The development of pore water pressure at the subgrade-subbase interface of a highway pavement and its effect on pumping of fines

The migration of fines from a subgrade soil into the overlying granular layer of highway or railway pavements in the presence of water depends mainly on the pore water pressures developed at the interface between the subgrade and Subbase/ballast layers. A new laboratory test was developed which enabled the measurement of the mean and cyclic pore water pressures at the centre of the contact area of a single subbase particle with the subgrade soil. A finite element analysis was carried out to estimate the pore pressure distribution under the contact area and thus the hydraulic gradients under different interface conditions. It was found that the placement of a geotextile layer at the subgrade subbase reduces the penetration of the Subbase particles into the subgrade soil and reduces the mean pore water pressure under the subbase particle. However, the geotextile was found to allow for quick dissipation of the cyclic pore water pressure, thereby causing erosion of the subgrade surface and migration of fines with water across the geotextile into the subbase layer. Dissipation of cyclic pore pressure within the time of a loading cycle was found to increase with the increase of permeability, decrease of thickness and increase of compressibility of the geotextile layer.