幌延深地層研究所の250m調査坑道における掘削損傷領域の経時変化に関する検討

Investigations for av change of an excavation damaged zone with time at the 250 m gallery in the Horonobe Underground Research Laboratory

青柳 和平  ; 津坂 仁和; 窪田 健二*; 常盤 哲也; 近藤 桂二; 稲垣 大介

Aoyagi, Kazuhei; Tsusaka, Kimikazu; Kubota, Kenji*; Tokiwa, Tetsuya; Kondo, Keiji; Inagaki, Daisuke

幌延深地層研究所の250m調査坑道において、弾性波および比抵抗トモグラフィにより、掘削損傷領域の弾性波速度や見掛比抵抗値の経時変化を計測している。弾性波トモグラフィ調査では、坑道掘削に伴い、壁面から約1mの範囲で弾性波速度が低下した。一方、比抵抗トモグラフィ調査では、掘削に伴う顕著な見掛比抵抗値の変化は見られなかった。さらに、坑道壁面の割れ目の観察結果に基づいて、調査領域の三次元割れ目モデルを作成して、坑道掘削後の弾性波トモグラフィ調査で得られる各測線の弾性波速度と割れ目密度の関係を分析したところ、割れ目密度の増大とともに弾性波速度が低下することが明らかとなった。このことから、弾性波トモグラフィ調査と割れ目密度の検討により、掘削損傷領域を適切に評価できることが示された。

The authors have been conducting seismic and resistivity tomography surveys in a gallery of the Horonobe Underground Research Laboratory in order to investigate an extent of an excavation damaged zone (EDZ) along time. The objective of this paper is to discuss an influence of fracture distribution and water saturation of a rock mass on variations in seismic velocity and the value of apparent resistivity in an EDZ. Based on the result of seismic tomography survey, the extent of a layer which has low seismic velocity was about 1.0 m from the gallery wall after excavation of the tomography area. From the results of resistivity tomography survey, the value of apparent resistivity has not changed remarkably along time. To investigate a relationship between variations in seismic velocity and density of fracture in the survey area, the authors built a three dimensional fracture model around the tomography area. From the comparison of seismic velocity with density of fracture, seismic velocity decreased almost linearly as the density of fracture increased. Also, it was found that density of fracture in the layer of low seismic velocity could be estimated using a simple numeric model. From this result, seismic tomography survey and investigation of density of fracture are suitable method for evaluation of an EDZ.