Phenomenon of Mud Pumping in Rail Tracks

Fundamental Concepts and Practical Implications

Distinguished Professor Buddhima Indraratna

Over the years, rapid urbanization has led to an increase in the demand for faster and sustainable heavy-haul rail service worldwide. With increasing axle loads of trains, the subgrade foundation experiences higher cyclic excess pore water pressures. At high axle load exceeding say 30 tonnes, the yielding of soft subgrade soil can occur, causing significant softening and undrained failure. In certain cases, fluidization of the soil (slurry) may occur, especially if the water content approaches the liquid limit of the soil, and the fluidized soil will pump-up rapidly (hence the term, mud pumping) towards the overlying substructure, adversely affecting the shear strength and permeability. While this phenomenon is commonly reported beneath heavy haul tracks, the underlying mechanism of subgrade fluidization under cyclic loads has not been given adequate attention. In this presentation, some critical studies investigating the mechanisms of mud pumping will be presented, including: (i) an experimental programme to investigate the mechanisms of subgrade fluidization; (ii) modelling mud pumping based on the novel approach CFD/LBM-DEM coupling; and (iii) site investigations and testing. The test results indicate that under adverse combination of cyclic loads, the excess pore water pressure increases rapidly in low-medium plasticity soils that are more prone to mud pumping. It is accompanied by a redistribution of moisture along with the depth of the specimen, where the water content increases towards the top layers while the bottom layers tend to densify (cyclic consolidation). When the moisture content reaches the liquid limit of the subgrade, the specimen transforms to a fluid-like state with a dramatic reduction in stiffness. Advanced computer simulations demonstrate that increased hydraulic gradients cause a reduction of inter-particle contact areas thus destabilizing the soil fabric. The presentation will also show some ongoing efforts towards deeper understanding of this complex geo-hydrodynamic phenomenon of mud pumping through extensive site investigations and laboratory-based testing.

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