Drainage system with unitary void-maintaining geosynthetic structure and method for constructing system

Unitary void-maintaining geocomposites, and drainage systems utilizing those geocomposites, are provided for location below a road surface or below a large structure such as a building, retaining wall or parking lot. In some embodiments, the geocomposites include at least one fluid-transmissible layer, preferably a geotextile, attached adjacent an upper or lower surface, or both surfaces, of a polymer-based core element. In other embodiments, the core element is constructed such that no geotextile layer is needed. Characteristic of the core element is high transmissivity, that is, a high rate of horizontal flow of gases or liquids through the core. Characteristic of the at least one fluid-transmissible layer is high permittivity, that is, a high rate of vertical transmission of liquids and gases through the geotextile layer and into the core element. Also characteristic of fluid-transmissible layers of the invention is high exclusivity with respect to solid materials. Core elements and fluid-transmissible layers can be formed simultaneously as a unit, or the several components may be bonded to one another by welding or adhesives. The systems include further drain means that communicate with the geocomposite structures of the system, such as perforated pipes and the like, which can connect further to culverts, ditches or waterways. The geocomposite structures and drain means may be placed below the subgrade, over the subgrade, or at the top and bottom of the base underlying a roadway or large structure. Unitary void-maintaining geocomposites can form part of a greater subsurface geosynthetic system that provides reinforcement, separation, and drainage for the structure to thereby extend its useful life and decrease maintenance costs.