Approaches to Determine Fault Shear Strength in Large-scale Direct Shear Test Simulations using Discrete Fracture Networks

In this study, 3DEC modeling in conjunction with the Discrete Fracture Networks (DFNs) technique was performed to better understand the behavior of shearing faults in a relatively large-scale direct shear test model. The DFNs explicitly generated two different geometries of the fault plane surface indicating the stepped or smooth waviness of the surface using probability distribution functions to generate orientation, fracture density (or frequency), and size of the discrete fractures. A 3-dimensional block model of hundreds of meters was constructed for the direct shear test using 3DEC modeling software. After the completion of the direct shear test simulations, a shear strength of the fault plane was estimated by back calculation. The DFNs with subhorizontal/vertical dip angles created an imbricate structure that resulted in a stepped geometry and strong shear response in the fault plane. Under direct shear, the fault plane needed to break the asperities or block before any frictional behavior was allowed or the fault plane would dilate significantly. This resulted in higher shear strength of the fault plane. On the other hand, the DFNs with oblique dip angles generated a relatively smooth surface of the fault plane. The smooth surface of the fault plane allowed deformation principally parallel to the fault plane, and only a few failures developed across the rock blocks in the model. Consequently, the 3DEC model explicitly replicated the complicated geometries of the fault plane using the two very contrasting DFNs and allowed researchers to investigate different shear behaviors of fault planes as a function of surface conditions.