Treading a fine line to build TEL Stage 2

For most of the past six years, as diners blissfully tucked into their meals at popular eateries such as Springleaf Prata Place and Ngee Fou Restaurant, intense construction work was taking place less than 10m away.

Out of sight behind towering noise barriers, heavy machinery chugged away to dig 20m into the ground to build the new Springleaf MRT station on the Thomson-East Coast Line (TEL).

With the site being so near to shophouses and the adjacent Thong Soon private housing estate, failure to put in place proper precautions could have been catastrophic. For instance, the excavation could have led to the ground around the site shifting, affecting the structural integrity of the buildings nearby.

Close proximity to existing structures was a common challenge that engineers had to deal with in constructing the second stage of the TEL, the Land Transport Authority (LTA) said in a media briefing last Thursday.

TEL2 will start running on Aug 28. It will have two interchanges - Caldecott, with the Circle Line; and Bright Hill, with the future Cross Island Line.

TEL1 - the first stage of the line, a three-station stretch from Woodlands North to Woodlands South - opened in January last year.

Citing the example of Springleaf station, LTA's deputy project manager Goh Heng Tak said several precautions had to be taken to protect existing buildings.

Contractors carried out ground improvement works around the site in areas deemed less stable. This was done through measures such as mixing cement into the ground.

Workers installed an earth retaining stabilising structure to prevent the ground around the site from shifting, said Mr Goh. Devices were installed along the structure to monitor its performance.

Meanwhile, cranes and other heavy machinery were deployed on the site in such a way that they would not land on the shophouses if they toppled in an accident.

Cranes used to lift objects at the construction site were barred from swinging outside of the site boundary.

Mr Goh said LTA also had to implement measures to reduce noise caused by construction.

On top of the 20m-tall noise barriers erected at the site, workers wrapped heavy machinery such as cranes with noise curtains to reduce the noise from the engines.

The barriers were also seen at the other new stations - Lentor, Mayflower, Bright Hill, Upper Thomson and Caldecott.

Multiple road diversions were necessary to create the space needed to build the stations.

LTA's senior project engineer Foo Chee Chong said each station posed its unique set of challenges.

For example, the sloping ground southwards along Ang Mo Kio Avenue 4 on top of Mayflower station had a difference in elevation of 12m from north to south. This is equivalent to four storeys of a Housing Board block.

Due to this slope, engineers faced difficulties in providing safe working platforms for boring rigs and cranes, said Mr Foo.

He added that the most challenging from an engineering perspective was the construction of mined tunnels for Caldecott station, as workers had to carry out the construction in ground made up of both rock and soil.

The presence of more groundwater as well as weaker soil also made the work tough.

At Upper Thomson station, the construction required the diversion of a canal. A major consideration in the construction of the station was minimising traffic disruption on the busy Upper Thomson Road, said Mr Foo.

The contractor for the station used a retractable micro-tunnel boring machine brought in from Japan. Mr Foo said this machine enabled the installation of an earth retaining structure called pipe roofing, so that workers could excavate and complete the subway underground. The use of this machine meant that only one side of Upper Thomson Road was closed during excavation instead of both sides.

The need to work around existing essential infrastructure in the ground, such as water pipes and telecommunication cables, complicated the workers' tasks. But Mr Foo said they managed to navigate through these without any major issues.

"If accidents happen, we have to quickly call up the relevant utility agencies to try to repair the damage that has been done and resume services as quickly as possible," he said.

"There were just minor disruptions, for example, to the traffic light cables, but not to residences."