Investigating particles migration caused by mud pumping in ballasted track subgrade under cyclic loading-wetting coupling

Abstract

To study the mud pumping mechanism in the conventional Chinese ballasted track subgrade, a physical modelling system which can simulate the cyclic loading-wetting coupling process was developed. The model specimen is compacted in a transparent cylinder with an inner diameter of 500 mm, including a 350 mm subgrade soil and a 200 mm ballast layer. The test system was equipped with four types of sensors to monitor the load, displacement, volumetric water content, and pore water pressure, respectively. A high-definition camera was installed to monitor the particle migration phenomenon. The results indicate that as the volumetric water content increases, the specimen gradually changes from strain hardening to strain softening. Dynamic pore water pressure is the key factor for particle migration. As the volumetric water content increases, the amount of migrated particle caused by dynamic pore water pressure gradually increases. In the saturated state, it will cause a large number of particles to migrate, the mud pumping phenomenon is significant. The void contaminant index (VCI) [1] of the ballast finally reached 38 %. In practice, such value implies the necessity to carry out subgrade replacement for the normal operation of the railway.