Compacted granular mixtures of high-density bentonite pellets have been evaluated as an alternative buffer material to fill the empty space around nuclear waste disposal canisters in horizontal drifts. Despite the obvious benefits of these compacted mixtures (the backfilling operation becomes easier and the gaps between the host rock and the buffer are minimised), there are several aspects of concern such as the effective blockage of the large inter-pellet pores due to granule swelling – this blockage improves the water permeability properties – and the tendency to develop initial collapses before reaching an adequate swelling pressure. Selected test results of a comprehensive laboratory experimental programme are presented to gain insight into the hydromechanical response of this multi-porosity compacted material. To improve the information on local transient behaviour, simulation-assisted techniques using a double-structure constitutive model are used. The paper presents a physically based one-dimensional model to simulate experimental results of different transient processes, such as the progressive loss of permeability during wetting and the occurrence of concurrent phenomena during fast flooding at constant stress (initial collapse of the granular arrangement and parallel expansion of granules).