Rezaei Tafti, Somayeh (2018-02). High Speed Train Geotechnics: Numerical and Experimental Simulation of Some Embankment Problems. Doctoral Dissertation.
Thesis
This dissertation explores some of the geotechnical challenges associated with implementing high-speed railway (HSR) systems. These challenges are mostly related to HSR embankments. Three special geotechnical issues addressed in this project are long-term softening of compacted unsaturated embankments due to water absorption, potential derailment when trains break through the Rayleigh wave barrier, and the tolerable bump (or any other type of irregularity) in the track. Soil softening due to water level rise in compacted embankments is investigated through an extensive experimental study that provides a better understanding of unsaturated soil and its impact on soil modulus degradation, which has a great impact on the other two issues. In fact, too much water content in the natural soil under the embankment can cause serious problems, such as unexpected large, uneven deflections. Through these experiments, it was confirmed that the soil modulus can considerably change as the water content in the soil varies. To evaluate both the track responses to breaking the Rayleigh wave barrier and bump along HSR lines, a series of four-dimensional (4-D) finite element models (FEMs) using LS-DYNA have been developed. These 4-D FEMs include all track substructural and superstrucural components. Breaking the Rayleigh wave barrier, especially when traveling on soft soils, results in large deflection in the track structures and substructure components due to the combination of static and dynamic loads. Using the developed 4-D FEMs, the effect of train speed on the large track deflection is investigated. Through this study, it was confirmed that at a particular speed called the critical speed, the maximum track deflection occurs. This train speed should be avoided because the resultant maximum track deflection leads to either high maintenance cost or, in the worst-case scenario, train derailment. These extensive numerical simulation results are compiled into proposed guideline charts to enable the design of safe embankment for HSR lines while keeping the train speed below 70% of the critical speed, which is considered as the safe train speed zone. The problems of stiffness transition and irregularity along HSR are investigated using a 4-D FEM of both faulted and non-faulted track, considering passenger safety and comfort criteria. Although the main source of bump development is found to be the track modulus differential alone, track modulus variation alone has only minor impact on the train/track responses. It should be noted that track modulus differential instigates the formation of different types of irregularities along HSR, which significantly increases the wheel/rail interaction force and train body acceleration. The parametric study conducted to look at the effect of different parameters such as train speed, subsoil modulus, and irregularity type and size on the train/track responses results in proposed guideline charts defining tolerable irregularity size for HSR lines such that the vertical train body acceleration and wheel/rail interaction force are kept below the permissible values.
