Structure-based hydro-mechanical properties of sand-bentonite composites

For the geological disposal of highly contaminated wastes, medical or other sorts, clay barrier systems are commonly designed and used. The engineered liners contain buffer material which is often carefully proportioned mixtures of pure bentonite and sand. Bentonite is an active clay mineral with very low hydraulic conductivity and extremely high expansive properties, which benefits in controlling the downward migration of hazardous contaminants to groundwater. In the design of such composite buffer geomaterial, deformation and pore-flow analysis is a pivotal matter and has therefore been thoroughly investigated in the decades past. When unsaturated, the coupling hydraulic-mechanical behaviour of sand-bentonite mixtures are complex. Among possible reasons behind this complex behaviour is the dependency of hydraulic hysteresis and consolidation properties on size, shape and sorting of solids and pores in the soil's skeleton, which are also rarely accounted for in most of the commonly used soil models.

In this contribution, the hydro-mechanical behaviour of saturated and unsaturated sand-bentonite soil is investigated in the context of the recently developed Concept of Double Porosity (CDP). The geomaterial under study is assumed to consist of an incompressible, rigid, elastic solid skeleton surrounded by viscous water and gas fluids, and connected via a network of elastoplastic clayey bridge/buttress units. Roundness and sorting are varied for the sand constituent. The clay fraction (CF) is also varied across testing specimens. The experimental work here introduces two micromechanical models (small clay and large clay) which facilitates interpretation of macro-scale coupled hydromechanical behaviour of composite sand-bentonite geomaterials. The findings from this work will aid design practitioners through a tentative decision support system proposed in closing remarks.