Failure analysis and treatments of a loess tunnel being constructed in ground fissure area

Tunnelling in ground fissure areas is prone to collapse even with reasonable reinforcement. Based on the collapse of a tunnel in Xi'an Metro Line 3 near a ground fissure, potential causes and corresponding mechanisms were analysed. Preliminary in-situ investigation indicates that the hydraulic erosion induced loose pressure and crack-rich soil below the unclosed initial support were mainly attributed to be the triggering force of the collapse. Compared with the two common failure modes in tunnels, this collapse is of unique characteristics, showing that special attention to the arch feet is required in similar strata. The collapse mechanisms were subsequently analysed in aspects of geology, soil meso-features and construction. In addition, The failure process was simulated based on discrete element method in MatDEM and stress chains have been observed, showing results consistent with the above analysis. To prevent a secondary disaster, necessary appropriate remedies with considerable resources were implemented at three sites: the tunnel, portal and surface. Combined with the monitoring data during the treatment, their effects and shortcomings were discussed. The result facilitates prevention and treatment of collapse induced by underground tunnelling near ground fissures.