Degradation of Mechanical Properties and Micro-Scale Damage Mechanisms of Mudstone-Type Soft Rock Under Hydro-Mechanical Coupling: A Case Study of the Baoshishan Tunnel, Western Yunnan

Hydro-mechanical coupling governs the stability of tunnel surrounding rock in soft-rock strata. Mudstone-type soft rocks exhibit abrupt strength loss and time-dependent large deformation under groundwater infiltration, posing severe threats to construction safety and long-term service performance. Most existing studies focus on oven-dry and fully saturated end-members, leaving continuous mechanical data across the unsaturated-saturated spectrum scarce, and the quantitative correlation between macroscopic mechanical degradation and microstructural damage remains poorly constrained. This study investigates mudstone intervals from the Baoshishan Tunnel in western Yunnan, where frequent groundwater-related support failures have been recorded. A series of laboratory tests, including uniaxial compression, triaxial compression, and triaxial creep tests, are performed on specimens prepared at six saturation levels ranging from oven-dry to fully saturated. Hydraulic property tests and microstructural analyses (SEM, MIP, XRD) are conducted complementarily to characterize water sensitivity and track pore-structure evolution. The derived degradation relationships are validated against 20 valid rounds of advanced geological prediction (AGP) records. Results show that full saturation reduces uniaxial compressive strength by 62–71%, and cohesion degrades far more significantly than the internal friction angle. The steady-state creep rate increases nonlinearly with rising saturation, while long-term strength follows an exponential decay trend. Hydration swelling of clay minerals and subsequent formation of a connected pore network are identified as the dominant micro-mechanisms driving macroscopic degradation. On this basis, a full-process degradation model, macro-micro damage correlation equations, and a composite water sensitivity index (WSI) are established. The findings provide a quantitative basis for surrounding rock classification revision, support timing optimization, and groundwater control in the Baoshishan Tunnel and similar soft-rock tunnels.