An experimental investigation of failure mechanical behavior in cylindrical granite specimens containing two non-coplanar open fissures under different confining pressures

Abstract

Fissures play a significant role in predicting the unstable failure of rock mass engineering. For deep rock underground engineering, rock mass containing pre-existing fissures is usually located in triaxial stress state. Therefore, not only pre-existing fissure but also confining pressure affects the failure mechanical behavior of rock material. In this research, the granite specimens containing two non-coplanar open fissures were investigated by a series of conventional triaxial compression tests. First, the effect of bridge angle and confining pressure on strength and deformation characteristics of granite specimens was evaluated. Results show that the triaxial compressive strength, failure axial strain, and crack damage threshold increased nonlinearly with confining pressure. Under high confining pressures, elastic modulus was insensitive to bridge angle. Then, an X-ray micro-CT scanning technique was used to analyze the internal fracture characteristics of granite specimens with respect to various bridge angles and confining pressures. Five typical crack coalescence modes were identified, namely, indirect coalescence, shear coalescence and three types of tensile coalescence. The reconstructed 3-D CT images indicated that under uniaxial or low confining pressures, the bridge angle had a significant effect on crack evolution behavior, while under high confining pressures, shear-dominated failures occurred with the development of anti-wing cracks.

摘要

裂隙在岩体工程失稳破坏预测中具有重要作用。对于深部岩体地下工程,含裂隙岩体通常处于三轴应力状态。因此,岩石材料破坏力学特性不仅受到原生裂隙的影响,还受到围压的影响。本文采用常规三轴压缩试验对含两个非共面张开裂隙花岗岩试样进行了研究。首先,研究了岩桥倾角和围压对花岗岩试样强度和变形特性的影响,结果表明:三轴抗压强度、破坏轴向应变和裂纹损伤阈值随围压的增大呈非线性增加; 高围压下,弹性模量对岩桥倾角不敏感。然后,利用X射线微米CT扫描技术分析了不同岩桥倾角和围压下花岗岩试样内部断裂特征,定义了5 种典型的裂纹贯通模式,即间接贯通模式、剪切贯通模式以及三种拉伸贯通模式。重构的三维CT图像表明,单轴和低围压条件下,岩桥倾角对裂纹演化行为有显著影响; 而高围压条件下,随着反向翼型裂纹的扩展,剪切主导最终的破坏。