Mechanics of Sheeting Joints and Near-Surface Stresses

片状节理是地球内部应力与地形相互作用的普遍表现形式，是在凸起地形下形成的近平行于地球表面的张开型断裂。它们与岩石板块结合，岩石板块的曲率随着深度的增加而减小。它们出现在所有主要的岩石类型中，达到数百米的尺寸，并且在景观发展，边坡稳定性和地下水流动中非常重要。节理被广泛认为是由于覆盖层的去除而形成的，但是仅仅通过释放压力不会在岩石中打开大的裂缝：需要张应力或高流体压力。本项目测试的假设，薄板接缝的形式，由于地形引起的拉应力，薄板接缝发展垂直于当地最大的拉应力。大量文献表明，在板状节理出现的地方存在高的平行于地表的压缩，在压缩足够强且地形凸起的地方，必须形成垂直于地表的拉应力以保持平衡。该研究结合了详细的现场表征，现有的应力测量，曲率和斜率的计算，以及应力分析。现场调查发生在约塞米蒂国家公园，在那里的近地表应力已被测量和板状节理是极好的暴露在不同大小和形状的可访问的基岩表面。使用高分辨率数字航空照片和数字机载激光测高（LIDAR）数据绘制关节和地形图。在现场测量了节理的方位和相对位移。利用微分几何方法从激光雷达数据确定地形曲率。基岩中的应力场进行了调查，精确的解析解和二维和三维的边界元程序，占地形，重力，水平区域应力，以及裂缝的存在，以测试如何以及可以预测的关节的分布曲率测量和现有的应力测量在公园。薄片状节理，也称为剥落节理，引起地质学家的兴趣已有两个多世纪。每年都有成千上万的学生通过教科书介绍薄板接缝，这些教科书将薄板接缝归因于卸载或压力释放。这项研究为这些节理提供了一种新的解释，应该解决构造地质学中的一个经典问题，应该重写教科书。对近地表应力的新认识，这对许多地质过程产生重大影响（例如，断裂、地下水流、质量浪费（包括约塞米蒂山谷的岩崩）、火山过程、物理风化等）将通过约塞米蒂国家公园的公众宣传获得和分享。研究生和本科生参与了项目的各个方面。

Sheeting joints, widespread manifestations of the interaction of the Earth's internal stresses and topography, are opening-mode fractures formed subparallel to the Earth's surface beneath convex topography. They bound slabs of rock whose curvature decreases with depth. They occur in all major rock types, attain dimensions of hundreds of meters, and are important in landscape development, slope stability, and groundwater flow. The joints are widely regarded as forming in response to removal of overburden, but large fractures do not open in rocks merely by relieving compression: tensile stresses or high fluid pressures are required. This project tests the hypothesis that sheeting joints form due to topographically induced tensile stresses, and that sheeting joints develop perpendicular to the local most tensile stress. High surface-parallel compression is widely documented where sheeting joints occur, and where the compression is sufficiently strong and the topography is convex, tensile stresses perpendicular to the ground surface must develop to maintain equilibrium. The research combines detailed field characterization, existing stress measurements, calculations of curvature and slope, and stress analyses. Field investigations occur in Yosemite National Park, where the near-surface stresses have been measured and sheeting joints are superbly exposed on accessible bedrock surfaces of different size and shape. The joints and topography are mapped using high-resolution digital aerial photographs and digital airborne laser altimetry (LIDAR) data. The orientations of joints and the relative displacements across them are measured in the field. The topographic curvature is determined from the LIDAR data using methods of differential geometry. The stress field in the bedrock is investigated with exact analytical solutions and with two- and three-dimensional boundary element procedures that account for topography, gravity, horizontal regional stresses, and the presence of fractures to test how well the distributions of the joints can be predicted from curvature measurements and existing stress measurements in the park. Sheeting joints, also known as exfoliation joints, have intrigued geologists for more than two centuries. Thousands of students each year are introduced to sheeting joints through textbooks that attribute sheeting joints to unloading or pressure release. This research offers a new explanation for these joints, should solve a classic problem in structural geology, and should rewrite the textbooks. New insights into near-surface stresses, which critically affect many geologic processes (e.g., fracturing, groundwater flow, mass wasting (including rockfall in Yosemite Valley), volcanic processes, physical weathering, etc.) will be gained and shared with through public outreach at Yosemite National Park. Graduate students and undergraduate students are involved in all aspects of the project.