Yutaka Sugita

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

Low alkaline cement used in the construction of a gallery in the Horonobe Underground Research Laboratory

In Japan, any high level radioactive waste (HLW) repository is to be constructed at over 300 m depth below surface. Tunnel support is used for safety during the construction and operation, and shotcrete and concrete lining are used as the tunnel support. Concrete is a composite material comprised of aggregate, cement and various admixtures. Low alkaline cement has been developed for the long term stability of the barrier systems whose performance could be negatively affected by highly alkaline conditions arising due to cement used in a repository. Japan Atomic Energy Agency (JAEA) has developed a low alkaline cement, named as HFSC (Highly Fly-ash Contained Silicafume Cement), containing over 60 wt% of silica-fume (SF) and fly-ash (FA). HFSC was used experimentally as the shotcrete material in construction of part of the 140m deep gallery in the Horonobe Underground Research Laboratory (URL). The objective of this experiment was to assess the performance of HFSC shotcrete in terms of mechanics, workability, durability, and so on. HFSC used in this experiment is composed of 40 wt% OPC (Ordinary Portland Cement), 20 wt% SF, and 40 wt% FA. This composition was determined based on mechanical testing of various mixes of the above components. Because of the low OPC content, the strength of HFSC tends to be lower than that of OPC. The total length of tunnel using HFSC shotcrete is about 73 m and about 500 m3 of HFSC was used. The workability of HFSC shotcrete was confirmed in this experimental construction.Copyright

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.

Coupled thermal, hydraulic and mechanical simulation with a theoretical model for swelling characteristics

Abstract Buffer material as a part of an engineered barrier system for high-level waste disposal is expected to seal the initial gaps between the buffer material and the surrounding rock during the resaturation process. Since the self-sealing ability represents the swelling characteristics of the buffer material, it is very important to estimate the swelling behavior during the resaturation process. This paper expands the theoretical model for evaluating the swelling characteristics of saturated buffer material proposed by Komine to deal with unsaturated media, and it is applied to the coupled thermal, hydraulic and mechanical (THM) model used in Japan. The new model is validated by comparison with a series of laboratory experiments quoted from the literature.