Rock spalling and countermeasures in shaft sinking at the Horonobe Underground Research Laboratory

幌延深地層研究所での立坑掘削における岩盤崩落とその対策工

津坂 仁和; 稲垣 大介; 名合 牧人*; 青木 智幸*; 重廣 道子*

Tsusaka, Kimikazu; Inagaki, Daisuke; Nago, Makito*; Aoki, Tomoyuki*; Shigehiro, Michiko*

原子力機構は、幌延深地層研究計画において3本の立坑を建設している。深度250m以深の立坑掘削においては、立坑の掘削径以上の幅を有する断層が複数出現したため、立坑壁面岩盤の崩落が頻繁に生じるとともに、覆工コンクリートにクラックが顕著に生じた。本紙面では、3本の立坑のうち、換気立坑(仕上がり内径4.5m)の深度250mから350mまでの施工について報告する。同施工では、詳細な岩盤壁面の観察とともに、3次元レーザースキャナを用いた岩盤壁面の3次元形状の計測を実施した。これにより、立坑掘削に伴う岩盤壁面の崩落位置やその規模を定量的に分析することができた。さらに、同計測結果に基づいて、岩盤崩落を適切に抑制しつつ、経済的な施工を実施するために、事前に設計した4つの支保構造から最適なものを選定し、立坑の施工を実施し、その手順を崩落量に着目した支保構造選定フローとして整理した。ここで適用した手法は、後続する西立坑(仕上がり内径6.5m)の施工における支保構造の選定に使用されている。

The Horonobe Underground Research Laboratory is planned to consist of the Ventilation Shaft (4.5 m in diameter), the East and West Access Shafts (6.5 m in diameter). The host rock of the URL site comprises Neogene sedimentary rocks. The unconfined compressive strength of the rocks is less than 20 MPa on average. Anisotropic stress distribution around the URL is also confirmed. Because several highly permeable fractures (hydraulic transmissivity: approximately 10m/s) with the size greater than the shaft diameter develop under the condition of around 2 in competence factor (i.e., the ratio of the unconfined compressive strength of rock to the initial stress) below a depth of 250 m, shaft sinking is a challenging issue from the viewpoint of tunnel engineering in the Horonobe URL Project. In this paper, the construction of the Ventilation Shaft below a depth of 250 m at the Horonobe URL is reported. During shaft sinking, fracture mapping of the shaft wall was performed. The geometry of the shaft wall was also measured using a three-dimensional laser scanner in order to investigate the shape and volume of rock spalling in the shaft wall resulting from the excavation work. Rock spalling was predominantly observed on the south and north wall rock corresponding to the direction of the minimum horizontal initial stress. A large amount of rock spalling also developed along a large-scale fault. With respect to the lining span and the layout of rockbolts, several support patterns were designed and installed as the countermeasures to prevent the development of excessive rock spalling. A flowchart for selecting the optimum support design was then developed.