CHAPTER 2. LITERATURE REVIEW AND SYNTHESIS
2.2. Mechanism of Geotextile Filtration and Physical Clogging
Soil filtration by geotextiles involves complex interaction between the filter and the contiguous soil. Under the action of seepage forces induced by groundwater 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,
2 The filtration opening size (FOS) of a geotextile is similar in concept to the apparent opening size (AOS) but is determined by wet hydrodynamic sieving (see the ISO/DIS 12956 test standard) which is a method more representative of field conditions than the dry sieving method used for the AOS.
blinding, blocking (or plugging) and clogging (Rollin and Lombard, 1988, Lafleur, 1999). The first three are conceptually represented in Figure 2.1.
Piping is a typical case of soil internal erosion. Because a large fraction of soil particles is much smaller than the filter openings, they cannot be retained. 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 structure at the interface with the geotextile. In this case, fine particles smaller than the geotextile openings are lost only within a thin layer in contact with the filter. Then, coarser particles arching over the geotextile openings prevent the process to extend beyond the interface zone. Eventually particle migration is contained 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 a distance are retained and accumulate in the interface zone close to the geotextile. As porosity in the interface zone decreases and flow conduits are filled, hydraulic conductivity increases locally in the zone from where the fines originated but decreases in the interface zone with the geotextile. As a result, the system average permeability may decrease steadily without a satisfying equilibrium being reached.
The other two mechanisms, blocking and clogging, involve more locally or internally the geotextile.
Figure 2.1 Piping(a) , bridging(b) and blinding (c) mechanisms associated with different geotextile opening size and soil behaviors (after Lafleur, 1999) – Left hand side: soil grain size distribution (GSD) and its variation in the vicinity of the geotextile ( doted curve : initial GSD; plain curve : final GSD ; RR=Of/di ; Of :filter opening size; di : indicative particle size of protected soil ) – Center-left: schematics of resulting granular structure – Center-right: profile of resulting soil hydraulic conductivity in function of
distance to geotextile (kB : initial soil hydraulic conductivity (dotted line)) – Right-hand side: evolution of system average hydraulic conductivity in function of time, as compared to kF (virgin hydraulic conductivity of geotextile).
In the case of blocking, coarse particles directly in contact with the
geotextile surface obstruct the filter openings, preventing fine particles as well as fluid to penetrate.
Internal clogging, instead, occurs when migrating fine particles penetrate the filter fabric and encounter fiber constrictions too narrow for traveling farther. Fines can then accumulate within the geotextile and obstruct its drainage channels.
In practice, the terminology of clogging is often extended to designate not only internal clogging of the geotextile but blocking and blinding as well (Rollin and Lombard, 1988). Another form of geotextile blocking, by fine particles instead of coarse ones, can also be observed in situations where fine pumping due to pulsing of excess pore pressure takes place. This could be the case in roadway or railway construction, for instance, when a geotextile is used as separator between aggregate base course or ballast and soft saturated silt subgrade (Alobaidi and Hoare, 1999). In the present study, this particular mechanism will be referred to as plugging in order to avoid confusion with the classical case where blocking is caused by coarse particles larger than the filter opening size.
It is noted that, of the five mechanisms described above, only bridging can be considered a highly desirable condition, all the other leading to either sediment being transported to the drains (piping) or the system hydraulic conductivity being possibly decreased down to a level insufficient for adequate drainage (blinding, blocking and clogging). For well graded soils, geotextile blocking or internal clogging are usually considered the most sever problems and more investigation of these types of filter failure is needed for various of soil and geotextile conditions.
The time required for physical clogging to stabilize in a particular situation varies with the hydraulic gradient magnitude: the greater the gradient, the faster the process. In the laboratory this often takes up to 1,000 hours when the gradient
ratio test is used (Rollin and Lombard, 1988; Bhatia et al, 1995, Bhatia et al, 1998). As will be seen later, reliance on such long duration laboratory tests is due, to some extend, to the current lack of a general theory to integrating the various filtration mechanisms altogether.