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Abstract The groundwater level around the front section of the Kuyu water conveyance tunnel is far above the ceiling of the tunnel，causing groundwater infiltration to be so severe that it ‘rains’ constantly in the tunnel. During construction，accidental collapse occurred often and many collapse galleries were formed above the tunnel，which brought serious problems and delays to the construction. Based on investigation and analysis of some of the collapse incidents，the failure mechanism was studied systematically using the finite difference software FLAC3D，and the effectiveness of ground reinforcement on a typical collapse section was assessed. The study showed that the formation lithology was the internal cause of the collapses，and that the groundwater further exacerbated the instability potential，with the deformation almost triple that under anhydrous conditions. Lack of timely initial support and too large an advance per excavation cycle were two direct causes of tunnel collapses，as these practices caused additional deformation of 80% and 20% respectively. The rationality of the ground reinforcement schemes and the quality of construction of the ground support system are crucial to the long-term stability of the tunnel.
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2015, Vol. 34 
