Abstract: Rock is treated as a compound including three component parts: grain, interface and matrix. FLAC is used to model the distribution and evolution of the shear strain increment, the minor and maximum principal stresses in the surrounding rock after a circular tunnel is excavated. Results show that in hydrostatic pressure, higher shear strain increments are mainly distributed in the soft matrix, forming an intersecting shear band network. However, higher minor principal (loop higher compressive) stresses are found at grains, generating several annular or ring regions. Larger granular sizes lead to fewer and longer shear bands, less number of regions with higher minor principal stresses and larger spacing between them. When the lateral confining pressure coefficient is not 1, the envelope of the shear band network exhibits two triangles at two opposite sides of the tunnel. At larger granular sizes, the results are similar to the zonal disintegration phenomenon in the tunnel surrounding rock at depth. Thus, a possible mechanism is proposed for the phenomenon. In jointed rock mass, several rock blocks are compressed together to form a ring in the loop direction of the surrounding rock due to the self-organization. If the stress level is high enough, several loop cracks isolated by zones without failures can be formed.

Key words: circular tunnel, grain, strain, stress, lateral confining pressure coefficient, zonal disintegration