This work aimed to study the cavity mechanism of surrounding rock at the bottom of a tunnel under the combined action of heavy-haul train load and groundwater. The study location was the Taihangshan Mountain Tunnel in Wari Railway, which is the first tunnel in China, constructed according to the 30 tons heavy-haul railway standards. Based on the water and soil pressure data measured in previous, the surrounding rock deterioration range and depth at the bottom of the heavy-haul railway tunnel under different axle loads and surrounding rock conditions were analysed by using a combination of laboratory test and discrete element method. The results showed objective defects in the bottom surrounding rock that appeared after the construction was completed, which provided a channel for groundwater. The groundwater flushed the surrounding rock at the bottom of the heavy-haul tunnels as time passed, and the dynamic action of heavy-haul trains caused the surrounding rock particles to gradually become loose, which was especially noticeable for the defect location in the weak surrounding rock. An increase in the axle load and a deterioration in the surrounding rock condition could aggravate the tunnel bottom cavity. The findings can be used to generate ideas for defects prediction and treatment methods for heavy-haul railway tunnels. Heavy-haul railway tunnelTrain load-groundwater Discrete element Tunnel bottom cavity Remote monitoring Laboratory test CORRESPONDENCE Zheng Li

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In China, the first tunnel was built in accordance with the 30-ton heavy-haul railway standard. Based on the change in water and soil pressure obtained from long-term on-site monitoring, the cavity mechanism of the surrounding rock at the bottom of a heavy-haul railway tunnel under rich water conditions was explored in this study. The cavity characteristics and degradation depth of the three types of surrounding rock under different axial loads and hydrodynamic pressures were analyzed through laboratory tests. The structural defects at the bottom of the tunnel and local cracks in the surrounding rock were determined to provide a flow channel for groundwater. The dynamic load of heavy-haul trains causes groundwater to exert high hydrodynamic pressure on the fine cracks. The continuous erosion of the bottom surrounding rock leads to a gradual loss of surrounding rock particles, which would further exacerbate with time. The cohesive soil surrounding rock is noticeably affected by the combined action of heavy-haul load and groundwater in the three types of surrounding rock, and the surrounding rock cavity is characterized by overall hanging. In the simulation experiment, the particle loss of the surrounding rock reached 1,445 g, which is 24.2% higher than that of the pebble soil surrounding rock and 40.8% higher than that of sandy soil surrounding rock. The findings of this study could be helpful for developing methods for defect prediction and treatment of heavy-haul railway tunnels.

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