30 Sep 01
(b) Capable of accommodating settlement of the underlying material.
(c) Capable of maintaining the surface slope approximately the same as the underlying soil.
c. Protective Soil Layer. The composition
of the protective
soil layer depends
on the type of
erosion control layer that will be used. When a vegetative cover is used, the lower component should
consist of topsoil overlying select fill. In the case of an armored cover, topsoil is not required and the
protective soil layer will be entirely select fill. In either case, the EPA recommends that the protective
soil layer be composed of at least 600 mm (24 in.) of soil. A layer thicker than 600 mm (24 in.) may be
required to prevent freezethaw damage to underlying layers or to increase the water storage capacity
available to plants. Guide Specification CEGS-02140 should be used in the contract documents to
specify the topsoil and select fill for a landfill cover. General guidelines for the design of these
alternatives are discussed in the following paragraphs.
(a) General. A relatively thin layer of topsoil is provided in the cover system to promote seed
germination and plant root system development. Medium-textured soils such as loam soils have the best
overall characteristics to help seeds germinate and plant roots develop. Sandy or coarse-grained soils
are often a problem because of high erodibility, low water retention, and loss of nutrients by leaching.
(b) Design Criteria. Generally, the selected topsoil should have a pH value between 6.0 and
7.5 to ensure that it is not too acidic to sustain vegetation. In addition, topsoil should contain from 5 to
20% organic matter to promote and sustain plant growth through water retention and nutrient
availability. Topsoil should be uniformly distributed and evenly spread over the select fill material to a
minimum thickness of 150 mm (6 in.) with minimal compactive effort. The topsoil should also be free of
contamination and representative of soils in the vicinity that produce heavy growths of vegetation.
(2) Select Fill.
(a) General. Below the topsoil layer or armored surfacing is the select fill layer. The purpose
of the select fill is to provide a soil that can sustain the vegetative cover through dry periods and protect
the underlying geosynthetics and clay barrier layer from human activities, animals, frost penetration,
desiccation, and maintenance equipment. The select fill also provides water-holding capacity to
attenuate rainfall infiltration to the underlying drainage layer. As with topsoil, select fill should consist of
medium-textured soils, such as loams, for both function and constructibility. Cohesionless silts and
sands are undesirable because these soils are highly erodible, do not retain water well, and nutrients are
easily leached from them. Clayey soil types are more fertile than sandy soils. However, high-plasticity
clays can be difficult to place and can damage underlying geosynthetics during placement. The best
materials are cohesive but not highly plastic and include SC (clayey sand) and CL (lean clay) as