A series of undrained triaxial shear tests

mercury intrusion porosimetry (MIP) tests

and scanning electron microscopy (SEM) tests were conducted on red mudstone fill material (RMF) at varying water contents to investigate its microstructure and stiffness degradation. Based on the principle of energy conservation

a relationship between the stiffness degradation of the fill material and the increment of elastoplastic strain energy was derived. The results show that the cohesion of RMF decreases with increased water content

while the internal friction angle initially increases and then decreases on the wet side. The evolution of strain energy leads to structural damage and subsequent stiffness degradation. The normalized stiffness ratio linearly decreases with the ratio of plastic strain energy to total strain energy and nonlinearly decreases with normalized shear stress. With optimal water content as the boundary

the RMF exhibits better load-bearing capacity on the dry side

while significantly lower on the wet side. The contours of damage stress and constant water content lines define a state boundary surface

whose upper and lower bounds follow logarithmic curves. The pore size distribution (PSD) of RMF on the dry side exhibits a uni-modal pattern with strong inter-aggregate contacts and good structural stability. On the wet side

however

a bi-modal structure is observed with weaker orientation of aggregates. It is recommended that the fill material be compacted on the dry side

with a target water content ranging from 5% to 7%.