Study on in-situ mechanical response and energy evolution law of water-immersed mudstone

To investigate the mechanical properties and energy evolution mechanism of water-immersed mudstone under complex in-situ stress conditions, we selected the floor mudstone in Yitang Coal Mine as the research object and conducted triaxial compression tests under different immersion time and confining pressure conditions. The experimental results show that water immersion significantly degrades the peak strength and elastic modulus of the mudstone, and this degradation exhibits apparent time-dependent effects. With increasing immersion time, the stress-strain curve of the mudstone specimen transitions from an “elastic-plastic” to an “elastic-plastic-creep” behavior, and the confining pressure effect on the mudstone is weakened to some extent by water immersion; with increasing confining pressure, the elastic energy, dissipated energy, and total energy of the specimens all decrease; however, the proportion of dissipated energy increases and is positively correlated with plastic failure. The weakening effect of water on the mudstone diminishes with increasing confining pressure; with the increasing of immersion time, the failure mode of the specimen changes from shear failure to predominantly plastic failure characterized by radial swelling; with increasing confining pressure, the failure mode shifts from single shear surface failure to conjugate shear surface failure with higher dissipated energy.