Abstract: Based on the triaxial compression test under different confining pressures of red sandstone, the failure characteristics at the outside surface of rock samples is monitored by using digital image correlation technique, and the failure images of outside surface of rock samples at different stages are obtained. The images showing the failure of the rock samples are analyzed and the fractal dimension calculated by box-counting method was used to characterize the variation of cracks on the outer surface of rock samples. It is found that the fractal dimension of cracks is proportional to the degree of damage in the rock samples. In addition, at the final stage of the failure in the rock sample, the fractal dimension of cracks appears an abrupt change, demonstrating the failure characteristics of rock samples. Combined with the mineral composition and mesostructure distribution characteristics of the red-bed soft rock, the cascade failure model is introduced to generalize the random connection model and failure mode of the structure. Furthermore, the redistribution principle of "capacity-load" of internal nodular particles under loading, and the initial load transfer path of nodular particles after failure are also analyzed. Results show that the red-bed soft rock under loading presents a multiple stage failure process, i.e. cascade failure. The cascade factor is continuously propagated and increased through the cascade paths. Thus the cascade failure becomes more obvious than before. Combining the deformation and damage process of rock failure and the interval characteristics of its axial stress-strain curve, the cascade failure classification criterion of rock failure is revealed, which has a certain reference value for expanding the research in this field.

Key words: red soft rock, triaxial compression, fractal box dimension, cascade failure, cascade factor