Groundwater and ground movement around deep excavation

Abstract

The Tsuen Wan West Station of Kowloon Canton Railway Corporation was constructed on both previously and recently reclaimed land. The project comprised approximately 600 m of cut-and-cover approach tunnels and a 400 m long station structure constructed in a 20 m deep excavation. Extensive instrumentation monitoring was carried out to monitor groundwater levels, ground movements and displacement of adjacent structures during the course of construction. This paper presents background information on the construction site and the monitoring results. It was found that the groundwater regime on the seaward side was strongly influenced by tidal movements throughout the monitoring period, whereas the landward groundwater regime showed a diminished tidal influence soon after construction of the diaphragm wall. Rock grouting carried out beneath the diaphragm wall was effective in mitigating drawdown of piezometric level outside the wall. The magnitude of the drawdown was found to be a function of diaphragm wall depth. Data on ground settlement, diaphragm wall deflection and groundwater were interactively discussed as the construction progressed. Finally subsurface settlement data were reviewed to highlight the adverse effect of pre-trenching for diaphragm wall construction and excessive drawdown in the weak rock layer.