Weakening Laws of Rock Mass Parameters in the Excavation Damage Zone of Deep Rock Tunnels

The degradation of rock mass parameters within the excavation damage zone (EDZ) poses a significant threat to the stability of deep tunnels. Hoek–Brown (H–B) criterion is one of the effective approaches to estimating the damaged rock mass parameters. This study conducted a quantitative determination of the excavation disturbance factor D in the H–B criterion and its depth-dependent variation using empirical theory and the wave velocity method. Based on this, the weakening laws of rock mass deformation and mechanical parameters were investigated. The results indicate that within the excavation damage zone (EDZ), the disturbance factor D = 1 in the highly damaged zone (HDZ), and subsequently exhibits a linear decrease to zero as the depth increases. In the absence of the HDZ, the disturbance factor diminishes in an almost linear fashion from the excavation surface toward the interior of the surrounding rock. The HDZ is characterized by the most significant deterioration of rock mass parameters. As the disturbance factor decreases linearly, there is an approximately linear increase in the rock mass deformation modulus and uniaxial compressive strength. Conversely, the uniaxial tensile strength, equivalent internal friction angle, and cohesion of rock masses experience a nonlinear increase. Within the EDZ, it is observed that the uniaxial compressive strength exhibits the most pronounced degree of weakening, while the internal friction angle demonstrates the least magnitude of deterioration.