Shear strength characteristics of ballast subjected to particle breakage and mud pumping using parallel gradation technique

Abstract

Ballasted rail tracks are most commonly used in rail track structure and are designed to provide a stable, safe and economic foundation. Main components of ballasted rail track structure can be grouped as track superstructure and track substructure. The cyclic loading by the train will be distributed from the superstructure to substructure. Sri Lanka currently uses the Indian standard gradation for almost all the tracks. Most of the ballast material used in Sri Lanka is obtained from biotite gneiss rock which is available in abundance. However, ballast is constantly subjected to degradation throughout its service due mainly to ballast breakage and migration of fine particles from the subgrade into the ballast layer, which is known as mud pumping. In this study, the effect of ballast particle breakage and mud pumping on the shear strength characteristics of ballast is investigated. Parallel gradation technique was used to model the sample as it is difficult to handle large size ballast in the conventional direct shear box. Sample was collected from Elugoda quarry (in the Kandy District) from where the ballast material is supplied to upcountry railway construction. Scaled down sample was sieved and test sample was prepared according to Indian standard gradations using the parallel gradation technique and the direct shear test was carried out under three normal pressures of 15 kPa, 30 kPa and 90 kPa. The study revealed that the shear strength of ballast is decreased for lower degrees of fouling due to ballast breakage and mud pumping and starts to increase with increased degree of fouling, but not as much as that of fresh ballast.