Full-scale mud pumping test of ballastless trackbed under train loading

Abstract

This paper presents a full-scale physical modeling investigation on the mud pumping in ballastless trackbed. The physical model was equipped with a rainfall device to simulate the in situ precipitation and various testing sensors were installed to monitor the dynamic response of the track-subgrade system. Results showed that the whipping of the track structure induced by high-speed train passages caused a separation between the track structure and the underlying roadbed, which allowed rainwater to intrude into the roadbed. The intruded rainwater infiltrated vertically and then permeated laterally from the bottom of the roadbed, eventually saturating the entire roadbed. Train moving loads partially promoted this rainwater infiltration. The excess pore water pressure (EPWP) was formed in the saturated roadbed caused by train moving loads, and it increased with train speeds and loading carriages. A noticeable upward EPWP gradient and a slight longitudinal EPWP gradient towards the expansion gap were also created in the saturated roadbed. Accordingly, both the vertical hydraulic gradient (HG) and longitudinal HG increased with train speeds and loading carriages, most of which, especially the vertical HG, exceeded the critical HG for particle migration with a diameter smaller than 7.1 mm. It caused severe upward particle migration and slight longitudinal particle migration towards the expansion gap. This spatial fine particle migration process, known as mud pumping, was driven by the HG-induced seepage force and further promoted by the EPWP amplification induced by high-speed train passages. Mud pumping resulted in a significant decrease in stiffness of the trackbed and a noticeable increase in vibration velocity of the track slab.