Detection and Characterization of Precursors to Shear Failure

岩石是很难表征和监测的材料，因为它们高度不均匀的性质导致了复杂的时间和尺度相关行为。一些尺度相关的行为是由岩石中存在的“缺陷”或“机械不连续”引起的。这些不连续面的范围从原子尺度，其中原子晶格可能具有不规则性；到颗粒尺度，其中裂纹在颗粒接触处普遍存在，以及直到区域或大陆尺度，这种不连续从毫米尺度到数百公里长的断层，如北安纳托利亚断层或圣安德烈亚斯断层。岩石研究和工程实践中最重要的限制之一是我们无法准确检测岩石内部缺陷和不连续面的存在，以及无法确定在外部因素应力作用下不连续面的演化。目前对岩石损伤的认识在很大程度上依赖于实验室的实验，在实验室中，直接观察岩石试件的表面来观察新的裂缝形成。这项研究的目的是建立岩石在荷载作用下的力学响应和地球物理响应之间的关系，为岩石系统的地震监测技术提供基础，并使分析工具的开发能够评估施工期间和之后的岩体状况。利用弹性波在裂隙岩石中传播作为一种有效监测技术的前景源于两位Co-Pis之前的发现，他们发现在摩擦不连续的剪切破坏之前存在前兆，分别以透射波和反射波中不同的极大值或极小值的形式存在。我们检测新裂缝或沿现有不连续面滑动的新破坏即将出现的能力的进步，将防止施工期间的故障，确保废物储存的长期遏制，或避免由采矿、地下流体储存或深层废水注入等人为活动引发的地震。将进行一系列实验室实验，以检验以下假设：这些前兆(1)是由地震传感器监测的区域的局部断裂特定刚度降低造成的，(2)是由凹凸体的滑动和破坏引起的，以及沿整个不连续面的破坏是沿着不连续面的渐进滑动的结果。实验将包括对岩石节理施加双向压缩载荷，更具体地说，是直接剪切。将使用数字图像相关和地球物理方法来监测接头的滑动和滑动失效，其中包括波传播测量(包括透射波和反射波，以及其他转换模式)。测试将在花岗岩、石灰岩或砂岩和页岩上进行，每种岩石都有不同的粒度和各向异性，以及可能紧密或充满土壤沉积物的节理。将对干燥和饱和岩石样本进行调查，以确定与岩石饱和度变化相关的地球物理特征，并确定和量化最适合监测不连续面潜在剪切破坏的模式。

Rocks are difficult materials to characterize and monitor because of their highly heterogeneous nature that leads to complex time- and scale-dependent behavior. Some of the scale-dependent behavior arises from "defects" or "mechanical discontinuities" that exist in the rock. These discontinuities range from the atomic scale, where atom lattices may have irregularities; to the grain scale, where cracks are pervasive at the grain contacts, and up to the regional or continental scale where discontinuities such joints occur from the millimeter scale to faults that range over hundreds of kilometers, such as the North Anatolian fault or the San Andreas fault. One of the most important limitations in research and engineering practice on rocks is our inability to detect accurately the presence of defects and discontinuities inside the rock and to determine the evolution of discontinuities with stress from external agents. Current knowledge of rock damage strongly relies on experiments in the laboratory where direct observations of the surfaces of the rock specimens are made to observe new crack formation. The objective of the research is to establish the relationship between mechanical and geophysical responses of rock under loading to provide a basis for seismic monitoring techniques of rock systems, and to enable the development of analysis tools to evaluate the condition of a rock mass during and after construction. The promise of using elastic wave propagation through fractured rock as an effective monitoring technique is rooted in previous findings by the two Co-PIs who discovered the presence of precursors prior to shear failure of frictional discontinuities, in the form of either distinct maxima or minima in transmitted and reflected waves, respectively. Advances in our ability to detect impending new damage in the form of new cracks or slip along pre-existing discontinuities will prevent failures during construction, ensure long term containment for waste storage, or avoid earthquakes induced by anthropogenic activities such as mining, underground fluid storage or deep wastewater injection.A series of laboratory experiments will be performed to test the hypotheses that these precursors (1) result from a reduction in local fracture specific stiffness in the region monitored by the seismic transducers, and (2) are caused by slip and damage of the asperities, and the hypothesis that failure along the entire discontinuity is the result of progressive slip along the discontinuity. The experiments will consist of subjecting rock joints to biaxial compression loading, and more specifically to direct shear. Digital Image Correlation and geophysical methods will be used to monitor slip and slip failure of the joints, which include wave propagation measurements (both transmitted and reflected waves, as well as other converted modes). The tests will be conducted on granite, limestone or sandstone and shale, each rock having different grain size and anisotropy, and on joints that may be tight or filled with a soil deposit. Both dry and saturated rock specimens will be investigated to determine the geophysical signatures associated with changes in rock saturation, and to identify and quantify the modes that are best-suited for monitoring the potential shear failure of a discontinuity.