ABSTRACT

JAEA is developing two off-site underground research laboratory (URL) projects in Japan: one in sedimentary rock (Horonobe, Hokkaido) and the other in crystalline rock (Mizunami, Gifu). The Horonobe URL project is a comprehensive research and development project aimed at studying geological disposal in sedimentary formations. This project consists of three phases: surfacebased investigations (Phase I), investigations during excavations (Phase II) and investigations in the underground facilities (Phase III). In Phase I, aerial reconnaissance surveys, ground geophysical surveys, surface and borehole investigations were performed to select the URL location and to understand the geological environment in and around the URL. At present, Phase II has started and the ventilation shaft of the URL has been excavated to about 250-m depth. This paper summarizes development in Phase I of a geomechanical conceptual model and estimation of physical and mechanical parameters for prediction of mechanical behavior of the rock mass in response to excavation of the facility.

1 INTRODUCTION

Japan Atomic Energy Agency (JAEA) (formerly the Japan Nuclear Cycle Development Institute – JNC) is conducting the underground research laboratory (URL) project focused on Neogene diatomaceous rock at Horonobe, Hokkaido in Japan. The technical aims of the Horonobe URL project are to study the geological environment as an example of a sedimentary formation in Japan; confirming the reliability of technologies for geological disposal of high-level radioactive waste (HLW) presented in the H12 report (JNC, 2000). The Horonobe URL project consists of three phases: surfacebased investigations (Phase I), investigations during excavations (Phase II) and investigations in the underground facilities (Phase III). In Phase I, aerial reconnaissance surveys, ground geophysical surveys, surface and borehole investigations were performed to select the URL location and to understand the geological environment in and around the URL. The siting process of a final repository for HLW in Japan consists of three steps as defined by the "Specified Radioactive Waste Final Disposal Act" (the Act). NUMO, Implementing organization responsible for HLW disposal in Japan, presented the siting process as follows (NUMO, 2002). 1) Literature survey. 2) Preliminary investigation such as surface-based field investigations, including deep boreholes. 3) Detailed investigation such as further characterization, including construction of an underground test facility. Contents of Phase I are almost corresponding to the process of the preliminary investigation, and contents of Phase II and Phase III are almost corresponding to the processes of the detailed investigation. Therefore, development procedure of a geomechanical conceptual model in the Horonobe URL project will be useful information for siting process of the final repository site, especially for design of the underground facilities.

2 GEOLOGICAL CONDITIONS

Geology in and around the URL area of 3 × 3 km consists of Neogene formations as shown in Figure 1. They are divided into three a fine sandstone called the Yuchi formation, diatomaceous mudstone called the Koetoi formation and siliceous mudstone called the Wakkanai formation. The Omagari fault, which is high (about 70 degrees) dipping reverse fault, may exist at the center of the URL area.

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