Role of Gradation Curve in Description of Mechanical Behavior of Unsaturated Soils

This paper presents a framework for the incorporation of the gradation curve in the description of mechanical behavior of unsaturated soils at low to medium degrees of saturation. Unsaturated soils have self-equilibrating initial stresses arising from surface tension forces acting on water menisci, which depend on the microstructure of saturation, particle surface contact characteristics, and pore size distribution (POSD). The latter, in turn, can be related to gradation curve. Unsaturated soil is treated here as a three-phase composite material, and its mechanical response is derived from that of its constituents: soil skeleton, water, and air. This approach leads to an analytical expression for the soil water retention curve (SWRC) in which the critical parameter to be determined is the water menisci area per unit volume (Swma) of the soil. This parameter can be identified from the gradation curve using well-established methodologies in soil science and agricultural engineering. Numerical simulations of triaxial tests show that deformation and strength of unsaturated soils, at a given degree of saturation and confinement, are influenced by the gradation curve, and its incorporation in the description of mechanical behavior of unsaturated soils is rational.