Modeling Shrink-Swell Soil Behavior Application to Railway

Abstract

Railway embankments constructed on shrink-swell soil often encounter various challenges, including uneven settlements, crack development, ponding water, which may lead to safety issue. The triggering factors and development mechanism of them are still not well known. There were many literatures assessed the problem, however, no comprehensive hydro-mechanical (HM) coupled simulation was conducted considering shrink-swell soil properties. This research aims to fill this gap and gain a deeper understanding of the behavior of railway embankments in such conditions.

There are three difficulties encountered, including understanding coupled Hydro-Mechanical interaction mechanism, seeking for appropriate soil property parameters, as well as considering environment impact on embankment behavior for over five months. The understanding of coupled problem formation is embedded into the full-scale test simulation aspect of this research served as theoretical support of them. A specific railway embankment previously issued train slowdown and located in Texas, USA, is selected to provide experimental facts for the simulation.

The initial phase of this research focuses on the development of a constitutive model for shrink-swell soils. By analyzing and comparing existing laboratory experiment results, an overlooked phenomenon has been identified. It has been observed that the amount of reversible deformation decreases within a wide suction range during cyclic drying-wetting tests, despite the application of vertical load. To accurately capture these experimental findings, a new elastoplastic constitutive model based on the Double Structure Model proposed by Sánchez et al. (2005) has been developed in this research. The model has been primarily revised from a microstructural perspective. MATLAB has been used to implement the model, incorporating a specific function to handle this newly discovered feature. The numerical algorithm of the model employs an explicit stress integration technique. By running the newly developed code, various laboratory tests, including cyclic drying-wetting tests and tests involving complex combinations of hydraulic and mechanical loading paths, have been successfully simulated. The results of these simulations have exhibited satisfactory agreement with the experimental data.

The second part of this research is a numerical solution to a practical problem that can be seen as an unsaturated fluid-flow transport problem in deformed porous media. By running FEM software Code_Bright, the performance of the test embankment undergoing long-term water exchange with the environment is comprehensively reproduced. The comprehensive simulation is divided into preliminary simulation and complete simulation, in which the hydraulic model and the hydro-mechanical model are considered. The influence of different vertical stress to behavior of shrink-swell soils at different depths is considered. The environmental effect is simulated using daily evapotranspiration and precipitation. As a result, the water content and deformation simulation results are verified with field-scale tests on a real-world expansive-soil railway embankment. Other than that, the transverse uneven settlement is simulated by considering different infiltrations and runoffs on two sides of the railway embankment. As a result, the role of railway embankment and the rain as “natural barrier” is ascertained. It is found the saturation state and shrink-swell soil characteristics are responsible for the irreversible heave after ponding.