30 Sep 01
(d) Overlaps. Because of the great variation among geosynthetic drainage materials,
manufacturer's recommendations regarding overlap requirements should be followed. In general,
geonets and geocomposite drainage layers should be placed parallel to the line of slope and not across
it. Care should be taken to ensure that all flow remains in the drainage layer and that head does not
build-up in the overlying cover soils. For this reason, the flow capacity of any end seams on drainage
slopes should be considered. Geonet to geonet contact should be ensured at all geocomposite end
seam connections. End seams are not recommended on slopes steeper than 4 horizontal on 1 vertical.
(4) Granular Drains. Granular materials can also be used to construct the drainage layer. The
design requires choosing an appropriate drainage material, layer thickness, and location of lateral drains.
A designer should consider the following guidelines when designing a granular drainage layer.
(a) Thickness. A minimum thickness of 300 mm (12 in.) and minimum slope of 3% at the
bottom of the layer should be used. A greater thickness or slope, or both, may be necessary to provide
ample drainage flow.
(b) Gradation. Granular material should have a minimum of fines that can migrate and collect
in downslope areas, thus clogging the outlet. The hydraulic conductivity of drainage material should be
no less than 1 102 cm/s; however, higher hydraulic conductivities may be required to provide
adequate flow capacity. If large (> 9.5 mm [0.375 in.]) or angular materials are used for a drainage
layer, the underlying geosynthetics should be protected with cushion layer materials. Details regarding
cushion layer design are given in Design Methodology for the Puncture Protection of
Geomembranes (Wilson-Fahmy et al. 1996).
g. Drainage Collection and Removal System.
(1) General. Once the drainage layer has been designed, the next step is to design a drainage
collection and removal system to allow collected water to exit the cover. It is essential that the
collection and removal system be adequately sized so that water does not "build up" in the drainage
layer of the landfill cover. Water collected from the drainage layer may be released through a variety of
methods; however, perimeter toe drains, perforated pipe collectors, or a combination of the two are
most commonly used.
(2) Design Criteria. The outlet drainage system must be capable of removing the maximum flow
from the cover drainage layer. Often, the lateral drainage layer simply drains freely at the toe of the
cover into a perimeter ditch or swale. However, if the landfill cover is relatively large with long drainage
slopes, it may be necessary to install additional drainage collection pipes at some regular spacing
(typically every 150 to 200 ft) to prevent exceeding the capacity of the drainage layer (see Figure B-3).
These pipes are then routed via gravity flow to the perimeter of the landfill cover and discharged to
drain out and away from the landfill. Manning's formula for open channel flow can be used for sizing
these pipes. A 150-mm (6-in.) diameter pipe is typically the minimum size used for drainage systems to
allow for easy maintenance. In addition, cleanout risers are recommended at a spacing not to exceed