BALLAST AND SUBGRADE RESPONSE TO TRAIN LOADS

Ballast and subgrade play major roles in the maintenance life of track structures because they are the source of the cumulative permanent deformation associated with the deterioration of surface and line. Ballast is also the principal means of correcting for this deterioration, which is caused by traffic and environmental factors. Better methods are still needed for the prediction of the effects of the controlling parameters on track performance for more rational track design and maintenance planning. The purpose of this paper is to provide a better understanding of these problems and describe progress being made toward their solution. The functions of ballast and subgrade are briefly discussed, and the mechanisms of permanent deformation are described. Newly developed or improved methods to measure the in situ physical state of ballast are presented, and examples of results from field tests are given. The capabilities of existing analytical track structure models for the prediction of track deterioration are assessed. New instrumentation techniques used for measuring the dynamic and permanent strains and deformations in ballast and subgrade are described. Finally, the characteristics of the stress, strain, and deformation in ballast and subgrade are illustrated with results of both analytical and experimental studies.