ABSTRACT
The Deep Tunnel Sewerage System Phase 2 Project (DTSS2) is the final phase of Singapore’s flagship water project which will deliver a nationwide network of recycled water supply, enhance water security and support the developing domestic and industrial water demands of this island city-state. The project will be delivered under various contracts. Contract T09, located in the densely populated western part of Singapore, comprises the detailed design and construction of: 7.9 km of used water tunnels, including 7.56m diameter TBM tunnels (the largest in Singapore at the time); mined tunnels; multiple online and offline shafts; hydraulic structures and associated air management facilities.
This paper focuses on the design of an unusual structure in contract T09; a temporary offline twin-cell shaft. In a departure from conventional shaft excavation practices in Singapore, an innovative design and construction approach was used to develop a solution comprising a diaphragm wall peanut shaft with different cell sizes, wall thickness, formation levels with significant openings. This pioneering approach was tailored to satisfy the project’s specific challenges - an extremely constrained site, challenging ground conditions, unbalanced load conditions and hydraulic performance requirements – whilst offering an economical construction solution. Multi-cellular shafts typically have straight central walls or props; due to the very soft ground conditions on this site, a curved central wall was adopted, as this assisted with diaphragm wall trench stability, where the two cells meet at an acute angle. The complex geometry of the diaphragm wall panel layout was optimised using 3D finite element analysis to ensure suitably anchored reinforcement was placed in the centre of the curved wall where peak bending forces occur. Specially designed T-shaped panels where the cells intersect cater for the high thrust forces and difference in wall thickness, ensuring this critical construction detail is as robust as possible.
