Mechanics of Sheeting Joints

Sheeting joints, commonly known as “exfoliation” in granites, have an important impact on society. These near-surface fractures are geographically widespread, have a profound influence upon the landscape, and are found in numerous rock-types. The mechanics of sheeting joint formation bears directly on physical weathering, fluid flow, natural hazards such as mass wasting and rock fall, rock bursts in quarries, and the siting and design of dams and waste repositories. Although sheeting joints have been studied for centuries, their cause has remained enigmatic. They are usually attributed to removal of overburden. Removal of overburden, however, does not necessarily induce the tension required to open these surface-parallel fractures. Such a tension can arise from compressive stresses acting parallel to a curved topographic surface. We are testing the hypothesis that topographically-induced tension can account for sheeting joints. The key factors are: surface curvature, compressive stresses parallel to the surface, the unit weight of rock, and the slope. We use an expression derived from the differential equations of equilibrium to predict the presence or absence of sheeting joints based on the topography and surface stresses. We employ aerial LIDAR at ~1 m resolution, ground-based LIDAR at ~5 cm resolution, photographs, detailed fracture maps, and mechanical analyses in our treatment of sheeting joints in Yosemite National Park. Initial findings support our hypothesis and imply that the long-term strength of rock may be at least an order of magnitude less than that indicated in laboratory strength tests over short time scales.