Fin drains or drainage screens are also longitudinal drains, manufactured from composite materials. Their essential make-up is of two geotextile faces that provide a filter function between the surrounding ground and a rigid plastic core that is sandwiched between the geotextile faces — see Fig. 13.16c. The so-called “drainage core” is, typically, formed of a high-density polyethylene, HDPE, structure. Often this feeds into an integral collector at the bottom. The core permits the water to flow in the plane of the geocomposite (compared with most simple geosynthetics
in which only cross-flow can readily take place). These drains are usually placed at the pavement’s edge, allowing the collector of percolating water (Fig. 13.16a & b). Their main purpose is not to lower the groundwater level. The advantage of fin drains is their narrow thickness, allowing the construction of narrow trenches. This is particularly advantageous in improvement works. The disadvantage is that they are less able to carry high volumes of water and, thus, are less suitable where groundwater lowering of permeable soils is to be attempted.
A fin drain is normally supplied in the form of a roll of constant width. Thus, its height in the ground is constant, too. Care must, therefore, be taken that the base of the slot in which the drain is to be placed will fall evenly towards an outlet rather than following the vertical profile of the pavement alongside which it lies (Highways Agency, 2006).
