Evaluation of time-lapse behavior of excavation damaged zone by first-arrival tomography in Horonobe Underground Research Laboratory

Abstract

The drift excavation damages the intact rock near surface of the drift wall and changes the physical properties there. The damaged domain is called excavation damaged zone (EDZ). The precise estimation of depth and the understanding of time lapse behavior of EDZ is required for the technology of drift closure. In Horonobe Underground Research Laboratory (URL), we repeatedly performed the travel time tomography to investigate the time lapse behavior of EDZ generated in soft sedimentary rock over time. For the analysis of the acquired data for the travel time tomography, we performed numerical simulation and estimated the conceivable change in the arrival time. The results of numerical simulation indicate that the shotcrete on the drift wall has the great impact on the travel time because of its high stiffness whereas the effect of shotcrete fixing the ray path might be the advantage for the monitoring purpose. The analysis of the data acquired at 350 m suggest that the significant change in EDZ that surpasses the observation limit was not recognized.