Design & construction of pedestrian access tunnels below an existing operational metro tunnels & station

A new underground station is added into Outram Park Station as a part of Thomson Line MRT/metro in Singapore and this is built next to two operating metro lines. Once the underground station is completed, it is required to connect this infrastructure with the platforms of existing metro lines for the passengers to interchange between the metro lines. This paper presents design and construction issues of two such pedestrian linkway access tunnels built for this purpose. Two tunnels with cross-sectional area ranging from 50 m2 to 85 m2 are excavated below an existing operational rail tunnel (with vertical clearance of less than 2.7m) and below an operational metro station. The tunnel excavation below the operational metro tunnel is carried out using pipe roof method and the tunnel excavation below the existing station base slab is carried out by underpinning the structure as the excavation progressed. This paper gives an insight into the design considerations, construction methodology and summarizes the learnings based on instrumentation and monitoring results.