A Discrete Approach to Modelling Hydromechanical Rock Response of FEBEX Tunnel Excavation (Grimsel Underground Research Laboratory, Switzerland)

Abstract LAEGO and ANDRA have participated in the modelling of the task 1A of the “DECOVALEX” III international research project, which was devoted to modelling the hydromechanical effects of FEBEX tunnel excavation at the Grimsel Test Site in Switzerland. After presenting the site, this paper will focus on the 3D discontinuous modelling performed by associating 3DEC and 3FLO codes, which leads to a discontinuous semi-coupled approach. The modelling tools, choice of geomodel, the mechanical boundary and related initial conditions, the hydraulic boundary and related initial conditions, as well as the calibration process of hydraulic properties will all be discussed herein. A purely-hydraulic modelling first yields a rather good estimation of both total flow leading towards the tunnel and the general shape of pressure evolution in a nearby borehole. A hydromechanical computation representing the final stage of excavation reveals a difference of just 5% on the evaluation of total water flow rate and does improve the prediction of water head in the boreholes surrounding the tunnel.