Mahdavi Kharanaghi, Mohammad (2020-07). LONG-TERM BEHAVIOR OF BALLAST IN RAILWAY EMBANKMENT UNDER HIGH-SPEED TRAIN AND FREIGHT TRAIN LOADING. Doctoral Dissertation.
Thesis
The behavior of the railway ballast under the operation of high-speed trains and heavy freight trains is studied in this dissertation. For this purpose, a combination of laboratory testing and numerical simulations has been performed. This project can be categorized into three different phases: (1) behavior of the ballast under monotonic loading, (2) long-term behavior of the ballast under cyclic loading caused by train passage, and (3) stress and strain development in the track substructure under train loads. The monotonic behavior of the railway ballast is investigated through extensive laboratory tests by using large-scale direct shear, direct simple shear, and triaxial tests. The outcomes of this phase of the study are the magnitude of the shear strength properties of the ballast, which directly uses in the railway substructure design and the maximum shear strength of the material, which is a key factor in the further cyclic investigations. Moreover, the stress-strain-strength of the large aggregate ballast material has been compared by the three abovementioned tests, and the appropriate empirical formulas for calculating the shear strength properties of the ballast based on all three tests are proposed. To investigate the long-term behavior of the railway ballast under repeated loading, a series of large-scale cyclic direct simple shear and triaxial tests were conducted. The tests were performed under various normal/confining stress and cyclic stress conditions to be representative of the various railway geometry and trains. The long-term characteristics of the ballast represent in forms of permanent settlement, lateral spreading, and the resilient modulus. Finally, a series of empirical equations to predict the long-term characteristics of the ballast based on the initial shear strength of the material, cyclic loading amplitude, and the number of cycles are provided. For the third phase of this study, a four-dimensional (4-D) finite element model (FEM) of the ballasted railway by using LS-DYNA has been developed. This model, which validated by the field measurement in Sweden, was used to study the stress and strain development in the railway foundation under high-speed and heavy freight trains. Moreover, the impact of train characteristics, substructure geometry, and material properties on the track maximum deflection and dynamic stress distribution in the track substructure have been investigated.
