INTERNATIONAL COMPARISON OF COUPLED THERMO-HYDRO-MECHANICAL MODELS OF A MULTIPLE-FRACTURE BENCH-MARK PROBLEM - DECOVALEX PHASE-I, BENCH-MARK TEST-2

An international inter-code bench mark comparison has been carried out under the auspices of DECOVALEX, an international cooperative project for the development of; coupled (thermo-hydro-mechanical) models and their validation against experiments in nuclear waste isolation, The subject bench mark problem, designated BMT2, involves a system of nine blocks of intact hard rock separated by two pairs of relatively soft fractures. The coupled processes of conductive, convective and dispersive heat transport; nonisothermal fluid flow; and thermo-mechanical stresses and displacements in both the intact rock and the fractures were simulated, Five research teams representing four European and North American countries participated in this bench-marking exercise using two fundamentally different types of numerical models, i.e. finite-element models and distinct-element models. All teams simulated the fractures as discrete features, For the coupled hydro-mechanical behaviour of the fractures the modelling teams have used either the empirical, nonlinear Barton-Bandis-Bakhtar constitutive relation or a simpler Coulomb friction like stress-deformation relation in conjunction with Snow's parallel-plate fracture flow model. For this bench mark problem it was found that (1) heat convection significantly affects the distribution of temperature, thermal stresses and displacements; and (2) the predominant coupled effect is fracture closure caused by thermal expansion of the rock blocks, Comparison of the modelling results obtained by the various teams have indicated very good overall agreement, In particular, the results calculated using finite-element and distinct-element codes are quite similar, Practically all the discrepancies among simulation results can be ascribed to different idealization, constitutive relations, or parameter values adopted by the participating research teams, In-depth discussions among participants have enhanced our understanding of the simulated processes and provided guidance for model improvements.