DESIGN METHODS & CONCEPTS
The main reason for including a Leakage Detection System (LDS) in a landfill is to capture and to laterally convey liquid entering due to leakage of a primary liner. Figure 1 presents a schematic of a leak detection system, indicating the primary source of leakage. There can be additional sources of leakage into the LDS including (i) construction and compression water already present in the LDS; (ii) consolidation water from the upper compacted clay liner (if a compacted clay liner is present); (iii) infiltration water from leaks in the lower geomembrane; and (iv) liquid flow from leakage of pipes penetrating the LDS. This manual considers leakage through the upper liner as the primary, and only source of liquid in the LDS.
In general, leakage from a hole in a geomembrane depends on: (i) hydraulic head; (ii) the size and shape of the hole; and (iii) the nature of the media underlying and overlying the primary liner. For a composite liner system typically used in landfills, the underlying medium is a compacted clay liner or a geosynthetic clay liner. The material overlying a primary liner is of a permeable nature, such as waste or a drainage medium. For the case of composite primary liner with circular defect, the leakage rate can be calculated as [Giroud, 1997]:
n × 0.976 × C
1 + 0.1 ×
× d × h × k
C = contact factor (dimensionless)
Q = leakage rate (m /sec)
h = head of water over the geomembrane (m)
n = number of defects per considered geomembrane area (A)
A = Considered geomembrane area (m )
k = hydraulic conductivity of the soil component (m/sec)
t = thickness of soil or GCL (m)
Equation 6.1 is valid only with the units presented above and the given definition of the variables. Giroud  recommends using a value of 0.21 or 1.15 for the contact factor, C, for good or bad contact, respectively, as described below in general terms:
FIGURE 6.1 – TYPICAL CROSS-SECTION OF LEAK DETECTION SYSTEM IN LANDFILLS
The good contact condition corresponds 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.
The poor contact condition corresponds 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.
Additionally, for the leakage equation to be valid, the hydraulic conductivity of the soil underlying the geomembrane must be “low” generally less than 10 cm/s (10 m/s). The range of applicability for the leakage equation is affected by the head and diameter of the assumed defect. More discussion can be found in Giroud .
Input parameter values
Considered geomembrane area
Hydraulic head on top of geomembrane
Thickness of CCL or GCL component
Permeability of the CCL or GCL component
Contact quality (good or poor)
Number of effects per considered geomembrane area(A)
Diameter of the circular defects
Contact quality factor
Leakage rate through the considered geomembrane defect
gallons per acre per day
liter per hectare per day
J.P. Giroud,"Equations for Calculating the Rate of Liquid Migration Through Composite Liners Due to Geomembrane Defects", Geosynthetics International, Vol. 4, Nos. 34, pp.335-348, 1997