the first time this was ever done here.

Removing the viaduct, which was built in 2008, posed its own challenge, requiring a 200-tonne crane to dismantle it.

But engineers faced an even bigger diversion project when it came to DTL3, the final stage of the 42km, $21 billion Downtown Line, expected to be ready in 2017. They had to re-route part of the Singapore River to bore two train tunnels under it.

roysim@sph.com.sg