Thermal conductivity of compacted Opalinus Clay at different degrees of saturation

Abstract

Tunnelling in bedded shale rocks induces damage around the excavation. Consequently, the rock mass in the excavation damaged zone (EDZ) shows different thermo-hydro-mechanical (THM) properties from the intact rock. In turn, the damaged material close to the tunnel wall is affected by seasonal fluctuations in temperature and relative humidity (RH) variations associated with tunnel ventilation, impacting the total suction and the saturation state of the EDZ. The monitoring data at the new Swiss BelchenA2 highway tunnel excavated in Opalinus Clay (OPA) captured these seasonal variations in the total stressacting on the lining associated with this temperature and RH fluctuation. Within this context, assessing the temperature field in the EDZ requires determining the thermal conductivity of the damaged rock at different degrees of saturation. However, obtaining good-quality cores of OPA from the EDZ is not straightforward, and partially desaturating them using controlled RH techniques is time-consuming. Consequently, instead of testing damaged OPA, statically compacted samples of OPA powder at different degrees of saturation have been considered to study the thermal conductivity. In addition, two natural OPA samples (with different bedding orientations) retrieved from the Belchen tunnel have been tested to cross-check the experimental procedure and results, which indicated a consistent increase in thermal conductivity with the degree of saturation.