For the in situ surrounding rocks of underground excavations, the radial stress corresponding to the minimum principal stress σ₃ varies with increasing distance away from the excavated boundary. To characterize the rock failure along the radial direction of an excavation, true triaxial compression tests with different σ₃ values (0.5, 5, 10, 15, 20, 25, and 30 MPa) were performed. The characteristic stresses (including the crack initiation stress σ_i, crack damage stress σ_d, and peak stress σ_s) and failure mode of the tested specimen under different σ₃ conditions were systematically investigated. The experimental results indicate that with increasing σ₃, the characteristic stresses increase monotonically and continuously. The increasing rate of the characteristic stresses with σ₃ ranging from 0 to 10 MPa is higher than that with σ₃ ranging from 10 to 30 MPa, indicating that low σ₃ impose a greater effect on the characteristic stresses than high σ₃. In addition, the failure mode of the tested specimen changes from tensile-shear failure mode I (with vertical tensile cracks and oblique tensile-shear cracks developing along the σ₂ direction) to tensile-shear failure mode II (with oblique tensile-shear cracks developing only along the σ₂ direction) to tensile-shear failure mode III (with oblique tensile-shear cracks along both σ₂ and σ₃ directions) as σ₃ increases. Then, the failure characteristic of the surrounding rocks of the underground excavation was characterized based on the obtained experimental results. When maximum radial stress σ_r encountered near the excavated boundary is much smaller than the axis stress σ_a, the surrounding rocks will exhibit a “Ternary failure characteristic”, i.e., with an increase in the distance from the excavated boundary along the radial direction, the failure mode of the surrounding rocks after excavation changes from tensile/splitting failure mode to tensile-shear failure mode I and then to tensile-shear failure mode II. When the maximum σ_r encountered near the excavated boundary approximates the original σ_a, the surrounding rock masses will exhibit a “Quaternary failure characteristic”. Specifically, the corresponding failure mode along the radial stress direction away from the excavated boundary changes from tensile failure mode to tensile-shear failure mode I then to tensile-shear failure mode II and finally to tensile-shear failure mode III.

对于在周围地下开挖的岩石的原位，对应于最小主应力的径向应力σ _3个因人而异从挖掘边界的距离增加了。沿着挖掘的径向方向表征岩石破坏，具有不同的真三轴压缩试验σ ₃的值（0.5，5，10，15，20，25，和30兆帕）进行。特性的应力（包括裂纹萌生的应力σ_我，裂缝损伤应力σ _d，和峰值应力σ_小号）不同下和故障模式的测试试样的σ ₃条件进行了系统地调查。实验结果表明，随着σ ₃，特性应力单调和连续地增加。特征应力与增加率σ ₃范围从0至10MPa比具有更高的σ ₃为10至30MPa，这表明低σ ₃上不是高的特性的应力施加更大的效果σ ₃。另外，被测试的试样的故障模式从拉伸剪切失效模式我改变（与垂直拉伸裂纹和斜拉伸剪切裂纹沿着显影σ ₂方向）上拉伸剪切失效模式II（具有倾斜拉伸剪切裂纹仅沿着显影σ ₂方向）上拉伸剪切失效模式III（沿两个倾斜拉伸剪切裂纹σ ₂和σ _3个如方向）σ ₃增加。然后，根据获得的实验结果，表征了地下基坑围岩的破坏特征。当最大径向应力σ _[R挖掘边界附近遇到比轴应力小得多σ_一个，围岩将表现出“三元破坏特征”，即随着沿径向距开挖边界的距离增加，开挖后围岩的破坏模式从拉伸/分裂破坏模式变为拉伸剪切破坏模式I，然后到拉伸剪切破坏模式II。当最大σ _[R开挖边界近似于原始附近遇到σ_一，围岩将表现出“第四纪破坏特征”。具体地，沿径向应力方向远离开挖边界的相应破坏模式从拉伸破坏模式改变为拉伸剪切破坏模式I，然后变为拉伸剪切破坏模式II，最后变为拉伸剪切破坏模式III。