Cause investigation of side-wall cracking in an operational tunnel

The underground environment is extremely complex and variable, which is one of the main factors affecting the safety of the structure. This paper presents a case study of investigating side-wall crackings in existing tunnels, in which the changes of structure's stress state induced by geological changes is investigated by means of field observation, in-situ test and numerical analysis. This work demonstrates that the side-wall cracking is significantly influenced by the softening of mudstone. The softening can not only reduce the bearing capacity of bedrock under tunnel, but also enhance the squeezing action of the surrounding rock on the structure, which the tunnel lining without invert cannot resist. Moreover, the treatment measure based on the principal cause is proposed and its effectiveness is demonstrated by numerical analysis.