Modelling of an in situ ventilation experiment in the Opalinus Clay

The so-called Ventilation Experiment (VE), located in the Opalinus Clay formation of the Mont Terri underground rock laboratory in Switzerland (www.mont-terri.ch), is aimed at investigating in situ the effect of tunnel ventilation on the hydro-mechanical conditions in the rock around the excavations. For this purpose, a hermetically separated tunnel section is ventilated under well-controlled conditions. Humidity, temperature and volumetric airflow are measured in the tunnel, porewater pressure, water content and deformation are monitored in the surrounding rock. Three major experimental phases have been accomplished till end 2004: a saturation phase between July 2002 and April 2003, a desaturation phase between July 2003 and February 2004 and a subsequent resaturation phase.The two-phase flow code TOUGH2 has been used to model the VE and to derive the hydraulic properties of the Opalinus Clay and the excavation damage zone (EDZ). The investigation is based on a radial symmetric model configuration with a horizontal axis located at the axis of the ventilation tunnel (1.3 m diameter and 10 m length) and an external boundary in a radial distance of 8 m.The focus of this paper is on the interpretation of the desaturation phase comprising three periods of varying humidity and flow-rates of the air entering the tunnel. The permeability of the undisturbed Opalinus Clay and of the excavation damage zone (EDZ) of Opalinus Clay are derived by means of numerical modelling based on the measurements in the rock and the ventilated tunnel.The simulated desaturation depth inside the rock corresponding to the 95% water saturation level is 0.35 m from the tunnel wall and well in accordance with the experimental data. The permeability level of the undisturbed rock has been estimated to be about 3 × 10−20 m2, a value which is very consistent with measurements in boreholes at the Mont Terri underground laboratory.