Mechanisms of Pumping at the Subgrade-Subbase Interface of Highway Pavements

Research into the mechanisms of pumping of fines at the subgrade-subbase interface of highway pavements is presented and discussed with respect to the behaviour of the subgrade-subbase system without a geotextile/transition layer and with a geotextile/transition layer at the interface. When no geotextile layer is used, softening of the subgrade soil occurs both outside the subbase particle-subgrade areas of contact due to swelling, and under the subbase particle-subgrade areas of contact due to cyclic loading. This softening causes the penetration of the subbase particles into the subgrade soil with consequent reduction of subbase layer thickness. When a geotextile layer is placed between the subbase and subgrade layers, it acts as a separator and, thus, prevents the intermixing of the two layers. However, the geotextile allows for rapid dissipation of the cyclic pore pressures under areas of contact and this causes erosion (pumping) of the subgrade surface. The geotextile layer reduces the static stress under areas of contact, but has a less significant effect on reducing the cyclic stress. Thus, the net effect is to cause an increase in the amount of pumping of fines. Geocomposites, which have a high compression modulus, high in-plane extensibility, low flexural stiffness, and ability to spread point loads from a subbase onto the subgrade, are more effective in reducing pumping. An anti-pumping geocomposite should also have a low short-term permeability to reduce the rapid dissipation of pore pressures during a loading cycle and sufficient permeability to dissipate pore pressures in the long term.