Effect of flow boundaries on two physical characteristics of geosynthetic drains

This paper shows that two physical characteristics of geosynthetic drains can be significantly affected by the flow boundaries: the specific surface area and the average flow path diameter. These two physical characteristics are important, because they have a significant influence on the hydraulic characteristics of geosynthetic drains. A theoretical analysis leads to a quantification of the effect of smooth and rigid flow boundaries on the specific surface area and the average flow path diameter. Numerical calculations performed using the equations proposed in this paper show that the effect of these flow boundaries is negligible in cases where the geosynthetic drain consists of a needle-punched nonwoven geotextile. In contrast, the effect of smooth and rigid flow boundaries on geonet drains is usually significant. The fact that even boundaries that are smooth and rigid have a significant influence on the measured hydraulic transmissivity of geonets supports the usual recommendation that, to measure a representative geonet transmissivity in the laboratory, flow boundaries that are representative of the field conditions must be used. For the sake of comparison, the methodology is also applied to granular drains. It is shown that the effect of flow boundaries is generally negligible in the case of granular (sand and gravel) drains. More generally, it is shown that the effect of flow boundaries that are smooth and rigid is negligible when the drain thickness is at least 20 times the size of the solid constituents of the drainage material, such as the diameter of the granular particles, or the diameter of geotextile fibers and geonet ribs.