A METHODOLOGY TO PREDICT THE ELASTIC AND INELASTIC BEHAVIOR OF RAILROAD BALLAST
Over a period of time, railroad track will settle as a result of permanent deformation in the ballast and underlying soil layers produced by traffic loading. After track settlement is produced, maintenance will be needed to resurface and line the track. Maintenance is desired as soon as the loss of surface and line reaches a degree that interferes with proper vehicle operation or affects safety. The period of time between maintenance surfacing and lining operations is designated as "maintenance life" of the track. The purpose of this dissertation was to develop a methodology to calculate the elastic and cumulative inelastic behavior of the railroad track structures under traffic loading as a basis for maintenance life prediction.
The methodology involves a three-dimensional computer model to estimate the stress state in the roadbed materials caused by train loading, and laboratory testing on the ballast under static and repeated loading conditions equivalent to those in the field. Available field measurements from the U.S. Department of Transportation test track in Pueblo, Colorado were used to validate the predictions obtained with the proposed methodology. The predicted results were in approximate agreement with the field results.
The methodology developed appears to provide a reasonable and rational approach to predict the elastic and permanent deformation of railroad track systems. The method can take into account the main factors influencing the deformation behavior, including axle load and number of cycles, rail and tie characteristics, and properties and thickness of the ballast and underlying layers.
A review of analytical methods developed to design and predict the elastic performance of track structures was presented. Also, the available information about static and cyclic behavior of roadbed materials was summarized.
Field ballast density measurements were presented. It was found that the boundary condition in ballast materials plays a crucial role in determining the accuracy of the calculated sample density. A sample preparation method for the laboratory samples was developed.
The analytical model GEOTRACK was chosen to determine the stress paths to which the ballast samples were subjected in the laboratory property tests. In the model, the two rails are designated as beams supported by eleven ties. The ties rested on a series of elastic layers of infinite horizontal extent representing the ballast, sub-ballast and subgrade. The model for the layers was adapted from the Burmister's theory. An iteration scheme was included to represent the stress-dependent nature of the roadbed materials. The approach taken to determine octahedral stresses in the field and their equivalent triaxial stresses in the laboratory was presented.
Static and cyclic tests were performed on granite ballast following the stress paths previously determined. All the tests were isotropically consolidated, drained tests with saturated samples. A large triaxial cell was built and special preparation methods and testing techniques were developed. From the static tests, Mohr-Coulomb strength parameters and hyperbolic stress-strain curves were determined. Two sets of properties were obtained from the cyclic tests: first, the elastic behavior in terms of a stress-dependent, resilient modulus; and second, the permanent strain accumulation as a function of stress state, physical state and number of cycles of load application.
A step-by-step application of the method of predicting elastic and inelastic deformations for ballast layers was presented. The results were compared with the measured values from the field experiments.
Finally, conclusions and recommendations from the study were presented.