Non-dilatant deformation and failure mechanism in two Long Valley Caldera rocks under true triaxial compression
Non-dilatant deformation and failure mechanism in two Long Valley Caldera rocks under true triaxial compression
复制标题
DOI：
10.1016/j.ijrmms.2005.01.002
复制
发表时间：
2005-04
影响因子：
7.2
通讯作者：
Chandong Chang;B. Haimson
中科院分区：
文献类型：
--
作者：
Chandong Chang;B. Haimson
我们进行了实验室岩石强度实验，在两个超细颗粒的脆性岩石，角岩和变岩，这两个共同的长谷火山口（加州，美国）在2025- 2996米深度范围内的基底的主要组成部分。这两种岩石都是带状的，孔隙度很低。单轴压缩试验在不同的方向相对于带状平面显示，而角岩的抗压强度几乎是各向同性的，变岩具有明显的各向异性。这些岩石的常规三轴试验表明，它们各自在特定方向上的强度随围压近似线性增加。真三轴压缩实验中，试样的方向与条带成一致的角度，其中最小主应力（σ3）和中间主应力（σ2）的大小不同，但在试验过程中保持恒定，而最大主应力增加，直到破坏，表现出与以前在类似试验条件下观察到的其他岩石不同的行为。对于给定的σ3值，无论施加的σ2如何，两种长谷岩石类型的抗压强度σ 1均无显著变化，表明中间主应力效应很小或没有。加载过程中在所有三个主要方向上测量的应变用于获得σ 1与体积应变的关系图。这些都是一致的线性几乎到岩石破裂的点，这表明没有相干性。这一现象得到了失效试样的SEM检查的证实，在出现一个沿σ 3方向急剧倾斜的贯穿剪切破坏面之前，没有微裂纹发展。在其他结晶岩石中，抗压强度对中间主应力的强烈依赖性与微裂纹萌生有关，微裂纹萌生随σ 2的增加而增加，并延缓了破坏过程。我们推断，长谷岩石的σ 2强度独立性直接来自于它们的非塑性变形。
We conducted laboratory rock strength experiments in two ultra-fine-grained brittle rocks, hornfels and metapelite, which together are the major constituent of the Long Valley Caldera (California, USA) basement in the 2025–2996m depth range. Both rocks are banded, and have very low porosity. Uniaxial compression tests at different orientations with respect to banding planes reveal that while the hornfels compressive strength is nearly isotropic, the metapelite possesses distinct anisotropy. Conventional triaxial tests in these rocks reveal that their respective strengths in a specific orientation increase approximately linearly with confining pressure. True triaxial compression experiments in specimens oriented at a consistent angle to banding, in which the magnitudes of the least (σ3) and the intermediate (σ2) principal stresses are different but kept constant during testing while the maximum principal stress is increased until failure, exhibit a behavior unlike that previously observed in other rocks under similar testing conditions. For a given magnitude of σ3, compressive strength σ1does not vary significantly in both Long Valley rock types, regardless of the applied σ2, suggesting little or no intermediate principal stress effect. Strains measured in all three principal directions during loading were used to obtain plots of σ1versus volumetric strain. These are consistently linear almost to the point of rock failure, suggesting no dilatancy. The phenomenon was corroborated by SEM inspection of failed specimens that showed no microcrack development prior to the emergence of one through-going shear failure plane steeply dipping in the σ3direction. The strong dependency of compressive strength on the intermediate principal stress in other crystalline rocks was found to be related to microcrack initiation upon dilatancy onset, which rises with increased σ2and retards the failure process. We infer that strength independence of σ2in the Long Valley rocks derives directly from their non-dilatant deformation.