Influence of Holes in Geomembranes

Theoretical description and calculations of the influence of flaws and the implementation and interpretation of laboratory control experiments are always based on simplifying models. In the following, an overview will be given about the concepts. It should in particular become clear 

The flow processes, however, also depend on the interface between the geomembrane and its subgrade and on how intimate the contact is. The influence of overburden above the geomembrane on the hydraulic potential is neglected, their weight, however, very strongly influences this contact and interface properties. The load on the geomembrane is therefore another very important parameter. 

The task is to determine the volumetric flow rate Q for specified parameters, i.e. the volume of water flowing per imit time through the hole in a steady-state condition. Q will simply be called flow rate or leakage rate in the following. A volumetric flow density v is obtained when Q is referred to the liner sinface. 

If the water can flow off under the geomembrane freely then the flow through the hole in the geomembrane can be calculated using Bernoulli s law  

C is a geometry factor which depends on how sharp edged the boundary region of the hole is. Its value is C = 0.6 for a sharp edge and C = 0.99 for a strongly rounded-off edge (Richter 1962). As usual, g is the acceleration due to gravity. 

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Holes in the geomembrane can have a considerable influence on mass transport depending on the properties of the subgrade. However, different views exist as to how water flow through a hole should be correctly modelled. 

Under common field conditions the parameters will have the following typical values A some centimetres, I up to some tens of metres. Values >0.1 mm are assumed for the radius R of the hole. An evaluation of Eq. 7.62 and comparison with Eq. 7.37 with this parameter selection shows that with accumulated water above the geomembrane sufficient water will flow through the hole into the tube formed by the residual wave to completely fill the cavity with water and that the same hydrostatic pressure will develop everywhere. Therefore Eq. 7.50 can be directly applied for the influence of flaws within the range of residual waves, where A characterises the spatial dimension of the cavity cross-section under the wave and / is the length of the wave. Only for perforation sizes of < 0.1 mm is the flow resistance of the hole the decisive quantity, in which case Eq. 7.37 must be used. 

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