An experimental study on the hydraulic conductivity of compacted bentonites in geoenvironmental applications

Compacted bentonites and compacted sand-bentonite mixtures have been proposed as suitable barrier and backfill materials for the disposal of municipal solid waste and high level radioactive waste. Although unsaturated on placement, the barrier and backfill materials can become saturated subject to the availability of fluid. Detailed understanding of the saturated hydraulic conductivity of compacted bentonites and sand-bentonite mixtures is essential to ensuring the integrity of the waste disposal facility and the long-term protection of the geoenvironment. This thesis is concerned with the experimental determination of the hydraulic conductivity of compacted MX80 bentonite and sand-bentonite mixtures (30% MX80 bentonite to 70% sand). A high capacity fixed ring modified swelling pressure cell was used for carrying out the hydraulic conductivity tests. High precision pressure-volume controllers were used to apply a range of hydraulic gradients between about 1250 and 12500 under constant head conditions. The expansion of the measuring system was studied to account for differences between inflow and outflow water volumes during the hydraulic conductivity tests. The hydraulic gradient was increased and decreased during the hydraulic conductivity tests. Chemical analysis of fluid samples collected from the inflow and outflow reservoirs after each hydraulic conductivity tests provided information about the type and amount of exchangeable cations expelled from the specimens. The inflow volume was calculated based on the system expansions. The hydraulic conductivities were calculated from Darcy’s law. The saturated hydraulic conductivity of compacted bentonite and sand-bentonite specimens were also calculated based on the consolidation tests results. The gas permeability of compacted unsaturated bentonites was determined. The saturated hydraulic conductivity of compacted bentonites was assessed using various existing models. A model based on parallel plate flow was proposed in the current study. The proposed model considered the viscosity of water in the inter-platelet region and its influence on the hydraulic conductivity of compacted bentonites. The correction of the water inflow volume by accounting for the system expansion during the hydraulic conductivity tests provided good compatibility between the inflow and outflow water volumes. The equilibrated inflow and outflow rates were found to be similar during the hydraulic conductivity tests. A linear relationship was noted between hydraulic gradient and hydraulic flux indicating the validity of Darcy’s law for calculating the hydraulic conductivity of compacted bentonites. An expulsion of exchangeable cations from the compacted bentonite specimens occurred during the hydraulic conductivity tests. The amount of expelled cations was found to be less than about 6% of the total exchangeable cations present in the bentonite. The gas permeability of compacted unsaturated bentonite was found to decrease within an increase in compaction dry density. The calculated hydraulic conductivity of compacted unsaturated bentonite based on the measured gas permeability was found to be greater than the measured hydraulic conductivity of compacted saturated bentonite indicating that swelling reduces the hydraulic flow paths. The saturated hydraulic conductivities calculated from the Kozeny-Carman model were found to better describe the measured saturated hydraulic conductivities than the other available models. The model proposed in this study did not satisfactorily establish the hydraulic conductivity of compacted saturated bentonite due to uncertainties associated with the assumptions made regarding the viscosity of the bulk fluid