Constitutive model of structural alteration and swelling behavior for Old Alluvium

We describe a constitutive model for the compression and swelling behavior of Old Alluvium, a material with a complex microstructure characterized by Fe-oxides cementation and silt-sized aggregates of swelling clay minerals. Compression loading of this material breaks down the aggregate structure, altering the mineralogy, particle fracture properties, and physicochemical properties. We use cation exchange capacity as a measurable proxy to represent the microstructural evolution that can be linked to changes in compressibility with consolidation stress. At load reversal, the value of cation exchange capacity affects the microscale swelling pressures associated with osmotic repulsion between platelets of expansive nontronite. The micro- and macro-scale stiffnesses are linked through a parameter that relates the disjoining pressure to the macroscopic swelling pressure, and an interaction function. By relating macroscopic strains to microscale osmotic repulsion between expansive clay platelets, the proposed model builds directly on the physicochemical characteristics of the material and achieves a more consistent physical basis for describing the unloading response than prior formulations. We illustrate how the model can provide a framework for interpreting how engineering properties vary vertically with the state of weathering in the Old Alluvium soil profile. This approach should find broader application for other piedmont, transported, residual soils.