Tunnel Boring Machine (TBM) performance in Singapore's Mass Rapid Transit (MRT) system

Abstract

Singapore's Mass Rapid Transit (MRT) network is one of the largest public works projects undertaken by the Singapore government. This thesis summarizes and evaluates the performance of Tunnel Boring Machine (TBM) construction in Singapore's MRT network. Surface settlement induced by the tunneling process can cause damage to underground utilities and foundations and buildings and/or disrupt daily life by damaging roads and pavements, and is used in this thesis as a measure of performance. The influence of encountered geology and adopted construction methods (referring to the type of TBM used) on settlement is discussed. The dominant construction method on all four existing MRT lines involved the use of shield TBMs, with the main difference being the method of face support adopted. The North-South East-West (NS-EW) line employed largely compressed air as face support, while a move towards greater use of Earth Pressure Balance (EPB) was observed on the North-East line (NEL) and Circle Line (CCL). The use of EPB on the NEL resulted in 22 incidences of large, localized ground losses, which were of two types; 1. subsurface voids, or voids which formed above the tunnel face but were grouted before they could migrate to the surface, 2. surface sinkholes, or local depressions which were found by visual inspection to appear over the tunnels as the machine advanced. These large, localized ground losses usually occurred when tunneling through different weathering grades within a single formation or through mixed faces of different geological formations. The variability in ground conditions which resulted from these mixed weathering grades and mixed faces is a direct result of extensive tropical weathering of Singapore's soils, and poses a challenge to the performance of EPB machines during construction. The employment of slurry machines on the CCL was intended to mitigate some of the difficulties faced by EPB machines on the NEL, though with limited success. The use of slurry machines on the CCL resulted in the additional problem of slurry discharging to the surface and disrupting traffic. Given Singapore's plans to double the length of her rail network by 2030, the need to understand the impact of construction projects on adjacent structures and surface activities remains just as important, if not more so.