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

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

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.