In Situ Stress Measurements in Siliceous Mudstones at Horonobe Underground Research Laboratory, Japan

The stress measurement methods implemented during the surface-based investigations and during construction of the underground facilities in the Horonobe mudstones, as well as information on the initial stress state around the Horonobe URL, are described in this paper. During the surface-based investigations, determination of deep in situ stress was conducted using HF, BB information in deep boreholes and core-based methods such as AE and DSCA. During construction of the underground facilities, subsurface investigations utilizing CCBO, HTPF and the monitoring of spalling around the shafts were conducted in order to verify results from initial stress measurements in the surface-based investigations. HF results indicate that magnitude of the horizontal maximum and minimum principal stresses increases linearly with depth. The maximum principal stress estimated from the HF and borehole breakout data is almost E-W. This is similar to the tectonic movement direction in the vicinity of the Horonobe URL. Due to tectonic movement, horizontal maximum stress is almost 1.5 times larger than the horizontal minimum stress. The minimum horizontal principle stress is almost equivalent to overburden pressure. Stress condition determined from HTPF in the investigations during construction of the underground facilities is almost equal to the results during surface-based investigations.