Effect of confining pressure and frequency on the deformation of ballast

Increasing the speed and frequency of trains with the same static axle weight imparts higher dynamic axle loads more frequently. When this occurs on existing track which has not been designed for such loading there can be increased rates of ballast degradation, characterised by unacceptable deformation and lateral spread, leading to more frequent requirements for track maintenance. Recent studies carried out at the University of Wollongong highlighted that confining pressure and frequency have a significant influence on the permanent deformation and degradation of ballast. However, confinement required to keep the deformation and degradation of the ballasted track to an acceptable limit will depend on the train speed (frequency). In this context, a series of cyclic triaxial tests was conducted on latite basalt samples having an initial confining pressure of 120 kPa. After every 25 000 cycles, the confining pressure was decreased in steps to simulate the drop of confining pressure during heavy traffic. This test procedure was adopted to replicate the influence of train speed on the stability of ballast. Test results indicated that both the frequency and confining pressure have a significant influence on the permanent deformation of ballast. Resilient modulus is found to increase with an increase in confining pressure and number of cycles, but to decrease with increasing frequency. The results also showed that the ballast layer requires a minimum level of confinement for preventing an excessive amount of track deformation. An empirical equation is formulated to determine the required confining pressure and resilient modulus of the ballast layer for an allowable limit of track deformation at a given train speed.