真三轴条件下横观各向同性岩石强度特征及应力路径相关性研究

Transversely isotropic rock (TIR) commonly refers to the rock in which there exists one set of planar structure, e.g., bedding or schistosity. The strength characteristic of TIR under true-triaxial condition and its dependence on stress path are by far the crucial theoretical issues in rock mechanics, and are critical for reasonable evaluation and prediction of the stabilities of large-scale underground caverns in layered rock masses. In this study a true-triaxial experimental system for hard rock will be adopted as the main test apparatus. Based on tests under typical loading angle combinations, we aim at revealing the variation of TIR strength with loading angles under true-triaxial loading paths. A systematic experimental study of the possible effects of stress paths on strength, which puts particular emphasis on simulating the excavation-induced and cyclic loading-unloading stress paths, will then be carried out via independent control of the three principal stresses. Based on these results, we attempt to unearth the mysteries of the stress path dependence of TIR strength under complex true-triaxial stress paths. Failure mechanisms of TIR under different loading angles and stress paths will also be unraveled. We then develop a novel true-triaxial strength criterion for TIR incorporating the effects of both loading angle and stress path. This new criterion will be a major advancement in realizing quantitative descriptions of the loading angle effect and parameter evolution of TIR strength. Meanwhile, the stress path effect on the TIR strength will be interpreted via this new criterion. The new strength criterion will be verified by analyzing the stability of a typical large-scale underground cavern in layered rock masses. This study will provide theoretical basis for predicting the instability of projects in transversely isotropic rock masses.

具有层理或片理等面状构造的横观各向同性岩石的真三轴强度特征及应力路径相关性是目前岩石力学研究的重要理论问题，对于合理评估大型地下洞室层状围岩稳定性具有重要意义。本项目拟采用硬岩真三轴实验系统，基于典型加载角度组合试验，揭示横观各向同性岩石真三轴加载条件下强度随加载角度的变化特征；通过独立控制真三向应力开展开挖扰动及循环加卸载应力路径试验，揭示真三轴复杂应力路径下横观各向同性岩石强度的应力路径相关性特征，辨析横观各向同性岩石不同加载角度及不同应力路径下的破坏机理。建立考虑加载角度及应力路径双重影响的横观各向同性岩石真三轴强度准则，实现对横观各向同性岩石真三轴强度的加载角度效应以及强度参数损伤演化的定量表征，解译应力路径对横观各向同性岩石真三轴强度的影响程度。将新准则应用于典型大型地下洞室层状围岩稳定性分析以验证准则有效性。项目可为预测横观各向同性岩体工程失稳破坏提供理论依据。

随着我国交通及地下工程的迅猛发展，规划新建或在建的铁路、公路隧道、水工隧洞的数量及里程也呈较大规模的增长。其中有大量的隧道穿越了板岩、千枚岩、页岩等层状岩体。层状岩石在物质成分、结构构造等方面具有优势发育方位，通常被视为横观各向同性体。隧道开挖后切断了层状岩体结构的连续性，在重分布应力的驱使下，围岩不同位置会产生类型及深度各异的破坏特征，如木寨岭隧道倾斜岩体发生了严重的顺层偏压现象，杨家坪隧道陡倾岩体发生了岩层挠曲破坏等。这些工程活动引发的岩体破坏现象与层状岩体力学性质的各向异性特征密切相关。随着我国地下工程逐步向地球深部进军，深埋条件下，岩体多处于三向不等的高应力状态。隧道开挖后，岩体应力分异作用明显。若忽视层状岩体在真三向应力条件下的力学特性，势必会在层状围岩灾害机制分析，支护设计等方面缺乏科学的依据。基于此，本项目在层状岩石的试验方法建立、细观破坏过程、真三向应力条件下的各向异性效应、应力差效应、应力路径效应、破坏准则建立等方面展开了系统的研究。主要结论有：层状岩石的渐进性破坏过程与其内部层理结构的加载方位具有较强的相关性；层状岩石在真三轴加载下具有显著的ω强化效应。ω和σ2越大，试样的强度越大。层状岩石强度受σ2的影响比花岗岩、玄武岩、大理岩等各向同性岩石更为显著；当σ2及ω越小时，层状岩石的破坏越趋于受层理结构控制，反之则越趋于受应力控制。在深部工程中，应力水平较高，应力差较大，层状围岩的破坏则更可能受应力条件及层理结构的共同控制。本项目的研究成果可对现场层状围岩隧道的设计和支护提供指导。首先，若隧道轴线与岩层走向的夹角较小，则层面上的正应力释放程度较高，容易沿层面产生开裂或滑动。其次，隧道断面切向应力与层理的夹角不同，围岩的破坏机理也不同。在支护设计中可引入“靶向支护”的理念，即根据破坏位置、破坏机制和破坏程度，选择相应的支护措施，以降低支护成本，提高支护效率。