Experimental Study on Mechanical Properties of Surrounding Rock of Railway Tunnel in Operation and Reasons for Tunnel Defects

The Laifosi Tunnel is a railway tunnel in operation located in Chongqing China, which has suffered from many defects, such as lining cracks, and water leakage from 2010 to 2020. To reveal the mechanism of tunnel defects, on-site boreholes were drilled from some typical sections of the tunnel and specimens were prepared to test for the surrounding rock. A series of laboratory tests were carried out on the rock specimens. The testing results show that the soften coefficient of mudstone is as low as 0.45, which indicates the strength of mudstone decreases sharply. Meanwhile, the grade of surrounding rock is reclassified based on the testing results and field investigations. Compared with the rock grade during tunnel construction, it is found that the surrounding rock near borehole No. 3 has not deteriorated significantly, which is mainly composed of sandstone. However, the surrounding rock near borehole No. 4 has deteriorated from grade III to grade IV from construction stage to present operation stage, which is mainly composed of mudstone. In the tunnel area with abundant seasonal precipitation, continuous deterioration of mudstone caused by long-term water–rock interaction is one of the dominant factors of the tunnel defects. The present research results introduce a theoretical basis for the subsequent treatment of the tunnel defects. It is suggested to consider the rock deterioration during long-term operation when designing and construction a tunnel in areas with such similar geological conditions. For example, the surrounding rock is mainly mudstone and the area is rich in rainfall.