The rate of flow (flux – Q) per unit area of liquid through a liner is a function of the hydraulic conductivity (k) and the hydraulic head (i), and can be approximated by applying the following equation, attributed to Darcy:
In the case of vertical flow under gravity, the hydraulic head (i) is defined as the height of liquid standing over the liner divided by the thickness of the liner. If the liner is perfectly drained and no leachate accumulates above it, but the liner remains saturated, then the hydraulic gradient is 1.0 and the flux of liquid is independent of the thickness of the liner. However, if a layer of liquid is allowed to accumulate above the liner, the head (i) is greater than 1.0 and the thickness of the lining layer exerts an influence on the flux.
European Community requirements, and many other national standards, specify a maximum hydraulic conductivity of 1 x 10^-9 m s” and the Table below shows the maximum permissible leakages assuming either perfect drainage or a 1 metre head of leachate above the liner.
Synthetic and composite liners.
Synthetic lining membranes possess hydraulic conductivities of between 1 x 10^-15 and 1 x 10^-16 m per second, and small samples of the materials may be considered essentially impermeable. The materials are laid as strips from rolls, or as sheets, with joining being achieved by on-site seam welding.
Quality Assurance checking during the installation of membranes is a standard procedure. Nevertheless, some flaws may be anticipated and the American Society of Civil Engineers suggests that an achievable performance level for such liners is a seepage loss rate of 200 litres per hectare per day (Bonaparte and Goss 1991).
A leakage rate of 200 litre per hectare per day is equivalent to an annual flux of 7 mm, for natural clay or amended soil liners.