Myths & Facts #11 How different behaviors are between filter performances?
Soil filtration process involves complex interaction between the filter and the contiguous soil. Under the action of driving forces induced by seepage flow toward the filter (and the drain), soil particle movement and relocation lead to changes in grain size distribution, porosity and permeability within both the soil and the filter. Several mechanisms have been identified as piping, bridging, blinding, blocking (or plugging) and clogging (Rollin and Lombard, 1988, Lafleur, 1999). These three mechanisms are conceptually represented in Figure.
Piping?is a typical consequence of soil internal erosion. Because a large fraction of soil particles is much smaller than the filter openings, they cannot be retained above the filter. As a result, the fine fraction disappears from the grain size distribution. In the affected zone, the soil porosity as well as its hydraulic conductivity increase dramatically and quasi-uniformly.
Bridging?is a mechanism by which the soil forms a self-filtration pore structure at the interface with the filter. In this case, fine particles smaller than the filter openings are lost only within a thin layer above the filter. Then, coarser particles arching over the geotextile openings prevent the erosion process from extending beyond the self-filtration zone. Eventually particle migration is sustained and a state of equilibrium is reached where only the porosity and hydraulic conductivity of the interface zone have been locally increased as compared to the initial state. In consequence, the system average hydraulic conductivity increases slightly and stabilizes at a value, intermediate between the soil initial permeability and that of the geotextile.?
Blinding?occurs when fine particles migrating from upstream soil are retained and accumulate at the filter interface. As porosity of the interface zone decreases and flow conduits are eventually blocked, hydraulic conductivity increases locally in the zone from where the fines originated but decreases at the filter interface. As a result, the system average permeability may decrease steadily without a satisfying equilibrium being reached.