Initial stress estimation of diatomaceous mudstone using hydraulic fracturing test and borehole breakout method

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

Horonobe Underground Research Laboratory (URL) project aims at improvement of the reliability of technologies for geological disposal of high level radioactive wastes. In investigation during construction of the underground facilities, the updating and check of the validity of initial stress estimations made during surface-based investigations is one of the main issues. Thus, three boreholes were made from the niche (GL-140 m) of ventilation and east shaft, and a three-dimensional evaluation of the initial stress in Koetoi Formation was done by the method called hydraulic fracturing technique and the analysis of the borehole breakout. Moreover, the direction of the maximum horizontal principal stress had been found to be east/west in general. However, the present test showed that the direction was 26 or 46 degrees counterclockwise from east/west.