Abstract: To evaluate the large deformation of the Permian age slate interbedded with carbonaceous slate in Muzhailing Tunnel, the bedrock, rock mass characteristics and engineering mechanical properties of the slate interbedded with the carbonaceous slate are analyzed and studied in detail by means of tunnel face sketch, grinding plate identification, X-ray diffraction, electrical microscope scanning, point load strength index test and large shear tests. The internal cause of the large deformation is also investigated. Meanwhile, the analysis and the understanding of the squeezing bedrock are performed. The conclusions are as follows: (1) the slate and the carbonaceous slate are composed of variable residual debris, peat and sericite, and the mineral particles form a “laminar flow structure” under the strong directional stress, with the remarkable anisotropy; (2) the integrity of the rock mass is poor and the shear strength is low; the main type of the deformation and the fracture are bending and plastic flow; (3) it is a high geostressweak expansion composite in the soft rock mass, which is a typical squeezing bedrock mass.

Key words: Tunnel, Slate Bedrock, Rock Mass Integrity Index, Squeezing Rock Mass, Large Deformation