ABSTRACT
There is a growing interest of many African countries to revamp their neglected and unsustainable railways to attract a transportation modal shift from the roads. In order to promote regional trade and transportation integration with border concessions, investors are faced with problems of railway track gauge conversions to match the needs of the rolling stock technology of the 21st century at the same time maintain business operations with the old rolling stock on the same track structure. The objective of the work documented in this paper was to numerically evaluate the impact of track gauge conversions on traffic load induced Permanent Deformation (PD) of low embankment on soft sub-grade. A method to predict the traffic load induced settlement of low embankment on soft sub-grade is proposed. Using the user-defined material subroutines (UMAT) in ABAQUS, a 2-D Finite Element (FE) model was formulated. These models are converted into a numerical formulation for implementation in FE analysis and the traffic load induced dynamic stress in the sub grade are calculated by using the multi-layer elastic theory. Then the plastic vertical strain in the sub-grade is calculated by an empirical equation, whose constants are related to the physical and mechanical properties of the sub-grade soil. The method was applied to analyze a 700 m long section of a low embankment on the soft black cotton soil of Nakuru plains in Kenya using single and dual track gauge respectively. The corresponding results showed that the application of traffic loads on alternate rail tracks due to gauge conversions have a significant effect on the permanent deformation of the sub grade soil. The depth significantly influenced by traffic loading was found to be close to 6 m below the base of the embankment. The analysis also shows that increasing the thickness and stiffness of the sub grade is a very effective way of reducing the traffic load induced permanent deformation of soft sub grade soil. The proposed method can be used for settlement analysis on low embankments as well as a useful tool for making decisions on railway track gauge conversions.
