Underground mining or tunnelling activity is always associated with the composite geological formations. The mechanical properties of layered composite coal–rock subjected to true triaxial stress conditions are significantly different from those under conventional triaxial or uniaxial stress conditions. In this work, we conducted a series of true triaxial tests using the self-developed true-triaxial apparatus to investigate the mechanical response (e.g., deformation, strength, and failure characteristics) of the layered composite coal–rock (CCR). The results show that the uniaxial strength of CCR lies between the strength of pure sandstone and coal, and the direction of the bedding affects the overall strength of the samples. The true triaxial strength of both the pure rock and CCR increases first and then decreases with the increase of the intermediate principal stress. Moreover, for a given loading direction, as the thickness of the sandstone layer increased, the strength of the CCR increases. The deformation of the CCR shows more obvious plasticity than that of the pure sandstone due to the coordinated deformation of the coal and sandstone layers. In addition, a new true triaxial strength criterion expressed by the first and third equivalent principal stress invariants was proposed, which can well describe the strength characteristics of different coal rocks. The stress states, weak structural planes, and localized stress have a great influence on the failure modes of CCR. The local stress concentration near the contact surface promotes the development of the secondary failure fractures. These findings are of great significance in stability designing in deep underground engineering.

中文翻译：

---

真三轴应力作用下层状复合煤岩力学性能

地下采矿或隧道掘进活动总是与复合地质构造有关。真三轴应力条件下层状复合煤岩的力学性能与常规三轴或单轴应力条件下的力学性能有显着差异。在这项工作中，我们使用自主开发的真三轴仪进行了一系列真三轴试验，以研究层状复合煤岩（CCR）的力学响应（如变形、强度和破坏特性）。结果表明，CCR的单轴强度介于纯砂岩和煤的强度之间，层理方向影响试样的整体强度。纯岩和CCR的真三轴强度均随着中间主应力的增大先增大后减小。此外，对于给定的加载方向，随着砂岩层厚度的增加，CCR 的强度增加。由于煤层和砂岩层的协调变形，CCR的变形比纯砂岩表现出更明显的塑性。此外，提出了一种新的真三轴强度准则，由第一和第三等效主应力不变量表示，可以很好地描述不同煤岩的强度特征。应力状态、弱结构面和局部应力对 CCR 的失效模式有很大影响。接触面附近的局部应力集中促进了二次破坏断裂的发展。这些发现对深部地下工程的稳定性设计具有重要意义。