Abstract: The deformation-bearing law of rock at the post-peak stage is of great significance to the stability control of the surrounding rock in the fragmented zone of the roadway. To study the rupture evolution and bearing characteristics at the post-peak stage of the rock, the uniaxial compression tests with different multiples of the peak strain value as the control strain were carried out on sandy mudstone, and the nonlinear fractal theory was applied to elucidate the distribution pattern of the post-peak ruptured masses. Based on the test results, a mechanical analytical model for rock samples containing penetrating fracture surfaces in the post-peak phase was established. The results show that: (1) The complete stress-strain curve of sandy mudstone presents two forms of post-peak stress drop, i.e. "single peak" and "multi-peak". (2) The post-peak specimen damage mode gradually changes from tensile damage under low-multiples peak strain to shear slip damage under high-multiples peak strain. (3) The fractal dimension of the internal fragments of the specimen is greater than that of the surface fragments for the same strain, and both of them have a significant positive correlation with the multiple of the post-peak strain of the specimen. (4) The continuous failure mode of rock at the post-peak stage is related to the characteristics of the contact fracture surface between fragments, and the fragments will only slide along the fracture surface within a certain dip angle range. The established analytical model can accurately explain the fracture-bearing characteristics and fragmentation distribution of the sandy mudstone at the post-peak stage.

Key words: rock mechanics, excavation damaged zone, post-peak bearing, fragment, fractal dimension