ABSTRACT

The construction of facilities on and in rock masses requires an understanding of rock mass behaviour so that the effects of excavation and support can be predicted with the necessary accuracy for design purposes. The DECOVALEX Project (DEmonstration of COupled Models and their VALidation against EXperiment) aims at developing the appropriate characterisation and modelling capability in the context of radioactive waste repository design plus the associated performance and safety assessments. The DECOVALEX Project has been in operation since 1992 with an international collaboration of waste management organizations and regulatory authorities supporting research teams from Canada, China, Czech Republic, Finland, France, Japan, Germany, Korea, Spain, Sweden, UK, and USA. In this paper, we summarise the coupled modelling advances made in the previous DECOVALEX phases (Phases I, II, III and THMC) and describe the current DECOVALEX-2011 work on modelling (a) the ventilation experiment at the Mont Terri Underground Laboratory in Switzerland, (b) the pillar stability experiment conducted at the Äspö Hard Rock Laboratory in Sweden, and (c) the hydro-mechanical-chemical interactions for single fractures and fracture networks. The DECOVALEX Project has firm plans until 2011 when the results will be disseminated at the time of the ISRM Congress in Beijing.

1 INTRODUCTION

An important aspect of the performance and safety assessment of disposal systems for radioactive waste and spent nuclear fuel is to evaluate the impact on repository performance of the coupled effects of mechanical stability, groundwater flow through the repository, and thermal loading from the decaying waste. It is recognized that to be able to conduct such an evaluation, there is a need to improve the theoretical background and to develop models capable of simulating coupled thermo–hydro–mechanical (THM) processes. More recently, chemical (C) processes have also been added, thus enabling the study of coupled THMC processes in geosystems. The term ‘coupled processes’ implies that one process affects the initiation and progress of another process. Thus, the response of a rock mass to radioactive waste storage cannot be predicted with confidence by considering each process individually or in direct succession (Tsang, 1987). In the field of rock mechanics and rock engineering, the main focus of such studies to date has been on binary couplings TM and HM but, for the repository performance problem, it is essential to study the full ternary THM coupling, and also the THMC coupling.

The coupling of THM and THMC processes is a major challenge to the science and engineering community because the processes have widely different characteristic time constants and spatial scales. The thermal gradient for rock material has relatively large time and spatial scales. Mechanical effects, on the other hand, have a short time scale, since changes in the mechanical response can propagate through the rock mass with the speed of sound, and the deformability is controlled mainly by the presence of large discontinuities, such as faults and shear zones. Groundwater flow and transport are sensitive to small-scale heterogeneities and are characterized by large flow and solute transport times.

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