Migration of fine particles from subgrade soil to the overlying ballast

Migration of fines from the subgrade soil to the overlying ballast is a significant problem in the case of some soil types. It occurs in certain types of fine grained (cohesive) subgrade soils. The consequences of migration of fines into the ballast layer are that the shear strength of ballast reduces, increasing its mobility under dynamic loading and hence reduces its capacity to support the track in terms of both plan position and level.
The most common solution include construction of a sand blanket layer between the ballast and the subgrade, designing the sub-ballast layer as filter to arrest movement of fines from the subgrade soils. Both these solutions have proven to be effective and are well established. However, construction of these layers may warrant importing large quantities of materials to site and it is time consuming to construct these layers. In order to reduce both the cost of importing materials and construction time, geotextile composites have been developed to replace the sand blanket and the sub-ballast layers. These can result in reduction in both the quantities of materials imported and construction time. Since all soils do not erode when subjected to dynamic loading, and therefore the application of geotextile should be limited to only areas where susceptible soils exit.
Finding of the study presented is evidence based analysis of erodible soils. It includes range of studies based one laboratory and field investigations. It was found that generally more dispersive soils exhibit greater propensity for migration.