At the Mont Terri Underground Research Laboratory (Switzerland), a field-scale investigation has been conducted in order to investigate the hydro-mechanical and chemical perturbations induced in the argillaceous formation by forced ventilation through a tunnel. this experiment has been selected to be used for processing model development and validation in the international project DECOVALEX-2011. the conceptual and mathematical representation of the engineered void, which itself forms a major part of the experiment and is not simply a boundary condition, is the subject of this paper. A variety of approaches have been examined by the contributors to DECOVALEX and a summary of their findings is presented here. Two major aspects are discussed. Firstly, the approaches for the treatment of the surface condition at the porous media/tunnel interface are examined, with two equivalent but differing formulations successfully demonstrated. Secondly, approaches for representing the tunnel with a...