This paper presents the findings from a series of constant suction triaxial tests conducted on compacted sand and silty sand under unsaturated conditions. These tests were carried out using a fully automated double-walled triaxial cell employing the axis translation technique. The net mean stresses applied ranged from 50 to 250 kPa, while matric suctions were maintained at 0, 100, and 200 kPa. The primary objective of this study was to elucidate the mechanical behavior of the two compacted soils under triaxial conditions, particularly focusing on the influence of suction on variables such as peak stress, apparent cohesion, critical state stress, postpeak softening, and strain-induced dilatancy. The experimental results were utilized to calibrate and validate two prominent critical state-based models for unsaturated soils: the Barcelona basic model (BBM) and the Morvan model. While the BBM accurately predicted the deviatoric stress values at the critical state under controlled suction conditions, it did not adequately capture the postpeak softening behavior. Conversely, the Morvan model, after appropriate calibration and validation, successfully replicated both the critical state and postpeak behaviors, demonstrating a strong correlation between its predictions and the experimental data for both soil types.

Due to continuous water level fluctuations and changes in climatic boundary conditions, river embankments experience frequent variations in the degree of saturation and suction distributions, which strongly influence earthworks performance, both in terms of infiltration regime and stability conditions. For these reasons, an experimental campaign aimed at investigating the hydro-mechanical response of a compacted silty sand mixture, representative for the embankment earthfills of the river Po tributaries (Italy), has been carried out and the main results are reported and discussed in this paper. To promote homogeneity and initial conditions consistency, the specimens were reconstituted at target dry unit weight and moisture content, by using the standard Proctor compaction energy. Suction-controlled triaxial and oedometer tests were designed and carried out to highlight the effect of drying and wetting stress paths, as well as confining stresses, on strength and compressibility characteristics of the tested material. The well-established axis translation technique has been used for controlling matric suction inside the samples. Typical suction ranges for the embankment, assessed through field monitoring of seasonal and daily hydraulic changes, have been imposed during the tests. Unsaturated specimens consistently exhibited a higher shear strength and stiffness with the increase of suction, compared to the saturated samples. The outcome of the present mechanical characterization may provide some meaningful geotechnical insights for the assessment of river embankment actual safety margins under transient seepage groundwater flow.