Why We Place Caps on Old Landfills and How It Is Done
Landfill Problems - A Landfill Site Anatomy
This article applies to all typical RCRA Subtitle C Landfill Cap Systems, and is an educational guide only read the USEPA guides before taking any action.
The placing of a sealing layer from rain over a landfill is usually done to:
* Reduce exposure on the surface of the waste landfill.
* Avert vertical penetration of water into wastes that would produce contaminated leachate.
* Confine waste while treatment is being applied.
* Keep in check gas emissions from underlying waste.
* Give rise to a soil surface that can support plant life and/or exist for added purposes.
Landfill Capping is the most widespread form of remediation as it is in general less costly than other technologies and in effect manages the human being and ecological risks coupled with a remediation location.
The strategy of landfill caps is location specific plus depends resting on the intended functions of the scheme. Landfill Caps can stretch from a one-layer system of vegetated top soil to a multifaceted multi-layer method of soils and geosynthetics. In general, a lesser amount of complex systems are necessary in arid climates and more intricate systems are required in damp climates. The material used in the building of landfill caps involve low-permeability and high-permeability soils and low-permeability geosynthetic products. The low-permeability materials divert water and avoid its passage into the garbage. The high permeability materials convey water away that percolates into the cap. Other materials can be used to enhance slope stability.
The most vital components of a landfill cap are the barrier layer and the drainage layer. The barrier layer can be low-permeability soil (clay) and/or geosynthetic clay liners (GCLs). A flexible geomembrane liner is placed resting on the barrier layer. Geomembranes are as a rule supplied in bulky rolls and are available in quite a few thickness (20 to 140 mil), widths (15 to 100 ft), and lengths (180 to 840 ft). The contender list of polymers commonly used is extensive, which includes polyvinyl chloride (PVC), polyethylenes of a range of densities, reinforced chlorosulfonated polyethylene (CSPE-R), polypropylene, ethylene interpolymer alloy (EIA), and several newcomers. Soils used as barrier materials in the main are clays that are flattened to a hydraulic conductivity no greater than 1 x 10-6 cm/sec. Compacted soil barriers are usually installed in 6-inch minimum lifts to accomplish a thickness of 2 feet or more. A composite barrier uses together soil and a geomembrane, taking advantage of the properties of each one. The geomembrane is in effect impermeable, but, if it develops a leakage, the soil component prevents noteworthy escape into the underlying trash.
For facilities over putrescible wastes, the gathering and control of methane and carbon dioxide, powerful greenhouse gases, must be part of facility design and operation.
Concrete Cap/Asphalt
The most effective single-layer caps are composed of concrete or bituminous asphalt. It is used to construct a surface barrier sandwiched between landfill and the environment. An asphalt concrete cap would lessen leaching through the landfill into an aquifer.
Subtitle C Capping Layer
The RCRA C multilayered landfill cap is a baseline design that is recommended for use in RCRA hazardous waste applications. These caps generally consist of an upper vegetative (topsoil) layer, a drainage layer, and a low permeability layer which consists of a synthetic lining over 2 feet of compacted clay. The compacted clay liners are effective if they hold on to a specific dampness content but are inclined to cracking if the clay material is dried. As a result alternate cap designs are usually considered designed for arid environments.
D Subtitle Cap – RCRA
RCRA Subtitle D requirements are for non-hazardous garbage landfills. The design of a landfill cover for a RCRA Subtitle D facility is usually a function of the bottom lining arrangement or native subsoils present. The cover must meet the following conditions:
* the material should maintain a permeability no greater than 1 x 10-5 cm/s, or equal permeability of any underside liner or natural subsoils there, whichever is less.
* The layer to keep the waste dry must contain at least 45 cm of subsoil material type.
* The erosion control layer must be at least 15 cm of earthen material capable of sustaining native plant development.
Different design can be considered, but must be be of parallel performance as the specifications outlined above. Every covers must remain designed to prevent the “bathtub” effect. The bathtub effect occurs as soon as a more permeable cover is positioned above a less permeable bottom liner or natural subsoil. The landfill subsequently fills up reminiscent of a bathtub.
