ABSTRACT
For construction of the new Thomson-East Coast Line (TEL) project in Singapore, twin bored tunnels in stacked configuration and a pedestrian link-way are being constructed directly under two operating MRT lines. Ground conditions consist of thick deposits of soft (under-consolidated) marine clay overlying Old Alluvium (OA), which at this location is relatively cohesionless. The construction scheme devised to enable construction of the TEL tunnels without interruption to the existing MRT lines, consisted of ground stabilization to allow underpinning of the existing lines and construction of the new tunnels by the Sprayed Concrete Lining (SCL) method. Both vertical and horizontal Jet Grouting Pile (JGP) as well as ground freezing were employed in the ground stabilization. The presence of existing MRT lines limited the coverage that could be achieved by conventional vertical JGP from the surface alone; as a result, Horizontal JGP was adopted to improve marine clay under the existing tunnels and allow driving of Adits by means of an Open Faced Rectangular Shield Machine. vertical JGP from the Adits was used to stabilize the lower zone of marine clay. Because of the density and potentially flowing characteristic of the underlying OA, the JGP could not be relied upon to stabilize this deposit and ground freezing was adopted to stabilize the zone at the interface of the marine clay and the OA. This was the first time that ground freezing had been applied for an underground railway project in Singapore. 2D and 3D FEM Analysis were carried out to predict the movement of the existing tunnels caused by each construction activity - Shield Excavation, ground freezing and SCL Tunnels excavation. Construction control was achieved based on actual monitoring data compared to predicted values, and the work was completed successfully without breaching the Work Suspension Level. The paper describes the impact assessment of the existing live MRT tunnels including overall integrity and stability due to the TEL construction.
