Strain-rate effects on dynamic failure and energy dissipation of strongly weathered granite in subsea tunnels

Strongly weathered granite is common in shallow-buried, ultra-thin overburden subsea tunnels, where low strength, poor integrity, and high disturbance sensitivity make the rock mass vulnerable to blast-induced damage. To clarify its dynamic failure behavior, quasi-static compression and impact tests were conducted over a wide strain-rate range to investigate mechanical response, energy evolution, and failure characteristics. Based on the identified rate-dependent damage mechanism, a synergistic control strategy combining vibration-damping holes and a non-blasting protective layer was proposed for tunnel excavation. The results show that strongly weathered granite exhibits significant strain-rate dependence. Under dynamic compression, the peak strength and absorbed energy generally increased with strain rate, accompanied by intensified strain localization. In contrast, dynamic splitting exhibited a reflected-energy-dominated response, with relatively low transmitted and absorbed energies that varied non-monotonically with strain rate, while failure was governed mainly by rapid tensile splitting and interfacial separation. These results demonstrate that the nonlinear energy evolution of strongly weathered granite is manifested primarily in the stage-wise accumulation and redistribution of energy during loading, rather than in a universally monotonic increase in absorbed energy with strain rate. Field results confirm that the proposed strategy reduces stress-wave transmission, limits crown damage, and improves excavation quality.