Abstract:
Contract T219 Thomson-East Coast Line (TEL) Orchard Station will serve as an Interchange Station between TEL and North-South Line (NSL) Orchard Station. Part of the project was the construction of a platform link, connecting the new TEL and the existing NSL Orchard Station. The section of the platform link below NSL Orchard Station was one of the biggest underpinning works in Singapore, covering a plan area of 92m (L) x 16m (W) x 10m (H), at the depth of 30m below ground level. The space for the new structure was created, underneath the existing underplatform slab, by sequential underpinning and mining supported by horizontal soil mixing blocks. The local geology is residual soil of Bukit Timah Granite origin and the water table is close to the ground level. Above the slab, the MRT trains were operating at the frequency of 2 to 3 mins in the peak hours, which required a very stringent settlement control during the construction stage. The excavation released the in-situ stresses of the natural soil underneath and would cause potential settlement of the existing slab, which was originally designed as a pile-raft. Ground improvement blocks were required to increase the natural soil parameters to Cu = 400 KPa and Eu = 200 MPa. 780mm diameter horizontal cement soil mixing columns were installed ahead of the excavation face, using low-head room multipurpose drilling rigs. The settlement at the track level and the actual loads on the underpinning columns were continuously monitored by real-time data acquisition system. The soil mixing columns successfully kept the slab deformations within the design limits and played a crucial role in the success of the mining/underpinning works.