LAUSR

The study of particle migration in porous media under cyclic loading is the key to understand the mechanism of mud pumping hazard in railway embankments. This paper presents a series of particle migration tests, in which soil particles migrate into an overlying gravel layer under cyclic loading. The results show that the increase in loading frequency and load magnitude leads to more particle migration upwards at a greater rate, implying that the train speed and axle loads affect the extent of mud pumping. The slurry turbidity in the gravel layer increases to a steady state value with time. Soil particles smaller than 5 μm have the potential to diffuse into the entire gravel layer, and larger particles tend to aggregate in the bottom layer of the gravel. The backward erosion gradually develops deeper into the soil layer, and there is a maximum erosion depth associated with each load frequency and load magnitude. As for the mechanism, the pore water pressure oscillates because of liquid sloshing. Its amplitude is much larger in the gravel layer than that in the soil layer due to their difference in permeability. The axial hydraulic gradient acts as a pumping effect to stimulate the migration of soil particles. Increasing load frequency is conducive to the generation of a stronger pumping effect at the gravel–soil interface. Increasing load magnitude does impact not only the extent of pumping effect, but also the development of an interlayer which plays an important role in promoting particle migration.