Interest in the physical processes controlling systematic fracture (joint) formation in sedimentary rocks has heightened in recent years, motivated by the importance of fractured rock masses to the petroleum industry, nuclear waste disposal efforts, and earthquake forecasting. This research focuses on: 1) characterization of joint geometries in sedimentary rocks by field mapping, 2) determination of the loading histories responsible for the geometries of multiple joint sets, and 3) understanding the influence of mechanical interaction on propagation paths, joint lengths, and joint intersections. The field work involves detailed mapping at scales of 1:50 to 1:100 of joint traces on bedding surfaces of well-exposed sedimentary units near Mexican Hat, Utah. The effects of stress state and crack interaction on joint development will be analyzed using solutions to boundary value problems of elasticity theory and the principles of fracture mechanics. Two dimensional laboratory model experiments of crack growth in PMMA plates using a computer-controlled, biaxial testing machine will investigate the influence of stress state on crack path and intersection. Experiments on PMMA plates with an easily fracturable brittle elastic coating will investigate the sequential development of multiple fracture sets as a function of loading history. The results of this research will provide guidelines for interpreting the loading history implied by certain joint patterns in sedimentary rock. It will also address important questions about hydraulic connectivity of joint sets and the mechanical behavior of a jointed rock mass.
