30 Sep 01
iterative process. Grading plans are normally developed for the random fill, clay barrier, and topsoil
layers. The grading plans should be well defined by horizontal and vertical control such that the cover
grades can be easily staked in the field. The final slopes must reflect minimum grade requirements after
settlement and must accommodate both internal and surface drainage requirements. The final slopes
must also reflect stability considerations. In general, minimum slopes after settlement should be greater
than 3% to maintain surface drainage and maximum slopes should not exceed 4H:1V to assure cover
stability and safe operation of maintenance equipment. To ensure that the various layers meet the design
grades, each layer must be surveyed after construction.
g. Leachate Control. Leachate seeps exiting from the landfill surface need to be identified and
located during pre-design and construction regrading activities. A leachate collection blanket, consisting
of either granular fill or a geonet coupled with a conveyance pipe and outlet, is sometimes required to
control these seeps. Uncontrolled leachate seeps can cause a build-up of hydrostatic pressure behind
the low-permeability layer, resulting in decreased stability of the cover system. After the leachate is
collected, it must be stored and ultimately treated, either on or off the site.
h. Cover Surface Runoff and Erosion Control Requirements.
(1) General. Soil erosion is a natural process when soil particles are displaced and carried away
by water, wind, or physical disturbance. The rate at which erosion occurs depends on the properties of
soil, terrain, climate, rainfall intensity and duration, and the volume and characteristics of the water flow.
Erosion is likely to occur at any area where flows are concentrated. Erosion commonly occurs on
slopes greater than 50-m (150-ft) or less depending on the percent slope and soil type, the outer banks
of curved channels, at culvert outlets or inlets and other areas of high flow concentration. Erosion
control features should be designed that are simple to construct and are effective in their operation.
After drainage is directed off of the cover, perimeter drainage features, such as ditches, gabions, or
storm sewers, are required to carry the water away from the toe of the landfill.
(2) Design Criteria.
(a) Revised Universal Soil Loss Equation. The USDA Natural Resource Conservation
Service developed the Revised Universal Soil Loss Equation for use in determining terrace requirements
for cropland on slopes. The equation has also been used for landfills to determine the maximum vertical
spacing between terraces and to estimate sediment transport into ditches and detention ponds.
Terraces and other permanent erosion control measures for landfill covers should be designed such that
soil erosion is no greater than 0.45 kg/m2 (2 tons/acre) per year. Some states have developed their
own criteria for terrace sizes and spacing. More information on the Revised Universal Soil Loss
Predicting Soil Erosion by Water: A Guide to Conservation Planning With the Revised Universal
Soil Loss Equation (RUSLE) (USDA 1997).
(b) Terraces. Terraces are used to reduce erosion, reduce sediment content in runoff water,