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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

El Naggar, M. Hesham

2nd Supervisor

Stark, Timothy D.

Affiliation

University of Illinois at Urbana Champaign

Co-Supervisor

Abstract

Railway transportation offers capacity, efficiency, and safety to serve the needs of modern societies for moving freight and people. Therefore, the sustainability and safety of railroad infrastructure are of paramount importance for securing transportation needs. Freight train passby loads increase the surface deformation of railroad substructure due to cumulative plastic strains, which lead to surface deviation of the railroad tracks. Correspondingly, railroad maintenance is necessary to sustain track geometry alignment and to prevent deterioration of the sub-structure over time, which can increase operating costs significantly. Therefore, this thesis evaluates the performance of fouled railroad superstructure through laboratory testing of fouled sub-structure material, field monitoring of fouled track section at the test site and finite element analysis of the fouled track performance. First, isotropically consolidated drained cyclic triaxial tests were conducted on fouled sub-ballast retrieved from the test site. Test results were used to assess the material shear stiffness during the loading cycles. Operational speed limits were deduced from the stress-strain behavior of the fouled sub-ballast to prolong the service life of the fouled foundation bed. Next, railroad superstructure performance due to track fouling was assessed through a field monitoring program. The normalized maximum rail-tie tensile force was employed to analyze the uplift resistance of a CWR section at the cross-tie. Results indicate that the tie-load ratio may misrepresent the superstructure efficiency in fouled ballast beds. Alternatively, the range of influence of wheels along with the uplift ratio may be used to assess fouled railroad performance using the proposed empirical models. Measured accelerations during the passage of various trains were analyzed in time and frequency domains. Train velocity guidelines are provided to minimize train-induced vibrations in the fouled tracks. Finally, a validated dynamic two-dimensional finite element model was developed to investigate the effect of train velocity on the generated positive and negative excess pore water pressures using loading time histories considering different train velocities. Results demonstrate that positive excess porewater pressure developed in the sub-structure due to cyclic loading. Undrained conditions prevailed in the sub-grade and sub-ballast. Capillary fringes were noticed to form in the sub-ballast layer.

Summary for Lay Audience

A railroad is comprised of two main components: the track and foundation. A typical modern railway track consists of the rail, fasteners, and cross-ties. The foundation bed consists of ballast, sub-ballast, and native soil. The rail made from steel carries train wheels and transmits train loads to the cross-ties. Fasteners are made from steel and hold the rail together with the cross-ties. The cross-ties (made from concrete or timber) provide support and transmit the applied train loads to the track foundation. The ballast is comprised of crushed stone and its main functions is to support train loads and drain water from the track. The sub-ballast (smaller diameter particles) acts as a diaphragm between the ballast and the native soil. The native soil is the local soil type present on the ground surface on top of which the railroad is constructed. Track fouling is a railroad condition where the ballast gets contaminated with fines (sands, silts, and clays). Progressive use of fouled track by traversing trains causes mud (fines) to be pumped and a gap to exist beneath the cross-ties a phenomenon called the voided cross-tie. Previous studies have highlighted the relationship between increased train speed and the rate of deterioration of the foundation bed. However, these studies did not provide an insight into the extent of deterioration of railroad foundation due to heavy haul trains travelling at high speeds. Therefore, the current study has provided an understanding of the behaviour of fouled sub-ballast under train loading and proposed guidelines to operators about the train speed limits in fouled railroad segments. In addition, the current study investigated the behaviour of the track through a 6-month duration field monitoring and quantified the deflection of the railroad track due to trains travelling in adverse environmental conditions. Recommendations were provided to manage trains travelling on fouled railroad segments in critical weather conditions. Moreover, the current study has placed special emphasis on assessing the response of voided cross-ties compared with supported cross-ties. The vibration of cross-ties in the fouled segment of the track is compared with the cross-tie vibration at the clean section to elucidate the effect of ballast fouling. Lastly, using software, the current study provided a critical review focused on the effect of mud pumping in railroad foundation and the magnitudes of water pressure due to actual measured wheel loads.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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