Seven-year history of vertical hydraulic diffusivity related to excavation around an underground facility

地下施設の掘削に伴う鉛直方向の透水性の時間変化

宮川 和也   ; 野原 壯   ; 常盤 哲也; 山崎 雅則*

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

地下施設の掘削により比較的大量の地下水が排水されることによって、岩盤中の透水性などの水理パラメーターが影響を受ける可能性が考えられる。本研究では、幌延深地層研究所において観測された大気圧と間隙水圧の長期間の観測記録を用いて透水性の経時変化を得るために、スペクトル解析を行った。本研究により、地下施設の掘削が周囲の帯水層へ影響を与え、鉛直方向の水頭拡散率を低下させていることが明らかになった。地下施設から約130m離れた距離では、地下施設の掘削開始から5年間で約70%の水頭拡散率の低下が観測された。また、地下施設から約860mの距離では、間隙水圧の記録には大きな変化が見られていないものの、約26%の水頭拡散率の低下が観測された。これらの結果は、地下水が移動しにくくなる傾向を示しており、地層処分の長期安定性に関して、安全側の評価が示された。

A large amount of groundwater drain induced by an excavation of an underground facility will influence parameters of site formations such as permeability. In this study, a spectral analysis was applied to long-term records of atmospheric response of pore pressure in order to obtain time history of rock permeability change at the Horonobe Underground Research Laboratory (URL) in Japan. This study revealed that the excavation of the URL has influenced aquifers around the URL, and has reduced the vertical hydraulic diffusivity in the aquifers. At the point of ca. 130 m distant away from the URL, the hydraulic diffusivity has decreased in approximately 70% after 5 years from start of excavation of the URL. At the point of ca. 860 m distant away from the URL, the vertical hydraulic diffusivity has decreased in approximately 26% even though pore pressure has not changed remarkably. These results indicate that groundwater consequently becomes difficult to flow, and then safe-side estimation with respect to geological disposal system was obtained. This study confirmed that long-term history of permeability change can be monitored by using long-period records of pore pressure and atmospheric pressure. During the development of a geological disposal system, which takes several tens of years, the vertical permeability can be monitored continuously from the start of the excavation through the post-closure of the repository.