The effect of increased axle loading on the behavior of heavily overconsolidated railway foundation materials

Abstract

The benefits of increased axle loading in the railway industry are well-appreciated to such an extent that increased axle loading has become a themed topic of interest across all disciplines of railway engineering. However, it remains important to understand the engineering behavior and performance of each track component under increased axle loading. This study was particularly interested in the behavior of railway foundation materials under increased axle loading, starting from a base load of 20 tonnes per axle for general freight to increased loads of 26, 30, 32.5 and 40 tonnes per axle for heavy haul. The methodology followed involved formulation of a theoretical model for analyses, characterization of the cyclic railway loading via finite element modelling, experimental laboratory work using a cyclic triaxial apparatus followed by detailed analyses, interpretation and discussion of the test results.

Based on the test results from the monotonic and cyclic loading, it was found that the critical state line was related to the various phase transitions in soil behavior and the resilient strain was found to be inversely related to the permanent strain. Stress states slightly above the critical state line resulted in a double-phase transition in soil behavior from dilation to contraction and then work softening with cyclic mobility. Stress states on the critical state line resulted in a single-phase transition in soil behavior from dilation to contraction with a combination of cyclic mobility and shakedown. Stress states below the critical state line resulted in a no-phase transition in soil behavior accompanied by dilation with shakedown. In relation to the effect of increased axle loading on saturated heavily overconsolidated railway materials, it was therefore concluded that resilience can be characterized by a no-phase transition predominated by linear permanent deformations and shakedown behavior while failure is characterized by a double-phase transition predominated by exponential permanent deformations and cyclic mobility behavior.