Surface settlement of backfilled shafts and rebackfilling of settled shafts at the Mizunami Underground Research Laboratory

Kokubu, Yoko   ; Takeuchi, Ryuji  ; Nishio, Kazuhisa*; Ikeda, Koki

The Tono Geoscience Center of the Japan Atomic Energy Agency has undertaken backfilling and restoration activities at the Mizunami Underground Research Laboratory (MIU) site since fiscal year 2020. These activities are being conducted in accordance with "The MIU Project from FY2020 Onwards," outlining the procedures for backfilling, restoration, and environmental monitoring at the MIU site. The backfilling activity was completed in January 2022, and thereafter, the observation of the backfilled shafts was commenced. On November 6, 2023, the settlement of the backfilled surface was observed in the Main Shaft and the Ventilation Shaft. By December 5, 2023, the depth of the settlement reached 12.9 m in the Main Shaft and 27.7 m in the Ventilation Shaft. After an evaluation by the MIU safety confirmation committee, which assessed the settlement condition and recommended countermeasures, the affected areas were backfilled for safety reasons. This report summarizes the observed settlement of the backfilled surface, the subsequent rebackfilling efforts, and the condition of the surface settlement areas. The condition of the backfilled sections has been confirmed up to June 2024.