Unsaturated soils are often used as a construction material in transportation infrastructures. In this situation, unsaturated soils are subjected to cyclic mechanical loading from traffic loads or wetting-drying cycles in seasonal climatic conditions. While mechanical hysteresis is a common feature of soils in general, hydraulic hysteresis is associated with unsaturated soils. Although several constitutive models for unsaturated soils have been proposed, the mechanical and hydraulic hysteresis behavior of unsaturated soils has been little studied. A modified state surface approach (MSSA) was first proposed for investigating the mechanical behavior of unsaturated soils. It was then extended to study the coupled hydro-mechanical behavior of unsaturated soils with a special focus on the consistency between different soil phases. However, hydraulic and mechanical hysteresis were neglected in MSSA formulations. In this paper, based on evidence from experimental results, the MSSA is extended further to study the coupled hydro-mechanical hysteresis behavior of unsaturated soils. The extended MSSA can reproduce several forms of mechanical and hydraulic behavior observed in experimental results that cannot be represented by existing constitutive models. To demonstrate the capabilities of the extended MSSA, typical behaviors are simulated and compared, qualitatively, with the characteristic trends of the behavior of unsaturated soils. Experimental results from the literature are then used to evaluate the model to predict, quantitatively, the observed behaviors. The agreement between measured and predicted results is considered satisfactory and confirms the possibility of the proposed approach to reproduce the hysteresis behavior of unsaturated soils.
This paper presents a consistent three-dimensional elasto-plastic model to study unsaturated soil behavior with consideration of coupled hydro-mechanical hysteresis. The model was first formulated under isotropic conditions with special consideration to the non-linearity of the hydraulic behavior. Only one yield curve is used to represent the yielding of both mechanical and hydraulic behaviors (i.e., the occurrence of plastic water content changes and mechanical strains). Later, the model is extended to general three-dimensional stress conditions. It was formulated in a way that a smooth transition between the saturated and unsaturated soil states is guaranteed. The model provides consistent predictions for different soil phases that is considered a significant limitation in many existing models. One of the characteristic features of the proposed model is the ability to represent the hydro-mechanical coupling during shearing. Moreover, the model is able to represent the degree of saturation increase or decrease during shearing that is closely related to the soil’s contractive or dilative behavior, respectively. The model is validated through the prediction of several hydro-mechanical behavioral features. The paper also compares the model predictions with published experimental results performed under different loading conditions. The response of the model is satisfactory in relation to both mechanical and hydraulic behaviors.
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.
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