Author(s)

HIROYA MATSUI, RYOJI YAHAGI, HIKARU ISHIZUKA, TOMOHITO TOGURI

Abstract

In-situ backfilling experiment using spray method in the small scale drift (approximately 4 m by 3 m scale) was conducted at a 500 m depth in the Mizunami Underground Research Laboratory (MIU), established by JAEA (Japan Atomic Energy Agency). The aim of this experiment is to demonstrate backfilling by spray method deep underground and establish the quality control methodology as the one of the techniques applied for geological disposal in Japan. The backfill material consists of a sand and bentonite mixture. Specification for the backfill material was designed to satisfy the target permeability of a generic host rock (10_-8 m/s) assumed by NUMO. In this case, effective clay density should be 0.4 Mg/m³ or more. Quality control of the material before backfilling was performed by setting the initial water contents (14±2%) based on the results of the Laboratory testing and preliminary spray testing on ground surface. While the quality control of the backfilled material during the experiment was performed by density measurement with the standard method and a 3D-scanning method. Densities of the backfilled material measured at any points satisfied the specification (more than 0.4 Mg/m³) and the results suggested the establishment of the practical quality control methodology of the backfilling by spray method under actual deep geological environment. Furthermore, the measurement of the moisture transfer characteristics and the swelling of bentonite using several instruments, as well as the amount of the outflowing of bentonite from the backfilled material, was carried out.

Keywords

backfilling, spray method, in-situ experiment, deep underground, crystalline rock