Double structure hydromechanical coupling formalism and a model for unsaturated expansive clays

• New formalism for hydromechanical coupling in double-structure unsaturated soils is proposed.
• Separate hydromechanical model for each structural level, including separate effective stress measures.
• Microstructure swelling controls global expansion as well as global water retention curve.
• Volumetric rachetting eliminated from hypoplasticity.

A formalism for double structure hydromechanical coupled modelling of aggregated unsaturated soils has been developed. Independent coupled hydromechanical models are considered for each structural level, including independent measures of macromechanical and micromechanical effective stresses. The models are linked using a coupling function to obtain the global response. The individual components have been selected to represent the behaviour of compacted expansive clays. The macrostructural mechanical model is based on the existing hypoplastic model for unsaturated soils. Hydromechanical coupling at each structural level is efficiently achieved by linking the effective stress formulation with the water retention model. An essential component of the model is representation of microstructural swelling. It is demonstrated that its calibration on wetting induced expansion measured in oedometric (mechanical) tests leads to a correct global hydraulic response, providing a supporting argument for the adopted coupling approach. An interesting consequence of the model formulation is that it does not suffer from volumetric rachetting, which is often regarded as one of the main drawbacks of hypoplasticity. The proposed model has a small number of material parameters. Its predictive capabilities have been confirmed by simulation of comprehensive experimental data set on compacted Boom clay.