北山花岗岩在三轴压缩条件下的强度参数演化

采用MTS815岩石力学试验机对北山新场深部花岗岩进行三轴循环加、卸载试验,研究岩石强度参数的演化特征。基于Mohr-Coulomb相关理论推导与分析,探讨岩石发生屈服后的强度变化规律。在分析不同围压条件下岩石全应力–应变曲线的基础上,以塑性剪切应变为塑性参数,建立北山花岗岩黏聚力、内摩擦角和剪胀角随塑性参数变化的数学模型。研究结果表明:(1)在损伤应力点,岩石塑性剪切应变接近于0,损伤应力可作为北山花岗岩塑性参数的零点,其亦可作为岩石强度参数演化的起点。(2)在损伤应力点后,岩石黏聚力随塑性参数的增加呈指数函数形式衰减并最终趋近于0;内摩擦角随塑性参数的增加以对数正态函数的形式表现出先增加后减小的趋势,且岩石残余内摩擦角值与起始内摩擦角值接近。(3)损伤应力后的岩石剪胀行为与峰后剪胀行为相似,剪胀角随着塑性参数和围压的增加而不断减小,且对低围压条件更为敏感。(4)将建立的模型嵌入到数值模拟工具中,通过模拟岩石三轴压缩试验,可证实模型的准确合理性。

Variation of strength parameters of Beishan granite under triaxial compression

Triaxial cyclic loading-unloading tests on Beishan granite from deep underground were conducted using the rock mechanical test system MTS815 to investigate the variation of strength parameters of the rock. The variation characteristics of rock strength parameters were discussed in detail according to the Mohr-Coulomb theory. The measured complete stress-strain curves under the different confining stresses were analyzed. The plastic shear strain was regarded as the plastic parameter to describe the strength and dilation behaviors of rock. The results indicate that the damage stress can be regarded as the zero point of plastic parameter and as the start point of the strength parameter variation. When applied stress is larger than the damage stress，the cohesion of the rock decreases exponentially with increasing the plastic parameter and finally approaches zero. Meanwhile，the frictional angle increases to the peak value and then decreases gradually to a constant value as the plastic parameter increases. The logarithmic normal distribution can be used to fit the data. The dilation behavior of the rock after the damage stress is similar to that of the post-peak stage. The dilation angle decreases with increasing plastic deformation and confining stresses，and is sensitive under the low confining pressures. The established models were implemented in the numerical software(FLAC) to simulate the triaxial compression tests. The predictions reproduced the main features of actual mechanical and dilation behaviors of Beishan granite.