Particle size is an important factor affecting the Thermal-Hydraulic-Mechanical (THM) coupling behavior of graveled rock mass, especially for the shear mechanical properties. In this study, three groups of the particle size range and nine particle grading samples are designed for a large-scale direct shear test. The relationships between shear stress and shear displacement, shear strength, stress ratio, shear strength parameters, and particle breakage of pebble gravel are analyzed. The influence of particle size range and grade on the strength and particle breakage of gravel material is discussed. Results show evident particle breakage in the process of direct shear, and the degree of fragmentation is controlled by the normal load and the particle size distribution of the sample. The shear strength of the sample is no longer applicable to Mohr-Coulomb strength theory because of particle breakage that is more in line with the power function relationship. Shear strength of pebble gravel material has scale effect, and a corresponding relationship model between friction coefficient of material and characteristic particle size of the sample is proposed.

中文翻译：

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卵石碎石材料的直接剪切和颗粒破碎特性

粒径是影响砾石岩体热-水力-机械（THM）耦合行为的重要因素，尤其是剪切力学特性。在本研究中，设计了三组粒度范围和九个颗粒分级样品用于大规模直剪试验。分析了卵石砾石的剪切应力与剪切位移、剪切强度、应力比、剪切强度参数与颗粒破碎之间的关系。讨论了粒度范围和等级对砾石材料强度和颗粒破碎率的影响。结果表明，在直剪过程中，颗粒破碎明显，破碎程度受法向载荷和样品粒度分布控制。样品的剪切强度不再适用于莫尔-库仑强度理论，因为颗粒破碎更符合幂函数关系。卵石碎石材料的抗剪强度具有尺度效应，提出了材料摩擦系数与试样特征粒径的对应关系模型。