Detection and characterization of the disturbed rock zone in claystone with the complex resistivity method

Abstract

Underground excavations provoke in their vicinity a region, where the rock is disturbed, i.e., loosened due to micro as well as macro fractures. Shape, dimension and properties of this so-called ‘excavation damaged zone (EDZ)’ are of increasing importance for the planning and construction of geotechnical barriers in underground repositories for toxic and problematic wastes. One way of assessing the EDZ around a drift is the investigation of the geoelectrical complex resistivity. In addition to resistivity, the phase measurements yield information about the pore spaces and not only about the water content. Measurements of resistivity and phase are conducted in a newly built tunnel in the opalinus claystone of the Mont Terri underground rock laboratory. Special ring profiles, as well as a geometry-adapted inversion scheme, are used. Significant local and temporal changes are found in the resistivity as well in the phase, which are directly associated with the disturbed rock zone. Laboratory measurements allow the correlation and quantification of resistivity and water saturation of the rock. A geotechnical modelling and an analysis of the stress pattern around the drift show that the geoelectrical properties agree remarkable with the mechanical state of the rock.