Tunnelling through a frequently changing and mixed ground: A case history in Singapore

The Kranji tunnel is part of the Deep Tunnel Sewerage System in Singapore. It is approximately 12.6 km in length. Along the tunnel alignment, all the ground is composed of granite with different weathering grades (from fresh rock to residual soil). The changing ground from hard rock to mixed face and soft ground (and vice versa) at the tunnel level was anticipated. The tunnel depth along the route is between 15 m and 50 m. Two EPB TBMs were deployed at this tunnel with a bored diameter 4.90 m. These machines were designed so that both hard rock and soft ground could be excavated. The cutter head was equipped with a combination of both rippers and disc cutters. During the excavation, it was found that the frequency of the ground change between hard rock and residual soil is much higher than that expected. Due to the frequently changing ground, correspondingly the tunnel boring machine (TBM) operation mode had to be transferred frequently from hard rock tunnelling to transition mode and to earth pressure balance (EPB) close mode. It resulted in great difficulties for the TBM in an optimized operation condition. These difficulties included high cutter wear and flat cutters, tunnel face instability, water inflow at weathering interface, and time delays. In order to overcome these problems and speed up the tunnelling progress, the TBM used in the north drive was modified to attempt the frequently changing ground. The performance of the modified TBM was highly improved. However, the highly abrasive and frequently changing mixed face ground still caused high cutter wear, especially flat cutter wear. These posed many challenges to the equipment and the tunnel crew.