Hiroyuki Sanada

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.Copyright

Development of borehole multiple deformation sensor system

The multiple deformation sensor system for small diameter borehole is required for the long term monitoring of deformation of rock mass of high-level radioactive waste disposal site. The conventional electric monitoring systems are difficult to apply for a long term monitoring in many cases because of the sensor failure caused by lowering of insulation or other problems and only available for the large borehole and for 6 or less measurement sections. The Borehole Multiple Deformation Sensor System was developed based on the FBG (Fiber Bragg Grating) sensor technology that is expected to have longer life time than electric systems. The developed system can be set in a 66 mm diameter borehole and available for 9 or more measurement sections that can be hardly achieved by electric systems. The sensor system is applying for the monitoring of ground deformation in the Horonobe Underground Research Laboratory in Japan.

Seismic Tomography Investigation in 140m Gallery in the Horonobe URL Project

The Japan Atomic Energy Agency (JAEA) is conducting the Horonobe Underground Research Laboratory (URL) Project to enhance reliability of disposal technologies through investigations of the deep sedimentary environment at Horonobe, Hokkaido, Japan. An excavation disturbed zone (EDZ) is expected to occur around an underground gallery when it is excavated for disposal of radioactive waste. A number of in-situ experiments for rock properties and extent of the EDZ were carried out at the “140m Gallery” at a depth of 140m below the surface of the Horonobe URL. One of the experiments is seismic tomography survey using a hammer seismic source. Its observation area was 3m square on the horizontal plane along the sidewall of the 140m Gallery. The measurement was repeated with the progress of excavation of a tunnel. In this experiment, a decrease of seismic velocity in the rock around the new tunnel and its distribution were observed. It is considered that seismic tomography investigation captured the EDZ developed around the tunnel.