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  Singapore North-South Tunnel Contract NS2- Construction Challenges Met In Tunnelling and Mitigation Measures Adopted to Deliver on Time

  The Singapore North-South Cable Tunnel contract NS2 is approximately 5.5km long located between Ang Mo Kio and Mandai and is in close proximity to two man-made reservoirs with an elevated water table. Tunnel excavation was split into two drives, using slurry Tunnel Boring Machines (TBM), excavating through the Bukit Timah Granite rock containing fractures and major fault zones. It also passes under an underground mass rapid transit (MRT) station, located within the Nee Soon fault zone, under construction at the same time period. In order to overcome difficulties posed by some of these conditions, changes were made to the tunnel alignment soon after commencement of construction works, and the TBM was launched on a very tight radius curve; grout blocks were also formed in advance under the MRT station to provide safe havens for carrying out cutterhead interventions within the fault zones. However, delays required the implementation of a recovery plan to overdrive one of the TBMs and to dock the two TBMs a few hundred metres from the planned reception shaft location. This is an innovative approach and is a first in Singapore for hard rock tunnels. In order to seal against water ingress from the tunnel face, a new method of grouting called the chamber grouting method was planned. The paper describes the construction sequence, method and the planning aspects including the actual cycle times as well as the different catch up plans to get the schedule back on track. In addition, the paper also addresses the various challenges met during the implementation of this design and build contract and the solutions adopted to successfully overcome these and complete the works.

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