Physical modeling of train-induced mud pumping in substructure beneath ballastless slab track

This paper identifies the fundamental driving factors for the mobilization of mud pumping in railway substructure beneath ballastless slab track. Physical model tests on a compacted gravel layer overlying a permeable layer were performed under different conditions in terms of fines content, moisture (i.e., unsaturated, saturated with and without surface ponding), load patterns, and slit width between load plate and gravel surface. The loading unit primarily consists of a load plate and electric actuator for reproducing the train induced loads under normal operation. The model test system was equipped with various devices and sensors allowing the measurements of vertical displacement, stress, moisture content, and hydraulic conductivity. Visual observations were also made to evaluate the extent of mobilized mud pumping, including the particles migration on the gravel surface. It was shown that the permeability of compacted gravelly soils decreases rapidly with the addition of fines. Both the dynamic loading and hydraulic condition of the gravel layer are essential in the mobilization of mud pumping, and the slit caused by the detachment of concrete base from subgrade surface is recognized as the most fundamental factor for this phenomenon. The substructure becomes more susceptible to mud pumping under dynamic loading when slit width and fines content increase and ponding on the surface of gravel layer is available. The repeated erosion and scouring of subgrade surface (compacted gravel layer) driven by dynamic water pressure leads to a fully established mud pumping for ballastless track given enough load cycles.