This calculator computes the rate of leakage through defects in a composite liner, i.e. geomembrane/CCL or geomembrane/GCL. The thickness of a CCL is between 0.3 to 1.5 m whereas the thickness of a hydrated GCL  depends on the compressive stress applied during hydration. Typical values are between 5 and 10 mm; or in the order of 100 times less than the thickness of a CCL. Field evaluation, sponsered by USEPA, of leakage rate for double-lined landfills indicates that GM/GCL composite liners outperform GM/CCL liners (Othman et al.,1998.)

The rate of leakage through a geomembrane liner due to geomembrane permeability is negligible compared to the rate of leakage through defects in the geomembrane (Giroud and Bonaparte 1989.) Hence, only leakage through defects will be considered. If there is a defect in the geomembrane, the liquid first passes through the defect, then it flows laterally some distance between the geomembrane and the low-permeability soil, and, finally it infiltrates in the low permeability soil.

Flow between geomembrane and low-permeability soil is called interface flow, and is highly dependent upon the quality of contact between the two components (Bonaparte et al., 1989.) Contact conditions are defined as follows:

• Good contact conditions correspond to a geomembrane installed, with as few wrinkles as possible, on top of a low-permeability soil layer that has been adequately compacted and has a smooth surface.

• Poor contact conditions correspond to a geomembrane that has been installed with a certain number of wrinkles, and/or placed on a low-permeability soil that has not been well compacted and does not appear smooth.

The Help model provides guidance for estimating the defect densities (Schroeder et al., 1994). Some useful information on the Help model is given in the Technical Note on Using HELP Model (ver 3.07). There are mainly two types of defects, manufacturing defects and installation defects. Typical geomembranes may have about 0.5 to 1 (1 to 2 per hectare) pinholes per acre from manufacturing defects (Pinholes are defects with a diameter equal or smaller than the geomembrane thickness. The density of installation defects is a function of the quality of installation, testing, materials, surface preparation, equipment, and QA/QC program. Representative installation defect densities as a function of the quality of installation are given in Table 2 for landfills being built today with the state of the art in materials, equipment and QA/QC.

Studies by Giroud and Bonaparte (1989) have shown  that for geomembrane liners installed, with strict construction quality assurance, could have one to two defects per acre (4000 m²) with a typical defect diameter of 2 mm (i.e., a defect area of 3.14 * 10^-6 m² ).

Typical for liner performance evaluation one defect per acre (4000 m²) is considered with a defect area of 0.1 cm² (equivalent to defect diameter of 3.5 mm), for a conservative design a defect area of 1 cm² (equivalent defect diameter of 11 mm) can be considered (Giroud et al., 1994) 

Different geomembrane defect shapes will be considered:

Circular defect with diameter of d

Square defect with side length b

Infinitely long defect with width of b

Rectangular defect with width of b and length of B

Limitation of the equations presented (Giroud et al. 1997):

• If the effect is circular, the defect diameter should be no less than 0.5 mm and not greater than 25 mm. In the case of the defects that are not circular, it is proposed to use these limitations for the defect width.

• The liquid head on top of the geomembrane should be equal to or less than 3 m.