Top Seals Cap or Cover Barrier Layers

Top seals (i.e., surface seals, caps, covers), as shown in Figure 10, function to control surface water so as to minimize infiltration, maximize runoff, prevent direct contact (such as by burrowing animals, which may dig into the landfill areas), and thereby reduce leachate production

Infiltration Basin Schematic
Figure 9 Schematic drawing of a vibrating-beam slurry wall. (1) Top seal; (2) landfill material; (3) bottom seal.
Landfill Cross Section
Figure 10 Typical cross section of a solid waste landfill with a native clay top seal. (1) Surface erosion; (2) nonuniform settlement; (3) bottom erosion.

and/or contaminant transport potential. Several types of materials can be used for the top seal barrier layer, including compacted natural clay, bentonite clay, and synthetic membrane. Each of these is discussed briefly in the following.

1. Compacted Natural Clay Caps

The most cost-effective top seal barrier layer usually is one of compacted native clay material from locally available sources. If suitable native clays are not available, imported materials or synthetic membranes must be used. In general, clays are thought to last longer than synthetic materials, and, where feasible, clay caps rather than synthetic caps may be chosen. However, to avoid the "bathtub effect" (more water entering the facility than can drain out), the use of a synthetic membrane cap is required whenever the bottom liner is also a synthetic membrane [35],

Selection of clay materials for clay caps is based on the compacted hydraulic conductivity of available material as discussed by Johnson et al. [27]. As shown in Figure 10, the thickness of the clay cap is typically 0.6-0.8 m. The following are some of the factors to be considered in the design of a clay barrier layer.

1. The top several inches of clay cannot be as well compacted as the remainder of the thickness owing to the lack of confinement. Further, it may be difficult, in the long term, to maintain the clay density in the top few inches due to potential desiccation cracking, wet-dry cycles, and freeze-thaw cycles.

2. The bottom of the clay cap barrier layer may become somewhat intermixed with subgrade material during construction. Therefore, the effective thickness of a nominal 0.6-m cap is likely to be less than 0.6 m.

3. The compaction procedures for the clay barrier layers are, in many ways, similar to those followed for standard compacted fills as discussed by Hilf [36].

4. Protection of the clay barrier layer from degradation due to erosion by surface water runoff, cracking due to drying, rutting from moving vehicles, and penetration by tree roots and other vegetation must be provided. Some of these factors will be further discussed in Section 4 below.

2. Bentonite Clay Caps

Where local natural clay is not available in sufficient quantity or at an acceptable quality or price, processed clay is a common alternative. Processed clay is typically bentonite from sodium montmorillonitic clay deposits. Bentonite clay is a strongly hydrophilic colloidal clay that swells (expands) in water. The construction of a processed clay cap requires the application of the bentonite in powdered or granular form at a controlled rate (e.g., about 2 lb/ft2) followed by adequate mixing with the in-place soil to a predetermined loose thickness. The mixture of soil and bentonite is then compacted. The main advantages of this method are the low cost coupled with relatively low hydraulic conductivity. The disadvantages are (1) it is difficult to obtain uniform application and blending and (2) bentonite clay swelling is very sensitive to pore fluid chemistry. Thus if contaminated water permeates the bentonite clay barrier layer, significant increases in the hydraulic conductivity may occur.

3. Synthetic Membrane Caps

Synthetic membranes (Figure 11) can be used as the barrier layer in covers for waste containment [37,38]. The principal advantages of geosynthetics are their low hydraulic conductivity and their ability to deform and remain intact. Thus geosynthetics can often accommodate the large total and differential settlements that may occur in the landfill beneath the cover layer.

Polymeric Membrane Seal

Polymeric Membrane Seal

Regraded Fill

Figure 11 Typical cross section of a waste control system with a polymeric membrane seal. (1) Surface erosion; (2) nonuniform settlement; (3) bottom erosion.

Regraded Fill

Divers ion Ditch

Figure 11 Typical cross section of a waste control system with a polymeric membrane seal. (1) Surface erosion; (2) nonuniform settlement; (3) bottom erosion.

Membranes are available in a wide range of materials from numerous manufacturers. Additional information regarding geosynthetic materials is provided later in this chapter.

4. Other Surface Controls

In addition to surface sealing with barrier layers, surface water diversion and collection systems can provide short- and long-term measures to isolate waste disposal sites from surface water inputs. Techniques used to control flooding and off-site erosion transport of cover and surface seal materials include dikes and berms, interceptor ditches, diversion dikes and berms, terraces and benches, sheets and downpipes, levees, seepage ditches, and sedimentation basins and ponds [39].

In summary, surface seals provide multiple functions in the overall liquid and solid waste containment control system. Their main function has been discussed as the control of infiltration by minimizing water infiltration and/or maximizing surface runoff.

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  • Martha
    What seals the top of the landfill?
    2 years ago

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