Influence of loading system stiffness on unstable failure mode and energy evolution of sandstone under lateral constraints

In order to explore the influence of the stiffness of the loading system on rock failure mode and energy evolution law, the self-developed rigidity and variable stiffness test system is used to carry out biaxial compression tests, and the mechanism of the loading system stiffness on the post-peak unsteady failure and energy release law of rock under lateral constraints is analyzed. The results show that: The interaction between the rock sample and the testing machine system is analyzed from the perspective of energy dissipation and release, revealing the relationship between these factors during the rock failure process under the stiffness of the loading system. It can be divided into three modes: stable failure (W_d > W_u +W_e), critical state (W_d =W_u +W_e), and unstable failure (W_d < W_u +W_e). Under low stiffness conditions, the stress-strain curve of the rock sample shows significant stress drop and fluctuation, indicating local unsteady failure. In contrast, under high stiffness conditions, the post-peak stress of the rock sample decreases gradually, and the curve is stepped with a residual stage. With the increase of the stiffness of the loading system, the maximum strain energy release and maximum dissipative energy of the rock sample decrease nonlinearly, while the increase of the lateral binding force leads to the nonlinear increase of the maximum strain energy release and maximum dissipative energy. The application of lateral constraints strengthens the stiffness rebound effect and changes the energy release mode when the rock is fractured. In the early stage after the peak (∆W_pe > ∆W_pd), the energy release is relatively rapid, In the middle stage after the peak (∆W_pe < ∆W_pd), the energy is mainly dissipated, In the late stage after the peak (∆W_pe≈∆W_pd), the energy release and dissipation enter a stable stage. The research results provide a theoretical basis for understanding the post-peak failure mechanism of the loading system stiffness and dynamic disaster prevention, and propose measures such as lateral binding, phased energy control and reducing the energy storage capacity of the rock mass to improve the energy release mode, enhance energy dissipation, and enhance the stability of the surrounding rock of the roadway.