“Qualitative and time analysis of the gaseous constituents transport processes in a micro-tunnel on the basis of in-situ measurements”

Abstract

This paper describes air pressure, relative humidity (RH), temperature and oxygen molar fraction (xO2) measurements that have been carried out over several years in a hundred meter long and 75 cm diameter micro-tunnel (MT) experiment at the Andra's Meuse/Haute-Marne underground research laboratory (MHM URL). Sensors were placed all along, every 20 m inside the MT, and in the access drift. RH and xO2 time evolutions exhibit very complicated profiles, showing many singularities. These features can be well explained by taking into account mainly the corrosion of the steel liners, the water evaporation from the surrounding saturated clay and a leak between the head of the MT excavation and the access drift. Airflow through the leak is driven by the drift pressure seasonal and short time fluctuations. A simple model involving coupled infinite and finite reservoirs made it possible to reproduce xO2 evolution and to calculate the air fluxes associated. Drift pressure variations induce longitudinal compression-dilatation processes in the system that are closely estimated using suitable analytical formulas. By taking into account diffusion driven leaks at the liners interfaces, it was possible to build a global model from which the results compare very well with experimental data.