Saline materials for waste containment: constitutive and numerical modelling

The high ductility of salt rocks gives them a very high self-sealing capability, under favourable conditions, fractures and cavities tend to disappear given sufficient time. This property, together with the relatively high thermal conductivity and very low hydraulic conductivity, makes rock salts very suitable geological media for the safe underground disposal of toxic and dangerous waste and, in particular, of high level radioactive waste. Any underground repository will require sealing systems to close the openings required for waste emplacement. It is widely agreed that the most suitable material for constructing the seals will be aggregates made up of crushed salt. Crushed salt is an artificial material, generally obtained from tunnel boring activities. It is a granular material that can contain a natural or artificially added amount of moisture. In general its state is unsaturated and, because of the high solubility of salt in water, the resulting liquid pore fluid is brine. Another characteristic property is the hygroscopic effect where the salt concentration in water influences the relative humidity in the atmosphere. A proper understanding of the behaviour of repository requires the performance of numerical analyses simulating realistically in situ conditions. This requires the development of coupled formulations capable of accounting for the main specific features of behaviour of saline materials and the elaboration of appropriate constitutive models for crushed and rock salt. The formulation must include hydro-mechanical coupling under non-isothermal conditions. Under uncompacted conditions, the hydraulic conductivity of crushed salt is very high and the thermal conductivity is relatively low. Therefore porosity reduction is necessary to achieve more adequate properties. Conventional compaction reduces the porosity only to a limited extent. Additional porosity reduction must be entrusted to salt rock creep closing the opening, with the long-term objective that the seal material will eventually become barely distinguishable from the rock. Therefore, during its lifetime, a crushed salt seal travels from a state typical of a loose granular material to a situation similar to a soft rock. This provides a challenging problem for the mechanical constitutive law that must encompass the behaviour of the material over this range. In this paper a formulation capable of reproducing the coupled thermo-hydro-mechanical processes likely to occur is presented together with a mechanical constitutive law describing the behaviour of crushed salt over a wide range of porosities. The study of the closure of backfilled openings cannot consider the crushed salt and the rock independently but as a single system. Large and medium scale 'in situ' tests are being performed in underground laboratories to study this joint crushed salt/rock behaviour. Heat is supplied in order to achieve the temperatures likely to occur in a real repository for radioactive waste. The numerical tool derived from the theoretical developments described in this paper is applied to the simulation of two in situ heating tests carried out in the Asse salt mine in Germany. The tests simulate the emplacement of a nuclear canister in a borehole (DEBORA test) and in horizontal drifts (TSDE) excavated in salt rock. Model results are compared with observations. The analyses presented clearly demonstrate the applicability of the formulation and constitutive law to complex practical cases