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Report No.
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In situ stress measurements at the 350m pumping station in the Horonobe Underground Research Laboratory

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

The objective of this report is to investigate the three dimensional stress state in the 350m pumping station at the Horonobe Underground Research Laboratory. For the measurement, four boreholes were drilled; three 20.0m long boreholes and one 6.0m long borehole. 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, considering the stress condition causing breakouts, the value of the maximum principal stress was 12 MPa, two times larger than overburden pressure. Furthermore, the ratio between maximum to minimum principal stress was 6. On the other hand, without considering the stress condition causing breakout, the maximum principal stress was 6 MPa; almost same as overburden pressure; the ratio between maximum to minimum principal stress was 2.7, thus the result was almost coincide with the result of surface-based investigation. The orientation of the maximum principal stress was N30W, dipping 45$$^{circ}$$ from vertical axis.

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