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幌延深地層研究センター東立坑140m小型試錐座における水圧破砕法による初期地圧の評価

In situ stress measurements at 140m niche around east shaft in Horonobe Underground Research Laboratory

中村 隆浩; 真田 祐幸; 杉田 裕 ; 加藤 春實*

Nakamura, Takahiro; Sanada, Hiroyuki; Sugita, Yutaka; Kato, Harumi*

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、東立坑の深度140mの小型試錐座において3本のボーリング孔を掘削し、水圧破砕法により初期地圧の評価を試みた。水圧破砕試験の結果、ボーリング孔軸を含む縦き裂はどこにも生じなかった。これは、当該岩盤に天然き裂が多く含まれていることが一つの原因と考えられた。応力状態が測定位置によらず一様であると仮定し、き裂面の法線応力と初期地圧の関係から初期地圧状態を評価した。主応力はいずれも鉛直方向あるいは水平方向から30度程度傾いた結果が得られた。

For the purpose of studying the in situ state of stress around Horonobe Underground Research Laboratory, hydraulic fracturing technique was carried out three boreholes drilled from the 140m niche located near the bottom of the east shaft. The results of the measurements are summarized as follows. Evaluation of stress in a plane perpendicular to the borehole axis was impossible because transverse fractures which developed along a pre-existing plane of weakness were created at eleven depths. Therefore, the relation between the shut-in pressure of a transverse fracture and the in situ stress was used to obtain the in situ stress state on the assumption that all of the target zones of hydraulic fracturing were in a uniform stress state. The determined in situ stress state showed that none of the principal stress axis was coaxial with respect to E-N-V coordinate system where E, N and V represent east, north and vertical direction.

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