Repetition of spectral analysis on rock permeability applied to seven years record of pore-pressure response to atmospheric loading at the Horonobe URL in Japan

幌延深地層研究所における7年間の間隙水圧の大気圧応答のスペクトル解析

宮川 和也   ; 野原 壯   ; 常盤 哲也; 山崎 雅則*

Miyakawa, Kazuya; Nohara, Tsuyoshi; Tokiwa, Tetsuya; Yamazaki, Masanori*

本研究では、幌延深地層研究計画に基づき、地下施設近傍のボーリング孔において実施している長期水圧モニタリングのデータ解析により、地下施設の建設が周囲の帯水層へ影響を与え、鉛直方向の透水性を低下させることを明らかにした。地下施設から約130mの距離では立坑の掘削から約5年間で、鉛直方向の水頭拡散率が地下施設の建設前段階と比較して、約70%低下していたことがわかった。立坑の掘削に伴って大量の地下水が排水されるが、この透水性の変化は、地下水の排水に伴う間隙水圧の低下に因る岩盤中の間隙率の低下が要因と考えられる。本研究で提示した、透水性の時間変化の調査解析手法は、地下施設の建設が周囲の帯水層に与える水理的影響の定量的な評価に貢献できると考えられる。

Understanding of geological environment has been required for radioactive waste disposal, and the permeability of rock mass is one of the most important characteristics of disposal system. It is reported that rock permeability can be estimated by the spectral analysis of the records of pore pressure and atmospheric pressure. JAEA began the Horonobe Underground Research Laboratory (Horonobe URL) Project as part of efforts to enhance the reliability of technologies used during the geological disposal of HLW in Japan. In this study, a spectral analysis of vertical permeability in sedimentary formation around the underground facility has been carried out with seven years records of pore pressure and atmospheric pressure in the Horonobe URL. This study revealed that the excavation of URL has influenced groundwater layers around the URL, and has reduced the vertical hydraulic permeability in the layers. At a point approximately 130 m distant from the URL, it was found that hydraulic diffusivity has decreased by approximately 15 - 70% in five years since the excavation started. This study indicated that permeability change during the excavation of URL could be monitored via pore and atmospheric pressure records.