Abstract：Based on the study of the geostress state and the deformation characteristic of surrounding rock of tunnels in deep stratum，the mechanism of the layered fracture within the surrounding rock is put forward. The mechanism is as following. For the geostress in deep stratum is very high and the orientation of the maximum stress is possibly parallel to the axes of the tunnel，the radial tensile strain within surrounding rock of tunnels will occur under higher axial compressive stress，and the value of this tensile strain is smaller at the wall of tunnels but higher in rock mass at some distances from the wall of tunnels. The tensile failure of rock mass around tunnels is created when the tensile strain exceeds its ultimate value. Consequently one or more layered fractures are created within rock mass around tunnels；and the range of layered fractures is determined by the magnitude of the axial compressive stress. According to the results of the model test of circular and upright-arch tunnels，the above viewpoints are proven. The study on the layered fractures within surrounding rock of tunnels in deep stratum is very important to deep excavation engineering and some national defense works.
