Failure Region Identification and Shear Strength Model of Rock-Like Joint Surface under Cyclic Water Intrusion

The deterioration of jointed rock mass strength induced by cyclic water-level fluctuations constitutes a critical issue affecting the stability of shallow rock slope engineering. To study the influence of cyclic water intrusion on joint shear rock mechanical characteristics, direct shear tests were conducted on joint samples with varying degrees of roughness subjected to cyclic water intrusion. The shear critical angle icr1 and tensile critical angle icr2 were proposed by analyzing the stress state of the asperities. Based on the joint failure modes, a joint surface shear strength model for the jointed rock mass was proposed that considered the failure modes of the joint surface under cyclic water intrusion. The findings indicate that (1) as the number of water intrusion cycles increases, the degree of stress softening exhibits an initially steep, followed by a gradual declining trend, with the decline magnitude positively correlated to the joint roughness coefficient; stress–displacement curves transition from postpeak drop type to platform type, while both the compressive strength of the jointed rock mass and the basic friction angle show an exponential downward; (2) as the number of water intrusion cycles increases, the shear critical angle icr1 decreases, while the tensile critical angle icr2 increases, and asperities become more prone to shear-cutting failure. Based on this, a shear strength model was established by considering the failure modes of the joint surface; and (3) model validation via MATLAB (version R2021b) programming quantified failure mode contributions to shear strength. The contribution ratios for the cutting failure in the J5 and J10 joints are about 14% and 37%, respectively. Ignoring this contribution will lead to significant deviations in the predicted values of shear strength. The relevant research methodologies and findings can provide significant references for stability analysis and analogous model testing in shallow water–related geotechnical engineering.