Study on a Damage Constitutive Model for Surrounding Rock Under the Coupling Effects of Initial Damage and Cyclic Blasting

To reveal the cumulative damage mechanism of surrounding rock with initial damage under cyclic blasting loads during tunnel reconstruction and expansion, this study combines theoretical modeling, split Hopkinson pressure bar (SHPB) tests, and three-dimensional numerical simulation. First, based on the Z-W-T model framework, a dynamic damage constitutive model capable of uniformly describing the coupling effects of initial damage and dynamic disturbance is constructed by introducing a damage evolution equation based on the Weibull distribution and an initial damage variable D0. Second, SHPB impact tests are conducted on sandstone specimens with different D0 values under various strain rates to obtain their dynamic mechanical responses. The model parameters are calibrated and its validity is verified. Finally, the validated model is implemented in ABAQUS via a user material subroutine to establish a 3D finite element model of the tunnel reconstruction and expansion, and a numerical test with seven cyclic blasting events is performed. The results show that the dynamic compressive strength of the surrounding rock increases significantly with increasing strain rate, but D0 has a clear weakening effect, which is amplified under high strain rates. Numerical simulation reveals that the damage in the surrounding rock accumulates nonlinearly with the number of blasts. The incremental expansion of the damage zone after the first blast is 1.51 m, decreasing to 0.03 m by the seventh blast, indicating a successively diminishing incremental expansion per blast. This reflects the saturation characteristics of damage accumulation and the diminishing driving effect of subsequent blasts due to energy dissipation and compaction within the already-damaged zone. The study provides key theoretical and analytical tools for evaluating the long-term stability of surrounding rock with initial damage under cyclic blasting.