Comparative tests on strength and deformation of phyllite of northern tunnels of Longmen Mountains

The experimental tests are carried out on the strength and deformation characteristics of four typical rock samples of phyllite taken from the northern tunnels of Longmen Mountains of the Chengdou-Lanzhou Railway. The results show that the deformation of the phyllite is strongly influenced by the loading angles related to the beddings and the mineral components of the rocks as well as the confining pressure in 3-D compression. The uniaxial compressive strength of the phyllite changes with the changing of the loading angles, showing a “U” type and typical anisotropic behaviour. The anisotropic intensity of the four phyllite samples can be queued as follows: HP>MP>QP>CP. Dilation appears in the rock samples generally before the peak stress of the loading, and the starting stress and the formation of rock dilatancy are related with the loading angle as well as the development degree of the beddings. In the triaxial compressive tests, both the peak strength and the residual strength of the samples along the loading axis perpendicular to the beddings are remarkably higher than those parallel to the beddings, and the differences increase with the increase of the confining pressure. The failure modes of phyllite samples with different loading angles are classified into 4 types: bedding plane slip shear failure, tension shear failure, tensile failure and composite failure.