For rock fractures, the degradations in the strength of contacting asperities and the surface frictional resistance are responsible for the water-induced weakening in the shear strength. To quantitatively examine their independent roles, direct shear tests on sawtooth fracture samples of granite and sandstone under three moisture conditions: dry, surface wet and saturated, were conducted subject to three levels of normal stresses. The surface wet condition only resulted in the variation in the basic friction angle and the saturated samples underwent the degradation in both unconfined compressive strength (UCS) and basic friction angle, which were obtained via unconfined compression test and direct shear test, respectively. Two weakening coefficients that represent the reductions in UCS and basic friction angle, respectively, were proposed and incorporated into an analytical model, which quantifies the entire shear stress evolutions during shear based on a continuous yielding mechanism. The difference in the shear strength between dry and surface wet conditions originates from the lubricant effect of water represented by the change in basic friction angle with a weakening coefficient less than 10% for both rocks. Under the saturated condition, the weakening coefficient of the UCS varies significantly from 15.17% for granite to 50.39% for sandstone. A series of datasets that characterize the reductions in UCS and basic friction angle induced by water were collected from the literature, which were then incorporated into the analytical model to estimate the general weakening trend in the shear strength of the common rocks in practices. For crystalline rocks, the water-mediated lubrication seems to be the primary mechanism reducing the shear strength, while for sedimentary rocks, the remarkable degradation in UCS may dominate the weakening mechanism. The quantified weakening coefficients and the revealed weakening behavior of various rocks can be directly linked to the fracture shear strength estimation in engineering design.

中文翻译：

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干、地表湿和饱和条件下岩石裂缝的抗剪强度

对于岩石裂缝，接触粗糙体强度和表面摩擦阻力的降低是水致剪切强度减弱的原因。为了定量检查它们的独立作用，在三种水分条件下对花岗岩和砂岩的锯齿状断裂样品进行直剪试验：干燥、表面湿润和饱和，在三个水平的正应力下进行。表面湿润条件仅导致基本摩擦角的变化，饱和样品的无侧限抗压强度（UCS）和基本摩擦角均发生退化，分别通过无侧限压缩试验和直剪试验获得。两个弱化系数分别代表 UCS 和基本摩擦角的减小，提出并结合到一个分析模型中，该模型基于连续屈服机制量化了剪切过程中的整个剪切应力演变。干燥和表面湿润条件之间剪切强度的差异源于水的润滑作用，以两种岩石的弱化系数均小于 10% 的基本摩擦角变化为代表。在饱和条件下，UCS 的弱化系数从花岗岩的 15.17% 到砂岩的 50.39% 变化很大。从文献中收集了一系列表征 UCS 和水引起的基本摩擦角减小的数据集，然后将这些数据集纳入分析模型，以估计实践中普通岩石剪切强度的总体减弱趋势。对于结晶岩，水介导的润滑似乎是降低剪切强度的主要机制，而对于沉积岩，UCS 的显着退化可能主导了削弱机制。各种岩石的量化弱化系数和揭示的弱化行为可以直接与工程设计中的断裂剪切强度估计相关联。