Progressive Failure of Slopes and Excavations as Fracture Problems

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The objective of this research is to develop fracture models and provide experimental data that can be used to assess conditions associated with progressive failure in cohesive, frictional materials. The basis for the method involves analyzing the propagation of a fracture, where the theoretical model is an exact representation of the stresses and displacements, and the results will be presented for (1) shear banding within a volume (slopes), and (2) crack growth near free surfaces (excavations). The analyses for volume-type instabilities will include (a) various shapes of the fracture, e.g. circular and log spiral; and (b) nonlinear and time-dependent behavior, e.g.plasticity and viscoelasticity. The analyses for surface-type instabilities will include the (a)initiation and growth of cracks near a free surface, including interactions; and (b) curvature of the free surface associated with a borehole or tunnel. Novel experimental apparatus will be used to measure the fracture characteristics of propagating shear bands and spalling instability phenomena. The approach to evaluate the conditions for progressive failure is based on the use of the hypersingular boundary integral equation for a piece-wise homogeneous medium (layered systems are allowed), with features such as interfaces (joints, faults) and multiple openings (drifts, shafts). The main virtue of hypersingular integral equations is that they present a convenient means to deal with discontinuities and other discrete surfaces. Hypersingular boundary integral equations contain the displacement discontinuities and tractions, the values that characterize the interactions at the surfaces. This powerful technique will be useful to evaluate conditions of fracture within a volume and near a free surface for excavations at the Deep Underground Science and Engineering Laboratory. Knowledge transfer is tightly integrated into the research program, where exchange and engagement with the broader geotechnical engineering community will be accomplished through software for interactive analysis. A general K-12 education program will use the familiarity and esthetic appeal of slopes and excavations to engage a broad spectrum of learners. Small exhibits will be designed and fabricated to demonstrate progressive failure through experiments on fracture of a weakly cemented sandstone.