Thomson Line – Contract 208

Project Type: Construct Only

Owner: Land Transport Authority

Construction Cost: SGD 383 Million

Contractor / Client: Leighton – John Holland JV

Design Period: 2014 – Ongoing

Construction Period: 2014 – 2020

 

The Project

The project forms part of the Thomson Line (TSL) which is a fully underground MRT system, running approximately 30km along a north-south corridor in Singapore. Contract 208 includes the construction of approximately 4.4km of twin segmentally lined tunnels and two stations namely, Springleaf and Tagore Station.

At both sides of Tagore station, two launch shafts are constructed to allow early TBM launching, independently from the remaining station construction. The 6.35m outer diameter twin bored tunnels are constructed using three slurry tunnel boring machines (TBM’s). Six (6) mined cross passages are constructed to connect the twin bored tunnels and provide emergency escape exits. Additionally, two pump sumps are constructed.

The two stations are excavated using bottom up construction technique. Secant bored piles with several major strutting layers are constructed as part of the temporary works before construction of the permanent cast in-situ station structure can commence.

The station at Springleaf is approx. approx. 171m long, 23m to 46.5m wide and 19.5m deep and includes the provision of 5 strutting layers.

The tunnels, stations and ancillary works are located in varying ground conditions such as medium to high strength granite, highly/completely decomposed granite and residual soils.

Key Aspects

• Two stations using bottom-up construction method
• Temporary station retaining system consisting of secant bored piles including several strutting layers
• Twin bored tunnels of 4.4km length
• Excavation by three slurry TBMs
• Pre-cast concrete lining segments: 5 segments, 1 key, inner diameter 5.8 m, 275mm segment thickness
• Ground conditions varying from full face rock, mixed face to full face residual soil
• 7 cross passages and two pump sumps
• Ground treatment to ensure excavation stability and limit impact on adjacent infrastructure

Scope of Works

NOMA Consulting was engaged by LJH JV to carry out the detailed design of the following aspects:

• Design of temporary ground support requirements and excavation sequence for all cross passages, connection adits and pump sumps
• Design of temporary opening support requirements for all cross passages and pump sumps
• Assessment of surface settlements due to bored tunnel and cross passage construction
• Design of segments for temporary load cases (thrust, handling, stacking)
• Design of face and air pressure requirements
• Design of instrumentation and monitoring requirements for cross passage construction
• Detailed design of ground treatment for TBM launching and at cross passages
• Detailed design of foundations for temporary structures (slurry treatment plant foundation, gantry crane foundation)
• Design of TBM erection, launch and dissembling structures including foundations
• Design of gantry beams for gantry cranes to traverse station boxes