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  Design Considerations and Impact Assessment of Underground Construction in Close Proximity to Deep Tunnel Sewerage System (DTSS) Tunnel

  Underground construction in urban environment encounter added complexity due to existing above and underground structures found within and along project site. The design and construction methods adopted must ensure that these surrounding structures are well protected from damages. Singapore MRT railway construction under Contract T313 - Thomson East Coast Line involves the construction of Xilin Station and associated tunnels. A stretch of cut & cover tunnel over cross an existing Deep Tunnel Sewerage System (DTSS) tunnel. The excavation area is approximately 64m wide and 26m deep, and with just a 6m clearance between the final excavation and the DTSS. The ground conditions are very challenging, where thick layers of soft marine clay interbedded with fluvial sand are found above and around the crown of DTSS. The DTSS is a critical national infrastructure and tolerance for damage is extremely low. Various ground stability methods from ground improvement to proper sequencing of the excavation are adopted to minimise impact on the DTSS. The impact assessment of the DTSS tunnel involves both 2D and 3D Finite Element Analysis to predict the induced movements and forces on the tunnel, and assessments and structural checks in compliance with regulatory requirements are carried out. In this paper, details of the design considerations, impact assessment and site monitoring and control to ensure the stability and integrity of the DTSS tunnel are presented.

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