Ground fissures in Xi'an and measures to prevent damage to the Metro tunnel system due to geohazards

Xi'an City is located at the central region of the Fenwei Plain, and there are 14 ground fissures (labelled GF1-GF14) which have been closely monitored in the urban area of Xi'an City. When a shield tunnel is constructed in a region with ground fissures, problems (such as cracking or failure of the tunnel lining, leakage of groundwater into the tunnel, damage to the completed tunnels and shield, and long-term instability) may be encountered due to the activity of ground fissure. To mitigate the adverse impact caused by the activity of ground fissure, a series of countermeasures are suggested in this paper, which include installation of movement joints, installation of a flexible layer for controlling deformation, foundation reinforcement with cement grouting, and improvement of existing water resistance. A case history regarding the metro tunnel (Xi'an Metro Line 1) which has been intersected the ground fissure 5 (GF5) obliquely was analyzed. The aforesaid countermeasures were applied to this case history. After improving the existing water resistance, the permeability of the surrounding soils was effectively reduced to about 4.0 x 10(-8) cm/s and the groundwater leakage into the tunnel was minimal. Observations of the accumulative ground surface settlement near the fissure verified the effectiveness of the countermeasures applied to this case history.