Evaluation of temporal changes in fracture transmissivity in an excavation damaged zone after backfilling a gallery excavated in mudstone

Abstract

Abstract The long-term geological disposal of high-level radioactive waste relies on predictions of future changes in a disposal facility’s hydro-mechanical characteristics in order to assess potential leakage through fractures in the excavation damaged zone (EDZ) after backfilling the facility. This study evaluated the transmissivity of EDZ fractures using in situ hydraulic tests around the area of a full-scale, experimental, engineered barrier system in the Horonobe Underground Research Laboratory, Hokkaido, Japan. After their installation, the buffer blocks swelled, altering the stresses within the EDZ fractures. The effects of these changing stresses on the fractures’ transmissivity were assessed over a period of four years. The transmissivity continuously decreased in this period to about 26% of its value measured prior to the swelling. Using the Barton–Bandis normal-stress-dependent fracture-closure model, the decrease in transmissivity is quantitatively attributed to closure of the EDZ fractures, which was caused by the swelling pressure increasing up to 0.88 MPa. Evidence of fracture closure came from seismic tomography surveying, which revealed a slight increase in seismic velocity in the study area with increasing swelling pressure. The results show that EDZ fractures were closed by swelling of the full-scale buffer material. They also demonstrate the applicability of the Barton–Bandis model to preliminary estimation of the long-term transmissivity of EDZ fractures in facilities for the geological disposal of radioactive waste.