From Basic Particle Gradation Parameters to Water Retention Curves and Tensile Strength of Unsaturated Granular Soils

The long-debated effective stress definition of unsaturated soils is believed to be correlated to suction, the degree of saturation, and the less mentioned interfacial areas. The tensile strength of unsaturated soils is directly associated with the effective stress definition in theory, and it is also crucial in engineering practices. For unsaturated soils, the relationship between suction and degree of saturation can be described as the water retention curve (WRC), which is related to the pore-size distribution. In the meanwhile, the air-water interfacial area is also regarded as a function of the degree of saturation, and parameters of the function are determined by the soil's pore structures. For granular soils, the pore-size distribution is usually in a unimodal shape and, therefore, the strength properties can also be related to the particle gradation parameters. In this study, a preliminary estimation method is proposed for the tensile strength of unsaturated sandy soils based on basic particle gradation parameters. In this method, with basic physical features considered, WRC is estimated from a characteristic grain diameter (d60) and the coefficient of uniformity (Cu). The function to determine the air-water interfacial area is also formulated by soil gradation parameters of the mean grain-size (d50), the coefficient of uniformity (Cu), and the void ratio. The proposed tensile strength estimation is compared with experimental measurements on sandy soils, which shows fair agreement, especially for the conventional split plate method. Qualitatively, the tensile strength is inversely proportional to soil mean grain-size and is increased with particle-size polydispersity.