Abstract In deep underground coal mining, engineering activities are performed within anisotropic in-situ stress fields due to engineering disturbances and tectonic stress. Many such activities involve the development of excavations in soft rock and anisotropic coal. Accordingly, studying the mechanical properties of soft rocks is important for the stability of deep underground excavations. In this study, the deformation, strength, and failure characteristics of soft sandstone and raw coal under two different true triaxial loading paths were investigated using a self-developed true triaxial test apparatus. The results indicated that the inelastic strain in the pre-peak stage of sandstone and coal gradually increased with increasing intermediate principal stress. Also, the strength-drop in the post-peak stage increased. The crack initiation stress, crack damage stress, and peak strength of sandstone and coal first increased and then decreased with increasing intermediate principal stress for a given σ3. Moreover, with increasing intermediate principal stress, the failure mode of sandstone and coal changed from shear to tensile shear, and from brittle to semi-brittle. The linear Mogi criteria as found to characterize the true triaxial strength of coal well, while the modified Lade criteria was more applicable to soft sandstone. Owning to the symmetry assumption, the linear Mogi criteria predicted low strength when the intermediate principal stress coefficient exceeded to 0.5. In addition, the peak strength curve of rock on the π plane and the influence of weak structures on the failure mode of anisotropic coal was discussed. Weak structures have an important influence on the failure mode of coal, which depends on the strength difference between the structural plane and the coal rock mass. The strength envelope on the π plane had a significant stress Lode angle effect, which gradually decreased as the mean stress increased.

中文翻译：

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不同加载路径下立方煤和砂岩的真实三轴强度及破坏特征

摘要 在深部地下煤矿开采中，由于工程扰动和构造应力，工程活动在各向异性的地应力场内进行。许多此类活动涉及在软岩和各向异性煤中进行挖掘。因此，研究软岩的力学特性对于深部地下开挖的稳定性具有重要意义。本研究利用自主研发的真三轴试验装置研究了软砂岩和原煤在两种不同真三轴加载路径下的变形、强度和破坏特性。结果表明，随着中间主应力的增大，砂岩和煤的峰前阶段的非弹性应变逐渐增大。此外，高峰后阶段的强度下降幅度也有所增加。裂纹起始应力，对于给定的 σ3，砂岩和煤的裂纹损伤应力和峰值强度随着中间主应力的增加先增加后减小。此外，随着中间主应力的增加，砂岩和煤的破坏模式由剪切向拉伸剪切转变，由脆性向半脆性转变。发现的线性 Mogi 准则可表征煤井的真实三轴强度，而修正的 Lade 准则更适用于软砂岩。由于对称性假设，当中间主应力系数超过 0.5 时，线性 Mogi 准则预测低强度。此外，还讨论了π平面上岩石的峰值强度曲线和弱结构对各向异性煤破坏模式的影响。弱结构对煤体的破坏模式有重要影响，其取决于结构面与煤岩体的强度差异。π平面上的强度包络线具有显着的应力洛德角效应，随着平均应力的增加而逐渐减小。