Circular Shaft Construction Using Vertical Shaft Sinking Machine (VSM) in Singapore’s DTSS2 Project
Lai Lynn WOO, Cheng Boon NG, Aung Ko Ko SOE#, Angela ONG, Venkatesa PERUMAL, Guo Hao LIM+
DTSS2 Department, National Water Agency, PUB, Singapore

Shaft constructions are required in the Deep Tunnel Sewerage System Phase 2 (DTSS2) project at designated locations along the tunnel alignment for the launch and retrieval of Tunnel Boring Machines (TBM) and for the operation and maintenance of the used water conveyance system. Some of the shafts in the upstream stretch of the tunnel are to be constructed in heavily built-up urban areas. Therefore, shaft excavation sites are bounded by commercial/residential buildings, schools, hospitals, transportation infrastructures, major utilities services etc. In addition, hydrogeological profiles at shaft locations are challenging due to the presence of soft clay layers, permeable soil and highly to moderately weathered fractured rock along the depth of excavation horizon. Hence, it is very likely to cause surrounding ground to settle and overstress the foundation system of the existing structures in close vicinity if ground water level draws down or pore pressure drops due to seepage flow into the shaft or stress relief despite a good ground water management system is put in place. Since these existing structures/services are very sensitive to ground movement, it is of paramount importance to keep construction impact and associated geotechnical risks to a minimum. Since shaft excavation by Vertical Shaft Sinking Machine (VSM) has proven advantages and benefits over conventional shaft excavation methods, especially with rapid sinking rates and maximum safety and environmental control, the VSM method is proposed to construct five of the shafts in Contract T-11 of DTSS2 Project. This is the first known use of VSM for shaft construction in Singapore and currently 3 out of 5 shafts have been successfully constructed with this technology. This presentation outlines the background of the method used and intends to share the design approach, challenges encountered, benefits gained, and lessons learnt based on DTSS2 experience.