Just as a bathtub has a drain to remove bath water, the landfill has a mechanism to remove leachate. The liner in Figure 10.13.3(a) is graded to direct any leachate reaching the liner surface into a leachate collection system. Liner systems are not leak proof. Collecting leachate and removing it from the landfill reduces the hydraulic head on the liner, thus reducing fluid flow through the liner. To
Protective Layer (Soil)
Drainage Layer (Sand, tires, or geonet)
Barrier Layer — Clay and Geomembrane (Keeps water out, directs gases toward venting and collection system)
FIG. 1G.13.4 Liner system. ©1999 CRC Press LLC
speed the lateral flow of leachate once it reaches the bottom of the landfill, a drainage layer is placed over the composite liner (see Figure 10.13.4). The drainage layer can be made of coarse media such as sand or shredded tires, though geonets (high-strength geosynthetic grids less than Ate in thick capable of transmitting high quantities of water) are also common. Geotextiles minimize clogging of the drainage layers by excluding particles. Drainage layers slope toward collection pipes, which direct leachate toward a sump or directly out of the landfill.
Figure 10.13.5 shows a typical leachate collection pipe cross section. The leachate collection pipe is laid in a gravel trench wrapped with a geotextile which allows water to enter the leachate trench but keeps out small particles that could clog the gravel or pipe. Leachate collection trenches lay on top of the liner and travel along local hydraulic low points. The leachate collection system carries leachate out of the landfill cell through the liner or dumps leachate into a sump which is pumped over the side of the liner.
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