Poisson’s Ratio Characteristic Curve of Unsaturated Soils

Abstract

Poisson’s ratio is an elastic property needed as input in a wide range of geotechnical engineering applications. Despite experimental evidence, there is currently no model in the literature to explicitly account for the effect of saturation on the Poisson’s ratio. This paper presents the concept of Poisson’s ratio characteristic curve (PRCC), which establishes a relationship between Poisson’s ratio and degree of saturation (or matric suction). The PRCC concept is developed based upon the observation that variations of Poisson’s ratio are mainly dominated by water retention mechanisms. A sigmoidal function is employed to describe the PRCC with two fitting parameters, funicular degree of saturation and pore fluid continuity, and both are related to the soil water retention curve (SWRC). The functional form is calibrated and validated against Poisson’s ratios calculated from measured compressive and shear wave velocities from experimental data sets for 22 different soils from the literature. Further, a set of laboratory tests is performed to measure wave velocities using bender elements and determine the Poisson’s ratio of Bonny silt at different suctions. The PRCC fitting parameters are shown to be linearly correlated with the SWRC fitting parameters. To illustrate the PRCC application, three sets of laboratory-measured data of at-rest earth pressure coefficient for different unsaturated soils are collected from the literature and compared against the predicted values using the PRCC model and those using constant Poisson’s ratio values. The values using the PRCC model closely match the measured values, whereas using a constant Poisson’s ratio can significantly underpredict or overpredict the at-rest earth pressure coefficient. The proposed model can readily be incorporated into analytical and numerical models, leading to more accurate assessments of unsaturated soil behavior.