EGS Collab Experiment 1 Site Geomechanical and Hydrological Properties by Triaxial Direct Shear

Abstract

We apply a triaxial direct-shear method to measure multiple geomechanical and hydrological properties of the Poorman’s schist in the EGS Collab project’s Experiment 1 (E1) test bed at the Sanford Underground Research Facility (SURF). Our method with integrated real-time x-ray imaging provides measurement of: 1) pre-shear natural fracture and matrix permeability, 2) natural fracture structure, 3) peak shear strength, 4) residual shear strength, 5) hydroshear critical pore pressures, 6) post-fracture permeability, 7) density, 8) acoustic velocity, 9) shear-dilation angles, and 10) aperture versus effective-stress correlations. The experiments use 25 mm diameter by 25 mm length right-regular cylinder core plugs. From these experiments, performed at actual site triaxial stress conditions, we have identified that natural foliation fractures containing graphite and/or aligned micas are the most likely features that could be hydroshear stimulated. We also find that the rock matrix permeability is very low, at less than 10 nD, which is less than previously expected. Hydroshear was successful after mechanically shearing the specimens but was not successful for bonded intact natural fractures or matrix rock.