Mechanisms of mud pumping and the effect of the fines content

Rail transport is one of the most efficient modes of transportation in terms of its technology and the economical way. It carries passengers and commodities around the world. The rapid development of urbanisation necessitates railways with higher permissible speeds and axle loads which will be widely constructed, even on soft compressible soil areas. Railways passing through saturated subgrade, particularly in coastal areas, frequently experience a severe loss of bearing capacity and excessive deformation. Subgrade mud pumping is one of the problems created as cohesive fines turn into the slurry and migrate upward to the overlying layer due to excessive cyclic pore pressure. This instability can cause track misalignment and in extreme cases, potential derailment. It is therefore, necessary to understand the mechanism of this phenomenon and how the properties of soft subgrade such as plasticity, as well as the void ratio and loading condition can influence its response to cyclic loading. In this study field investigations were carried out along the south coast railway line where mud pumping is a frequent occurrence. Samples were taken from the substructure to evaluate the characteristics of this problematic material. The laboratory results indicated that low plasticity soil is more susceptible to fluidisation. The fouling indices of different layers were discussed. A series of undrained cyclic triaxial tests were carried out to examine the cyclic response of subgrade. An artificial cohesive fine soil called Kaolin was used to investigate how different amounts of this material can alter the cyclic behaviour of subgrade. Samples of this natural subgrade and mixtures with 10% and 30% of Kaolin added were tested under undrained cyclic conditions. In these tests the cyclic stress ratio (CSR) varied from 0.2 to 1.0 at frequencies (f = 1 and 5 Hz), a low initial confining pressure of 20 kPa was applied to reproduce field conditions at shallow depths. The response of these samples after being tested were categorised as, (i) stable, (ii) fluidised, and (iii) cyclic undrained failure; all of which depended on the level of CSR applied. In the sample which experienced fluidisation, the shear stress decreased rapidly in the early stages, followed by a rapid increase in excess pore pressure and deformation. When Kaolin was added to the soil, the critical number of cycles increased and its resistance to fluidisation improved. Those samples with a smaller initial void ratio can withstand larger cyclic stress at a higher number of cycles, so they are less susceptible to fluidisation. There was also a large redistribution of water over the height of the test specimens, which fluidised due to cyclic loading. Th