A small amount of slurry can considerably reduce ballast life.
Subgrade pumping has always been a problem in ballasted track, particularly on weakly cemented mudstones or over consolidated clays. These soils have a high shear strength and as such do not need a deep trackbed to support track loading. If unprotected though, the upper surface degrades easily to a slurry when directly exposed to water.
The open texture of ballast allows free water to come into contact with the exposed formation. When the exposed surface contains fine grained particles, these can be readily eroded by the water accumulating in the voids, forming weak, highly mobile slurry. This slurry is then ‘pumped’ up into the overlying ballast by each passing axle load. Contamination of the clean ballast layer by the fine soil particles in the slurry very quickly reduces the load-bearing properties of the ballast and leads to loss of track alignment in the affected area.
Mud pumping results in ballast failure, that weakens the track modulus, and consequentially reduces the bearing capacity. Under extreme conditions the ballast will become unmaintainable within a very short time post-installation.