Reactive and non-reactive transport modelling in partially water saturated argillaceous porous media around the ventilation experiment,Mont Terri

At Mont Terri Underground Research Laboratory （URL） Switzerland, a specific experiment has been per- formed in a tunnel, in order to investigate the hydro-mechano-chemical （HMC） perturbations induced in the argillaceous formation by forced ventilation. This experiment has been selected in the international project DECOVALEX to be used for process model development and validation. The numerical simula- tion of the geochemical response to the ventilation experiment （VE） is the object of the present paper, focusing on the transport of chloride as a conservative species and sulphate as a reactive species. Uti- lising the validated hydro-mechanical （HM） results from earlier steps of the DECOVALEX task, reactive and non-reactive transport models, incorporating the current understanding of the geochemistry at the site, were successfully constructed for the whole experimental period. The associated parametric and process uncertainty analyses clearly demonstrate that the basic HM understanding must be sound. How- ever, to demonstrate this degree of robustness, the explicit inclusion of process representations of water desaturation, liquid vaporisation, species exclusion porosity, and redox processes, is required.

Analysis of hydro-mechanical processes in a ventilated tunnel in an argillaceous rock on the basis of different modelling approaches

In this paper, a modelling benchmark exercise from the DECOVALEX-2011 project is presented. The bench- mark is based on the performance and results of a laboratory drying test and of the ventilation experiment （VE） carried out in the Mont Terri Underground Rock Laboratory （URL）. Both tests involve Opalinus clay. The work aims at the identification, understanding and quantification of mechanisms taking place during the ventilation of a gallery in argillaceous host rocks on one hand and at investigating the capacity of different codes and individuals to reproduce these processes on the other hand. The 4-year in situ VE took place in a 1.3 m diameter unlined tunnel and included two resaturation-desaturation cycles. The test area was equipped with over one hundred sensors （including the global water mass balance of the system, relative humidity （RH）, water content, liquid pressure, relative displacement and concentration of some chemical species） to monitor the rock behaviour during ventilation. The laboratory drying experiment, carried out before the VE, was designed to mimic the in situ conditions. The work was organized in a progressive manner in terms of complexity of the computations to be performed, geared towards the full hydro-mechano-chemical （HMC） understanding of the VE, the final objective. The main results from the modelling work reported herein are that the response of the host rock to ventilation in argillaceous rocks is mainly governed by hydraulic processes （advective Darcy flow and non-advective vapour diffu- sion） and that the hydro-mechanical （TM） back coupling is weak. A ventilation experiment may thus be regarded as a large scale-long time pump test and it is used to determine the hydraulic conductivity of the rock mass.