Deformation analysis of a soft–hard rock contact zone surrounding a tunnel

Abstract The Mounigou tunnel crosses complex geological strata, mostly with surrounding rock of poor quality with an IV or V classification. Failure in the form of collapses or squeezing deformation occurred many times during the tunnel construction. Especially, in the soft and hard rock contact zone of phyllite and meta-sandstone, the deformation failure was most obvious. This paper uses the FLAC-3D modeling analytical software to establish a 3D-numerical model, on the basis of a field tracing survey and laboratory tests to analyze the stress and strain changes before and after excavation of the tunnel. After excavation there are obvious stress concentrations and differentiations near the interface of the soft-and-hard rock stratum. Stress transfers and concentrates towards the hard rock stratum while in the soft rock zone stress release could be observed to some degree. The range of plastic deformation is obviously larger in the soft rock zone than in the hard rock zone. Both differences in principal stress and principal stress ratio increase after excavation. The amplification of the difference in principal stress is larger than the augmentation of the principal stress ratio. Moreover these principal stress amplifications are smaller in the soft rock zone than in the hard rock zone. The difference in principal stress amplifications between the soft and hard rock zones, together with the fact that soft rock strength is far lower than that of hard rock, causes local “relatively high geostress” effects in the soft rock. It is the main controlling factor of the large deformation of soft rock in the tunnel. It is concluded that the large deformation in the soft and hard rock contact zone in combination with the surrounding rock deformation failure in the Mounigou tunnel and the damage of the supporting structure, belongs to the type of soft rock plastic-squeezing failure.