Mechanics of Sheeting Joints and Near-Surface Stresses

板状节理是地球内应力与地形相互作用的普遍表现，是在凸地形下与地球表面近平行形成的开口型裂缝。它们将弯曲度随深度而减小的岩石板捆绑在一起。它们存在于所有主要的岩石类型中，尺寸可达数百米，对景观发育、边坡稳定和地下水流动都很重要。人们普遍认为这些节理是由于覆盖层的移除而形成的，但是岩石中的大裂缝并不仅仅是通过解除压缩而打开的：需要拉应力或高流体压力。该项目测试了板状接缝的形成是由于地形诱发的拉应力，以及板状接缝垂直于局部最大拉应力发展的假设。高表面平行压缩被广泛记录在板状节理发生的地方，在压缩足够强且地形是凸的地方，垂直于地面的拉应力必须发展以保持平衡。该研究结合了详细的现场特征、现有的应力测量、曲率和斜率计算以及应力分析。在约塞米蒂国家公园进行了实地调查，在那里测量了近地表应力，并在不同大小和形状的可接近基岩表面上完美地暴露了板状节理。使用高分辨率数字航空照片和数字机载激光测高（LIDAR）数据绘制了接缝和地形。在现场测量了关节的方向和它们之间的相对位移。利用微分几何方法从激光雷达数据中确定地形曲率。基岩中的应力场是用精确的解析解和二维和三维边界元素程序来研究的，这些程序考虑了地形、重力、水平区域应力和裂缝的存在，以测试通过曲率测量和公园内现有的应力测量来预测节理分布的程度。两个多世纪以来，薄片节理，也被称为剥落节理，一直引起地质学家的兴趣。每年都有成千上万的学生通过教科书了解钢板接头，这些教科书将钢板接头归因于卸载或压力释放。该研究为这些节理提供了新的解释，解决了构造地质学中的一个经典问题，改写了教科书。近地表应力对许多地质过程（如压裂、地下水流动、物质浪费（包括约塞米蒂山谷的岩崩）、火山过程、物理风化等）产生了重大影响，将通过约塞米蒂国家公园的公众宣传获得新见解，并与之分享。研究生和本科生参与项目的各个方面。

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.