Coupled chemical-hydraulic-mechanical behaviour of bentonites

Bentonites are clay soils characterised by a high specific surface and a permanent negative electric charge on their solid skeleton. Their common use as hydraulic and contaminant barriers for landfill and soil remediation applications, including the final disposal of nuclear waste, needs to be supported by adequate theoretical modelling of their mechanical behaviour and transport properties, in order to assess the expected performance in the long term. To this end, a theoretical approach has been proposed in order to derive constitutive equations for their coupled chemical-hydraulic-mechanical behaviour. The phenomenological parameters that govern the transport of electrolyte solutions through bentonites - that is, the hydraulic conductivity, the reflection coefficient (which is also called the chemico-osmotic efficiency coefficient), and the osmotic effective diffusion coefficient - have been measured through laboratory tests on a bentonite with porosity of 0.81, over a range of sodium chloride concentrations in the pore solution that varied from 5 mM to 100 mM. The relevance of the osmotic phenomena has been shown to decrease when the salt concentration increases. The obtained results have been interpreted by assuming that the microscopic deviations of the pore solution state variables from their average values are negligible. In this way, it is possible to interpret the macroscopic behaviour on the basis of the physical and chemical properties of the bentonite mineralogical components.