Time differential development of destruction mechanism of bolt anchorage system in composite rock formation

This study systematically investigates the temporal evolution of destruction and debonding mechanisms in composite rock bolting systems. The time-dependent characteristics of failure progression are quantitatively characterized, and a novel assessment method for the time-differential development of destruction is proposed based on the butterfly-failure theory of roadway mechanics and the trans-permeability characteristics of the butterfly-shaped plastic zone. A method for assessing the time differential development of destruction in composite bolting systems is proposed. Employing an orthogonal experimental design combined with finite element simulations, this work explores the mechanisms of temporal failure differentiation from both tensile-shear and compressive-shear perspectives. The results demonstrate that, unlike anchorage systems in homogeneous rock masses, those in composite strata exhibit a distinct time-delayed failure phenomenon, which can be effectively captured by the proposed evaluation framework. By using interface stress as the primary criterion and unit elastic energy as a complementary indicator, the time lag between tensile and compressive failures in soft–hard rock combinations is determined. Across 18 simulation cases, approximately 50% of the configurations involving low-strength-hard rock contrasts exhibited a temporal delay exceeding ten frames, underscoring the pronounced asynchronous deformation behavior between lithologic layers. Through range and variance analysis, three key factors—strata position, height ratio between soft-to-hard rocks, and strength contrast—were identified as the dominant parameters influencing the temporal evolution of failure. Based on these findings, two optimized anchorage strategies were proposed: for tensile-shear-dominated failure, the optimal configuration follows the HSCH sequence with a height ratio of 2:2:1:1 and a strength ratio of 1:1:3, enabling the upper hard rock to exert a more effective suspension effect on the underlying soft strata; for compressive-shear-dominated failure, the optimal configuration also adopts the HSCH sequence but with a height ratio of 1:1:2:2, which mitigates excessive fracturing in the surface rock under high deviatoric stress. Overall, this integrated optimization approach enhances the synergistic deformation compatibility and load-bearing efficiency of composite bolting systems, effectively reducing the time delay in failure progression and improving structural stability under complex geological conditions.