Assessment of tunelling under existing MRT tunnel in Bangkok subsoil

Abstract

Tunnelling under existing tunnels in Bangkok has been undertaken in recent years. Unfortunately, monitoring in the vacant tunnel is inapplicable as most tunnels are constructed underneath the Metropolitan Water Tunnel Network. Hence, close investigation of any deformation has not been carried out and the deformation of existing tunnels in Bangkok geology is unknown. This research studies assessment of the Bangkok Metropolitan Admission (BMA) water diversion from Makasan Reservoir to Wat Chonglom pumping station. The tunnel is close to pile foundation of the expressway and also tunnelling under existing MRT subway tunnel in Bangkok area. Field monitoring shows that boundary of surface settlement is about two times of tunnel depth both in front and behind the TBM face. Most of the tunnel deformation and ground settlement, about 90%, has occurred over a short term period and is likely to achieve full displacement within 2 months. Fluctuation of excess pore-water pressure surprisingly changes only in the horizontal plane. The influence zone is limited to about one time of TBM diameter in vertical axis and limited to about two times of TBM diameter in horizontal axis. Moreover, similar to ground movement, most excess pressure is also fully dissipated over a short term period. The study also shows that setting up the test sections can provide validation of numerical analysis and prevent possible risk during construction. The TBM operator could use suggested TBM parameters as a guideline for ease of operation, while maintaining settlement in the construction criteria. Tunnel records show that additional face pressure and grout filling ratios could minimize ground movement and displacement of the surrounding underground tunnel. Tunnelling close to existing underground tunnels is applicable and has less effect on the structure