Civil and Environmental Applications of Geosynthetics - Reinforced Soil Engineering: Advances in Research and Practice - page 6

Figure 2 Geotextile as drainage layer or filter: (a) chimney drain in earth dam; (b) drain
behind retaining wall; (c) underdrain; (d) drainage layer in tunnel.
If the geotextiles (usually nonwoven needle-punched geotextiles) are used
as a drainage layer, the in-plane permeability is considered. Because the
thickness decreases with increasing normal stress acting on it, the term
“transmissivity” is used, where the coefficient of hydraulic conductivity is
normalized by the geotextile thickness.
3.3 Large-Capacity Flow with Geonets/Geocomposites
For drainage applications (such as landfills and surface impoundments), geonets
and geocomposites are preferable to geotextiles. These are specifically
manufactured to allow for large-capacity flow. Geonets have a parallel set of
ribs overlying similar sets at various angles for drainage of fluids. Most geonets
are manufactured from polyethylene. They are laminated with geotextiles on one
or both surfaces to form drainage geocomposites (Fig. 3) . Geonets are mostly
manufactured from polyethylene; thus they have high resistance to leachate.
In geonets/geocomposites, the flow is no longer laminar, and thus Darcy's
law is invalid. The flow rate is used in lieu of transmissivity or coefficient of