Geomechanical assessment of excavation damaged zone in the Horonobe Underground Research Laboratory, Japan

In a high-level radioactive waste (HLW) disposal project, it is important to understand the characteristics of an excavation damaged zone (EDZ). An EDZ can strongly influence not only rock mass deformation behavior but also can adversely affect the permeability around the underground openings. Therefore, it is important to evaluate the development and extent of an EDZ quantitatively. Japan Atomic Energy Agency (JAEA) has been conducting the Horonobe Underground Research Laboratory (URL) Project in order to enhance reliability of technology related to deep geological disposal of HLW in sedimentary rocks. As a part of the research, JAEA has been implementing comprehensive investigations of EDZ development at 250 m depth in the facility. In an earlier study, a conceptual model of fractures induced by gallery excavation (EDZ fracture) was developed. Although the conceptual model helps us to understand the size, geometry (strike and dip) and frequency of EDZ fractures, their extent and geomechanical properties have not been described in detail yet. To understand these properties, the authors conducted P-wave tomography surveys during the gallery excavation, and examined rock core from the tomography area. In addition to these measurements and observations, the authors conducted numerical analyses to understand the distribution of stress and strain to identify the location of a damaged zone. As a result of P-wave tomography surveys, a low velocity layer, one with less than 80% of the seismic velocity of the rock mass before excavation, has been detected. This layer extends about 0.6 m into the drift wall. In the layer, several EDZ fractures can be observed in rock core. Furthermore, the numerical analysis showed that the microcracking region, the non-linear-elastic region that developed prior to failure in the stress-strain curve from uniaxial compression tests, extends about 0.2 m into the drift wall. From these results, the authors concluded that non-linear-elastic behavior would occur with development of EDZ fractures in the low velocity layer detected by P-wave tomography surveys. (author)