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幌延深地層研究センターの250m大型試錐座(西)における初期地圧測定

In situ stress measurement at the 250m Niche off the West Shaft No.1 in the Horonobe Underground Research Laboratory

青柳 和平 ; 櫻井 彰孝; 丹生屋 純夫*

Aoyagi, Kazuhei; Sakurai, Akitaka; Niunoya, Sumio*

幌延深地層研究センターの地下施設周辺岩盤の応力場を把握するため、250m大型試錐座(西)にて、方向、傾斜の異なる3本のボーリング孔を掘削し、水圧破砕法による3次元初期地圧測定を実施した。初期地圧を評価するにあたっては、14深度で観測されたき裂に作用する法線応力、5深度で生じたブレイクアウトの破壊位置および2深度で生じた縦き裂の開口圧に関する観測方程式を用いた。結果として、調査地点の初期地圧状態は、主応力の方位に関しては、両ケースにおいて、最大主応力が北方向から約70$$^{circ}$$西寄りで、鉛直方向下向きから約70$$^{circ}$$傾き,最小主応力は南から約20$$^{circ}$$西寄りで鉛直下方より約60$$^{circ}$$傾く結果となった。最大主応力の値は2.6MPa、最小主応力は2.1MPaであった。

The objective of this report is to investigate the three dimensional stress state in the 250 m Niche off the West Shaft No.1 at the Horonobe Underground Research Laboratory. For the measurement, three 20.0m long boreholes were drilled. Hydraulic fracturing was applied as a stress measurement method. For the analysis, shut-in pressure of a transverse fractures, reopening pressure of longitudinal fractures and stress condition causing borehole breakouts were integrated into the equation; then stress state was calculated by inversion technique. As a result of the in situ stress state measurement around the experimental area, the orientation of the maximum principal stress was estimated to be ESE-WNW, dipping 70$$^{circ}$$ from the vertical axis. The orientation of the minimum principal stress was NEN-SWS, dipping 60$$^{circ}$$ from vertical axis. The orientation of the principal stresses is almost coincided with the direction of borehole breakouts and longitudinal crack induced in the boreholes. The values of the maximum and minimum principal stresses were 2.6 MPa and 2.1 MPa, respectively.

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