# Capacity for drainage geocomposite (GCD) underneath a layer of soil

## Calculation method

The minimum drainage capacity requirement for a drainage geocomposite (GCD) are calculated for the conditions of a given slope angle (β), the normal component of the load (e.g. saturated soil layer) and the inflow of water through the soil layer. The in-plane transmissivity of the soil layer itselve is not taken into account for calculation of the total drainage capacity, but can be considered as an additional safety factor in case its permeability coefficient (k) is relatively high (e.g. sandy soil).

## Styles of drainage geocomposites

Texions’ drainage geocomposites (GCD’s) have a high in-plane permeability. The transversally permeable drainage geocomposite consists of an in-plane drainage core made from a three-dimensional structure (jumbled structure of threads, a geonet, a cuspated structure) with a geotextile filter fabric (GTX) attached to one or to both sides. When installed on a slope on an impermeable geomembrane (GMB), the drainage geocomposite is placed underneath the reinforcement geogrid (or geotextile).

## The hydraulic gradient (HG)

The hydraulic gradient (HG) is the height of water (head height H) divided by the horizontaly measured distance (l) between the points where the highest and lowest heads on the drainage geocomposite (GCD) are considered. Therefore HG = 0.1 for example means a head height of 3m with a GCD horizontaly projected distance of 30m. A 45° slope is the equivalent to a HG = 1.0, so the HG is the Tan(β) of the slope angle and as the head height increases, it is possible to get HG's above 1. The way to calculate the slope from the HG is to use the inverse Tan of the HG (Tan^{-1} as it is on most calculators), e.g. Tan^{-1} (1.0) = 45°, Tan^{-1} (0.5) = 26.60°, Tan^{-1}(0.1) = 5.70°