Study on the micro mechanism of damage caused by unloading confining pressure of silty mudstone

The excavation and unloading of silty mudstone slopes often precipitate slope instability and collapse. While abundant research addresses the macro deterioration of rock strength, the micro-damage mechanism of silty mudstone remains largely uncharted. To bridge this gap, our study innovatively utilized nano-materials to prepare silt-like mudstone subjected to the triaxial unloading test considering multiple paths. Then we delved into the variation in microstructure using Scanning Electron Microscopy (SEM), binarization, and fractal dimensions. Our findings reveal that: (1) The peak stress, residual stress, and residual strain of silt-like mudstone display a decreasing trend with an increase in initial deviatoric stress while the peak strain initially increases and subsequently decreases; (2) Following unloading, the crack propagation on fracture surface is dictated by both confining pressure and initial deviatoric stress, which profoundly impact porosity, particle fragmentation, and pore expansion; (3) As confining pressure and initial deviatoric stress elevate, the average morphology coefficient rises with a reduction in rate of change. The impact of unloading confining pressure on pore morphology diminishes; (4) The fractal dimension increases with increased confining pressure and initial deviatoric stress, with a transition in the pore surface from smooth to rough.