Abstract
Upon drying, matter and energy are exchanged between the atmosphere and porous media through evaporation, which is a coupled process that involves the simultaneous transport of liquid water, water vapour and heat. At shallow depths, evaporation controls the water content and suction of both natural soils and earthworks, affecting their hydraulic response. This impact is particularly relevant when the earthworks are aimed at the containment of aqueous or non-aqueous pollutants, as in the case of cement bentonite cut-off walls. A coupled model for the transport of liquid water, water vapour and heat through cement bentonite mixtures upon evaporation was formulated. The model considers flow of water driven by both total suction and temperature gradients. Model predictions were compared to experimental results obtained in the laboratory on samples having different sizes and imposed boundary conditions. A good agreement between predicted and measured volumetric water contents was obtained, once defined a suitable dependency of the relative permeability of the mixture on degree of saturation. The results suggest that the proposed formulation correctly accounts for the underlying physical processes, and that it might be used to model the real scale behaviour of cut-off walls.
Highlights
The contemporaneous movement of heat, water vapour, and liquid water plays an important role in many geotechnical and geo-environmental applications
Matter and energy are exchanged between the atmosphere and porous media through evaporation, which is a coupled process that involves the simultaneous transport of liquid water, water vapour and heat
In the context of waste disposal, the prediction of the volume of leachate percolating from landfills requires the analysis of the heat and of the water mass exchanged through the landfill cover
Summary
The contemporaneous movement of heat, water vapour, and liquid water plays an important role in many geotechnical and geo-environmental applications. In the context of nuclear waste storage, the radioactive decay may generate high temperatures and temperature gradients, which are expected to cause evaporation of water, soil desaturation, and consequent migration and condensation of water vapour in cooler areas This work focuses on the study of the evaporation processes occurring in cement-bentonite mixtures. A set of differential equations, defining the mass balance of liquid water and vapour and the heat balance, is proposed. The parameters of these differential equations, describing the water retention and the transport laws were calibrated upon previous characterization [10] and laboratory data from evaporation experiments [11]
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