The evaluation of hydraulic properties in the excavated damaged zone is very important in the geological disposal of high level radioactive waste. Aiming at the development of evaluation method of permeability change for rocks showing swelling property like Opalinus clay, pore air pressure measurement has been started in Mont Terri rock laboratory. The pore air pressures of three different depths and the air pressure inside the tunnel are recorded for half a year with temperature and relative humidity. As a result, the pore air pressure data from shallowest borehole recorded the transient process of gaining phase shift and amplitude attenuation. However the other two data show almost the same trend with ambient environment. The observed phase shift and amplitude attenuation were consistent with the behavior previously reported by Wassermann, J. et al. (2011).

1. Introduction

When a tunnel is excavated, an excavated damaged zone (EDZ) is formed near the wall. The evaluation of hydraulic properties in the EDZ is very important in the geological disposal of high level radioactive waste. Opalinus clay, which is distributed in the Mont Terri Underground Rock Laboratory in Switzerland, is a candidate host rock for sedimentary rock, and its characteristics are studied in detail (Bossart and Milnes, 2018). However this rock contains swelling minerals and shows remarkable swelling property. When water is getting into contact with a shaly matrix, undesirable and uncontrolled processes may be induced, and then the in-situ hydraulic property must not be obtained. Therefore alternative testing methods should be developed which do not use water and which are able to identify zones of different permeability.

In this study, for the purpose of investigating the transient change in hydraulic properties in the in- situ EDZ, long-term observations of pore air pressure, temperature and humidity in three isolated spaces inside the rock at different depths from the tunnel wall were started in the Mont Terri URL. This paper introduces the methodology and the results obtained so far.

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