Characterisation of the powder/pellets mixture of bentonite: hydro-mechanical properties and diffusion parameters
A granular material composed of bentonite powder mixed with high dry density bentonite pellets have been developed for sealing and backfilling purposes. Such a material has the advantage that it can be easily applied to backfill irregularly shaped volumes and that it is rather cheap to produce in comparison to compacted blocks, generally used for this purpose. It also requires less compaction effort to achieve a given average dry density because of the very dense pellets of the mixture.The properties of this heterogeneous material will be studied and compared with those of the homogeneous compacted powder material. In addition to the data on this material obtained during the first phase of the RESEAL, the powder/pellets mixture is further studied in this second phase, especially the hydro-mechanical and migration properties. The FoCa clay, Ca-bentonite from the Paris Basin (France), was selected as reference material to study the powder/pellets behaviour. This characterisation programme should give parameters for repository design and performance assessment calculations. Status: in progress To analyse the hydration mechanisms of the pellets/powder mixtures and their coupling with the mechanical behaviour, different infiltration tests with measurement of the swelling pressure are being performed with a mixture of 50% pellets and 50% powder of FoCa clay. As expected, the thickness of the sample has an influence on the kinetics of water intake and swelling pressure development and in the final value of swelling pressure reached. The results obtained up to now point also to a possible influence of the pellets/powder water content ratio. The analysis of the results obtained up to now allows drawing some preliminary explanations on the powder/pellets behaviour during hydration. These tests will provide to the modelling teams a large database (including the results obtained during RESEAL I), which is required in order to identify the parameters to be used to model the hydration mechanisms of this mixture. The diffusion coefficients for tritium and iodide are being determined in 50/50 pellets/powder mixtures of three different dry densities saturated with synthetic Boom clay water. The tests are currently running. The results obtained up to now are being analysed. To confirm that no preferential pathways for tracer diffusion remain in the mixture after saturation, the results of diffusion tests on pre-compacted powder sample (currently running) will be compared to those obtain with the mixture.
Predictive and concurrent coupled hydro-mechanical and transport numerical analyses of the in situ tests
The performed predictive and concurrent analyses of the in situ demonstration tests will increase the confidence in coupled hydro-mechanical and transport numerical analyses. Agreement between the experimental results and the calculations should confirm the good understanding of the bentonite-host rock interaction. These results should also demonstrate that it is possible to predict the hydro-mechanical and transport mechanisms occurring in the seal from independently measured model parameters. Status: in progress The modelling of the shaft sealing test started taking as a starting point the scooping calculations made during RESEAL I and the operational characteristics of the test. The predictive stage showed the need to reassess the mechanical parameters for the constitutive modelling of the FoCa clay. This was accomplished and the preliminary results are promising. However new improvements must be made to reproduce the hydration delay because actual computations fail to agree precisely with measurements
The experimental results of the in situ test give an opportunity for further validation and development of the computer code CODE_BRIGHT, which can be then used for other scientific and industrial projects. In particular, the following areas will be especially affected by the project: - Interaction rock/fill material; - Gas and water flow through porous material; - Hydromechanical coupling; - Transport numerical analyses. Results of the in situ test also give an opportunity for further validation and development of the computer code CASTEM 2000, which can be then used for other scientific and industrial projects.
The interpretation of the experimental results of the in situ demonstration tests has a direct consequence on performance assessment studies of geological repository sealing and backfilling for radioactive waste or for other products from the chemical industry. The experimental results obtained for both in situ tests (the borehole and the shaft sealing tests) during the hydration phase of the bentonite seal are the evolution in time of the swelling pressure, of the pore water pressure, of the water intake and of the suction potential (for the shaft seal test only) of the bentonite. In addition to these results, experimental results should be obtained on conservative tracer diffusion through the borehole seal of compacted FoCa clay. From the shaft sealing test, experimental results should be also obtained on the hydro-mechanical behaviour of the clay surrounding the seal, on the radionuclides migration through the EDZ (Excavated Disturbed Zone) and on the effect of bentonite swelling on the host plastic clay. Moreover, results on the water, gas and radionuclide migration through the seal and consequently on the sealing capacity of the swelling clay powder/pellets mixture should be obtained. Status: in progress. Results: - Shaft sealing test The shaft seal installation was the opportunity to observe the behaviour of the Boom clay during the removal of the shaft liner, which is similar, from a hydro-mechanical point of view, to an excavation. The removal of the liner caused an important hydro-mechanical perturbation of the surrounding host rock leading to the desaturation and fissuration of the clay. The combined effect of the self-healing properties of the plastic Boom Clay and the hydration of the bentonite seal should help to restore the perturbation. The behaviour of the bentonite seal and of the surrounding host rock during hydration of the seal is currently monitored. - Borehole sealing test (diffusion test through the FoCa clay seal of the borehole)