Instability mechanism of shallow tunnel in soft rock subjected to surcharge loads

The tunnel instability accidents caused by surcharge loading could even lead to ground settlement or collapse, damage to the existing municipal pipelines or inclination of the surgical buildings. Series of large-scale physical model test was conducted in this paper to reveal the failure mechanism of tunnel in soft rock subjected to surcharge loading. The non-contact full-field displacement measurement (i.e. Digital Image Correlation, DIC) was employed to obtain a detailed deformation of surrounding rock after excavation. A coupled numerical method combing the continuum (Finite Difference Method, FDM) and dis-continuum (Discrete Element Method, DEM) analysis was used to deal with the failure mechanism of tunnel excavation in rock mass. Good agreements between physical model WA and numerical analysis approve the accuracy of the proposed FDM-DEM numerical model. It is found that the collapse mainly occurred in tunnel roof rather than in tunnel sidewall. The collapse zone shows a step-type increase with the increase of surcharge loads. The stress loosening zone (SLZ) in tunnel roof is larger than that in tunnel sidewall. The thickness of SLZ in tunnel roof can be used to design the length of the formed pressure anchors. The proposed large-scale model WA with DIC method is applicable to the tunnel engineering study. Moreover, the results in this paper give some insights to secure excavation works and optimize supporting structures.