Restoring initial conditions in a deep argillaceous formation with induced suction on retrieval

Abstract

The deep and plastic Ypresian clays (Ycs) are one of the potential host rock formations in Belgium for the geological disposal of heat-emitting radioactive waste. Consequently, their thermo-hydro-mechanical behaviour has to be exhaustively studied at different depths. With this aim and considering that there is no underground research facility, deep core samples of Ycs have been retrieved between 300 and 450 m depth. The water-undrained sampling induces high matric suctions on the retrieved samples despite their nearly saturated state. Microstructural features, and particularly the larger dominant pore sizes with relatively low air-entry values, may also affect this induced suction. An experimental campaign has been launched to analyse the induced suction at different depths, which is complemented with pore size distributions carried out with mercury intrusion porosimetry. The study is accompanied by a constant volume test, in which a retrieved cylindrical sample with high initial suction is subjected to positive water pressure at the bottom boundary. The evolution of pore water pressure and total stress at different heights allows detecting the development of effective stress (swelling pressure) close to the initial suction. Finally, the paper emphasises the importance of following adequate experimental protocols on these high-suction samples to restore in situ conditions.