Test scheme.

In order to study the shear mechanical properties of rock joint under different unloading stress paths, the RDS-200 rock joint shear test system was used to carry out direct shear tests on concrete joint specimens with five different morphologies under the CNL path and different unloading stress paths. The unloading stress paths include unloading normal load and maintaining constant shear load (UNLCSL), unloading normal load and unloading shear load (UNLUSL), unloading normal load and increasing shear load (UNLISL). The results show that the peak shear strength, cohesion, internal friction angle, pre-peak shear stiffness and residual shear strength of concrete joints under CNL path increases with the increasing JRC and normal stress. Under the UNLCSL path, under the same initial shear stress τ1, instability normal stress σi decreases with the increasing JRC, and normal stress unloading amount Δσn increases with the increasing JRC. Under the same JRC, σi increases with the increase of τ1, and Δσn decreases with the increasing τ1. Under the same JRC and σi, τi is significantly smaller under the UNLCSL path than the CNL path. Under the same JRC, the cohesion under the UNLCSL path is less than the CNL path, and the internal friction angle is higher than that the CNL path. Under the same JRC and σi, τi is the largest under the path of CNL and UNLISL, followed by the UNLCSL path, and τi under the UNLUSL path is the smallest. Compared with the CNL path, the variation range of the specimen internal friction angle is within 3% while the average decrease percentage of the specimen cohesion reaches 37.6% under the UNLCSL path, UNLISL, and UNLUSL. Therefore, it can be inferred that the decrease in cohesion caused by normal unloading is the main reason for the decrease in joint instability shear strength. After introducing the correction coefficient k of cohesion to modify the Mohr-Coulomb criterion, the maximum average relative error after correction is only 3.5%, which is significantly improved compared with the maximum average relative error of 56.9% before correction. The research conclusions can provide some reference for the accurate estimation of shear bearing capacity of rock joints under different unloading stress paths, which is of great significance to the stability evaluation and disaster prevention of rock mass engineering.

为研究不同卸荷应力路径下岩石节理的剪切力学特性，本研究采用RDS-200型岩石节理直剪试验系统，针对五种不同形貌的混凝土节理试件，开展常法向应力路径（Constant Normal Load, CNL）及不同卸荷应力路径下的直剪试验。所涉及的卸荷应力路径包括：卸除法向荷载且维持剪切荷载恒定（Unloading Normal Load and Constant Shear Load, UNLCSL）、卸除法向荷载且卸除剪切荷载（Unloading Normal Load and Unloading Shear Load, UNLUSL）、卸除法向荷载且增加剪切荷载（Unloading Normal Load and Increasing Shear Load, UNLISL）。试验结果表明：在CNL路径下，混凝土节理的峰值抗剪强度、黏聚力、内摩擦角、峰前剪切刚度以及残余抗剪强度均随节理粗糙度系数（Joint Roughness Coefficient, JRC）与法向应力的增大而提升。在UNLCSL路径下，当初始剪切应力τ₁相同时，节理失稳法向应力σ_i随JRC的增大而降低，法向荷载卸荷量Δσ_n则随JRC的增大而升高；在相同JRC条件下，σ_i随τ₁的增大而升高，Δσ_n则随τ₁的增大而降低。当JRC与σ_i均相同时，UNLCSL路径下的失稳剪切应力τ_i显著低于CNL路径。在相同JRC条件下，UNLCSL路径下试件的黏聚力低于CNL路径，而内摩擦角则高于CNL路径。当JRC与σ_i均相同时，CNL路径与UNLISL路径下的τ_i最大，UNLCSL路径次之，UNLUSL路径下的τ_i最小。相较于CNL路径，在UNLCSL、UNLISL及UNLUSL路径下，试件内摩擦角的变化幅度仅在3%以内，而黏聚力的平均降幅则达到37.6%。由此可推断，由法向卸荷引发的黏聚力降低是节理失稳剪切强度下降的主要原因。在引入黏聚力修正系数k对莫尔-库仑准则（Mohr-Coulomb criterion）进行修正后，修正后的最大平均相对误差仅为3.5%，相较于修正前的56.9%的最大平均相对误差，精度得到显著提升。本研究结论可为不同卸荷应力路径下岩石节理剪切承载能力的精准评估提供参考，对岩体工程的稳定性评价与防灾减灾具有重要意义。