Permeability of compacted saturated clays from high stress consolidation test

Document available in extended abstract form only. In the past years, compacted bentonites have been proposed as suitable barrier and backfilling materials in the toxic waste repositories. In the waste disposal repositories at depths ranging from 500 m to 1000 m, the saturated host rock serves as the source for supplying fluid to the compacted bentonites and also acts as confinement against the volume increase. Considering the stress level at the location of the waste disposal repositories and the stress convergence of the host rock with the compacted saturated bentonites, it is necessary to study the permeability of compacted saturated bentonites for these conditions. The behaviour of compacted expansive clays has been intensively studied since last three decades with respect to key aspects such as swelling pressure, swell potential, compressibility, water absorption and desorption, temperature dependent behaviour of the material, micro-structural features. In contrast, less information about fluid flow in compacted bentonites is available in literature. In the present study, the permeability was determined indirectly based on consolidation tests of compacted saturated clay specimens. The material used was Calcigel bentonite and Spergau kaolin in order to investigate the effect of different mineralogy. De-ionised water was used as the fluid. The initial dry densities?d (in Mg/m3) and water contents w (in %) of the compacted bentonite specimens (CB1 to CB9) and those of the compacted Spergau kaolin specimens (CK1 to CK6) are presented. The initially compacted clay specimens were allowed to saturate at constant volume condition (phase 1) and were then submitted to vertical stresses higher than the swelling pressure measured (phase 2). Maximum applied vertical stress was 25 MPa. The special high stress odometer device used is described in detail in Baille et al. (2010). For each loading step, the coefficient of consolidation, cv was determined from the time-compression curves using Casagrande's log-time method. The coefficient of permeability k was computed from the values of volume compressibility mv and the cv values for each incremental pressure. The coefficient of consolidation was plotted against applied vertical stress and the coefficient of permeability void ratio relationship of the clays is shown. For the sake of comparison, the results for the initially saturated (slurry) bentonite (SB1, SB2) and the initially saturated Spergau kaolin (SK) are included. The results revealed the following: (1) With an increase in the applied pressure, for Calcigel bentonite, cv was found to decrease for both initially saturated specimen and compacted saturated specimens; however, at large pressures, cv increased for the initially saturated bentonite specimen, whereas it remained nearly constant for the compacted saturated bentonite specimens. For the initially saturated specimens the results are similar to those obtained by Marcial et al. (2002). The effect of initial compaction conditions on cv was found to decrease with increasing vertical stress, indicating the similarity in the fabric and structure of the bentonite specimens at very large pressures. (2) The cv values of the initially saturated Spergau kaolin first increase and then remain constant for stresses greater than about 500 kPa. The influence of compaction condition is minor in case of Spergau kaolin. (3) The variation of k with the void ratio was found to be distinctly bilinear for the initially saturated bentonite specimen, whereas for the compacted saturated specimens the relationship was found to be nearly linear and remained within a narrow band. The compacted saturated bentonite specimens were found to be more permeable than the initially saturated specimen at all void ratios considered in this study. (4) The variation of coefficient of permeability with void ratio was found to be linear both for the initially saturated and the compacted saturated specimens