Backfill formulations for a nuclear waste disposal vault

In the program of study conducted by Atomic Energy of Canada Limited and Ontario Hydro for nuclear waste disposal in an underground vault, a suitable backfill material has to be formulated to ensure low hydraulic conductivity in order to prevent contamination of groundwater. The candidate backfill materials selected for this study consisted of a mixture of coarse aggregates and a naturally occurring swelling clay. Trial mixtures were tested by varying the gradation, with respect to maximum grain size and clay content, and evaluating the density-moisture content relationship. Various trial mixtures were subsequently subjected to the constant head permeability test, which led to the findings that the hydraulic conductivity of the aggregate-clay mixtures could be close to that of the clay (by itself) when the clay content was in the range of 25% or more. Selected mixtures were evaluated for free swell and swelling pressure, both of which increased with increasing clay content. Based on the aforementioned results, the candidate backfill material was proposed to contain about 25% clay with the maximum grain size of 19.1 mm. The proposed backfill material appeared to perform satisfactorily according to the criteria demanded