30 Sep 01
(1) General. The primary function of a drainage layer in a landfill cover is to intercept water that
percolates through the top layer of the cover and drain it laterally to a collection system and away from
the cover. Adequate cover soil drainage is necessary to reduce hydraulic head on the underlying barrier
layer and to reduce seepage induced slope failures. It is generally recognized that most cover system
slides and failures are caused by inadequate drainage layer system design. The drainage layer in a
landfill cap is usually constructed with either geosynthetic drainage products or granular materials. A
complex drainage system might incorporate a series of collectors, laterals, and main lines to direct and
discharge water from the drainage system.
Figure B -2. Water movement in a cover.
(2) Design Flow Rate. Figure B-2 depicts water movement on and through a landfill cover
system. For each precipitation event, some water moves off the cover as runoff and some infiltrates the
surface. Of the water that infiltrates, some is stored in the cover soils where a portion will return to the
atmosphere via evapotranspiration, while some will infiltrate down to the barrier layer and then move
laterally through the drainage layer system. The drainage layer should be designed to minimize head
build-up in the overlying cover soils. The drainage layer flow capacitySoilqKCiS)ements may be determined
Cover re (u r
by several methods, some of which are described below.
Drainage Layer (KD)
d = max depth of flow