Characterizing and Modeling the Coupled Hydro-Mechanical Cyclic Behavior of Unsaturated Soils Using Constant Water Content Oedometer and Direct Shear Tests

Abstract

Most transportation infrastructures are constructed on compacted soils that are typically unsaturated above the groundwater table. The soils are subjected to cyclic traffic loadings and seasonal wetting–drying cycles. Although problems associated with unsaturated soils are ubiquitous in the US, coupled hydro-mechanical analysis is rarely included in the design/analysis of transportation geosystems. This can be attributed to two main reasons: (a) there are no simple devices/methods which can be used to rapidly characterize stress–strain behavior for unsaturated soils, and (b) there is a lack of a constitutive model to study coupled hydro-mechanical cyclic behavior for unsaturated soils in a consistent way. This paper takes advantage of the recent advancements in suction measurement and constitutive modeling to overcome these limitations. In this paper, conventional oedometer and direct shear tests for saturated soils are modified to characterize stress–strain behavior of unsaturated soils under cyclic undrained loading conditions. Then, series of constant water content cyclic oedometer and direct shear tests are performed to characterize the hydro-mechanical behavior of a silty soil. Moreover, methods are developed to use results from both tests to calibrate parameters for a recently proposed hydro-mechanical constitutive model. The paper also compares the model predictions with the measured results. Results indicate that the model is able to satisfactorily predict the hydro-mechanical hysteresis behavior of the soil. With developments in equipment, constitutive models, and analysis, it is now possible to fully characterize the hydro-mechanical behavior of unsaturated soils in a more efficient manner.