Deformation and energy evolution of rock-like specimens under hydro-mechanical coupled stepwise confining pressure unloading

Abstract

To investigate the hydro-mechanical coupling failure mechanism during excavation-induced stepwise unloading in coastal areas, stepwise confining pressure unloading tests were designed and conducted under different water pressures and unloading levels. The deformation, fracture, and energy behaviors of the specimens were analyzed based on a deformation-based brittleness index, fracture surface, and microscopic SEM observations. Results show that both water pressure and unloading level reduce rock strength and increase post-unloading deformation, with deformation during the holding stage exceeding that during unloading. The deformation-based brittleness index captures brittleness degradation during stepwise unloading and decreases with increasing water pressure and unloading level. It exhibits an exponential decrease with water pressure and an approximately linear decrease with unloading level. Energy results indicate that higher water pressure accelerates the transition from elastic energy storage to energy dissipation, while higher unloading levels promote earlier energy release and shorten the elastic-dominated stage. Fracture results show that water pressure and unloading level jointly affect crack complexity and failure mode, supported by fractal dimensions and SEM observations. A conceptual hydro-mechanical coupling model is proposed based on seepage evolution, energy transformation, and fracture development. Instability during stepwise unloading is controlled by crack propagation induced by seepage and confinement reduction induced by unloading, providing support for stability assessment in underground engineering.