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. In this context, a series of cyclic triaxial tests was conducted on latite basalt samples having an initial confining pressure of 120 kPa. 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 it decreases 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.