Calculation of fill foundation deformation based on a modified unsaturated soil constitutive model

Deformations in high fill foundations comprising soil–stone mixtures must be accurately predicted to ensure construction quality and long-term operational safety. However, existing computational and analytical methods inadequately capture their complex mechanical behavior. We conducted a series of triaxial tests on unsaturated soil samples collected from a high fill project site in northwestern China under three stress paths. The incremental nonlinear and elastoplastic constitutive models for unsaturated soils were modified, and a calculation method was developed for the vertical and lateral deformations of high fill foundations using the layered summation approach. The results indicate that for soil samples with the same mixing ratio (m) and compaction coefficient (n), the strength of the sample and its tendency to exhibit shear dilation increase with the net confining pressure or matric suction. Additionally, the stress–strain curve of the soil sample gradually changes from the strain-hardening type to the ideal elastoplastic type as the compaction coefficient increases. Moreover, the compaction coefficient is an important factor influencing the magnitude of yield stress and yield suction in soil samples, and the yield points of both are similar in shape to the loading-collapse (LC) and suction increase (SI) yield lines obtained using the Barcelona model in the net mean stress-generalized shear stress (p-s) plane, respectively. The modified incremental nonlinear instantaneous model simultaneously considers the effects of the compaction coefficient, suction and mixing ratio, and the model parameter E_t can be simplified to the tangential modulus expression in the Duncan-Chang model when the suction is zero. Furthermore, the modified elastoplastic constitutive model, which considers the effects of the net mean stress, suction and partial stress, can be simplified to the elastoplastic constitutive relationship of saturated soil when the suction is zero. The proposed deformation calculation method, based on the layered summation theory, is applicable to both elastic and elastoplastic foundation states, as confirmed through numerical simulations. Our work can be used as a reference for the calculation of foundation deformation in similar mixed material high fill projects.