The redistribution of three-dimensional (3D) geostress during underground tunnel excavation can easily induce to shear failure along rockmass structural plane, potentially resulting in engineering disasters. However, the current understanding of rockmass shear behavior is mainly based on shear tests under 2D stress without lateral stress, the shear fracture under 3D stress is unclear, and the relevant 3D shear fracture theory research is deficient. Therefore, this study conducted true triaxial cyclic loading and unloading shear tests on intact and bedded limestone under different normal stress σn and lateral stress σp to investigate the shear strength, deformation, and failure characteristics. The results indicate that under different σn and σp, the stress–strain hysteresis loop area gradually increases from nearly zero in the pre-peak stage, becomes most significant in the post-peak stage, and then becomes very small in the residual stage as the number of shear test cycles increases. The shear peak strength and failure surface roughness almost linearly increase with the increase in σn, while they first increase and then gradually decrease as σp increases, with the maximum increases of 12.9% for strength and 15.1% for roughness. The shear residual strength almost linearly increases with σn, but shows no significant change with σp. Based on the acoustic emission characteristic parameters during the test process, the shear fracture process and microscopic failure mechanism were analyzed. As the shear stress τ increases, the acoustic emission activity, main frequency, and amplitude gradually increase, showing a significant rise during the cycle near the peak strength, while remaining almost unchanged in the residual stage. The true triaxial shear fracture process presents tensile-shear mixture failure characteristics dominated by microscopic tensile failure. Based on the test results, a 3D shear strength criterion considering the lateral stress effect was proposed, and the determination methods and evolution of the shear modulus G, cohesion cjp, friction angle φjp, and dilation angle ψjp during rockmass shear fracture process were studied. Under different σn and σp, G first rapidly decreases and then tends to stabilize; cjp, φjp, and ψjp first increase rapidly to the maximum value, then decrease slowly, and finally remain basically unchanged. A 3D shear mechanics model considering the effects of lateral stress and shear parameter degradation was further established, and a corresponding numerical calculation program was developed based on 3D discrete element software. The proposed model effectively simulates the shear failure evolution process of rockmass under true triaxial shear test, and is further applied to successfully reveal the failure characteristics of surrounding rocks with structural planes under different combinations of tunnel axis and geostress direction.

中文翻译：

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真三轴多级加载和卸载剪切试验下岩体的剪切破坏行为和退化力学模型


地下隧道开挖过程中三维 （3D） 地应力的重新分布很容易导致沿岩体结构平面的剪切破坏，从而可能导致工程灾难。然而，目前对岩体剪切行为的认识主要基于无侧向应力的二维应力下的剪切试验，三维应力下的剪切断裂尚不清楚，相关三维剪切断裂理论研究存在不足。因此，本研究在不同法向应力 σn 和侧向应力 σp 下对完整和层状石灰岩进行了真正的三轴循环加载和卸载剪切试验，研究了其剪切强度、变形和破坏特性。结果表明，在不同 σn 和 σp 下，应力-应变磁滞回线面积从峰前阶段的几乎为零逐渐增大，在峰后阶段变得最显著，然后随着剪切试验循环次数的增加在残余阶段变得非常小。剪切峰值强度和破坏面粗糙度几乎随σn的增加呈线性增加，而随着σp的增加，它们先增加后逐渐减小，强度最大增加12.9%，粗糙度最大增加15.1%。剪切残余强度几乎随 σn 线性增加，但随 σp 变化不显著。基于测试过程中的声发射特性参数，分析了剪切断裂过程和微观破坏机理。随着剪切应力 τ 的增加，声发射活度、主频率和幅值逐渐增加，在接近峰值强度的循环中表现出显著上升，而在残余阶段几乎保持不变。 真正的三轴剪切断裂过程呈现以微观拉伸破坏为主的拉伸-剪切混合物破坏特性。基于试验结果，提出了考虑侧向应力效应的三维抗剪强度判据，研究了岩体剪切断裂过程中剪切模量G、内聚力cjp、摩擦角φjp和膨胀角ψjp的确定方法和演变。在不同 σn 和 σp 下，G 先迅速下降，后趋于稳定;CJP、ΦJP 和 ΨJP 先快速增加到最大值，然后缓慢下降，最后基本保持不变。进一步建立了考虑侧向应力和剪切参数退化影响的三维剪切力学模型，并基于三维离散元软件开发了相应的数值计算程序。该模型有效模拟了真三轴剪切试验下岩体的剪切破坏演化过程，并进一步应用于成功揭示隧道轴线和地应力方向不同组合下结构面围岩的破坏特征。