An Observational construction management in the Horonobe Underground Research Laboratory Project

幌延深地層研究計画における情報化施工

津坂 仁和; 稲垣 大介; 常盤 哲也; 横田 秀晴; 名合 牧人*; 松原 誠*; 重廣 道子*

Tsusaka, Kimikazu; Inagaki, Daisuke; Tokiwa, Tetsuya; Yokota, Hideharu; Nago, Makito*; Matsubara, Makoto*; Shigehiro, Michiko*

日本原子力研究開発機構は、北海道幌延町にて幌延深地層研究計画を実施し、堆積岩を対象とした高レベル放射性廃棄物の地層処分にかかわる技術の信頼性向上を目指している。同計画では、深度500mの3本の立坑と4つの深度の調査坑道からなる地下研究所を建設中である。掘削対象の岩盤は、一軸圧縮強さが20MPa以下の珪藻質泥岩と珪質泥岩の堆積軟岩であり、その地層境界に厚さ約100mの高透水性の割れ目帯が分布する。このような深度の深い割れ目帯での立坑の掘削では、土木工学的な視点から、さまざまな対策が必要となる。特に、岩盤強度が低いうえに、立坑の掘削径以上の不連続面(断層)と遭遇する場合には、立坑掘削に伴って壁面の崩落現象が生じることが予測された。そのため、壁面の崩落に伴う作業の安全性や工程の遅延を可能な限り抑制するために、情報化施工を実施してきた。本論文では、3本のうち最も先行する換気立坑を対象として、立坑掘削前に実施した立坑周辺の断層分布の予測と実際の断層分布の比較を述べるとともに、実際の断層と遭遇した際の立坑の支保構造の損傷とその対応策について述べた。これらは、現在割れ目帯を後続する東立坑の施工に反映されている。

In the Horonobe URL Project, three shafts are planned to be excavated up to the depth of 500 m in the Neogene sedimentary rocks. The host rock of the URL site is comprised of diatomaceous and siliceous mudstones, which are the Koetoi and Wakkanai Formations, respectively. Approximately 100 m thick fracture zone with high hydraulic conductivity develops above about 400 m in depth in the Wakkanai Formation. The shaft sinking through the fracture zone is the most challenging issue from the aspect of tunnel engineering in the project. In the fracture zone, there is high possibility of severe breakout and spalling in shaft wall because the shafts might be intersecting faults with the size greater than shaft diameter in addition to low intact rock strength at great depth. In practice, prior to the construction of the Ventilation Shaft through the fracture zone below a depth of 250 m, the three dimensional fault distribution were predicted by integrating borehole investigation results and geological response to pre-excavation grouting operation. The countermeasure was also designed against massive spalling. During the shaft sinking, fracture mapping of shaft wall was carried out in order to evaluate the prediction of fault distribution. Roughness of shaft wall was also measured by three dimensional laser scanner in order to investigate the shape and volume of spalling resulting from the excavation work. Consequently, the Ventilation Shaft has successfully been constructed through the fracture zone. This is because the prediction of fault distribution was accurate, and the countermeasure against concrete lining damage due to spalling was promptly applied.