Designing engineered systems for use in a landfill using TDA is similar to designing systems using traditional materials like rock or gravel. The only difference is adjusting the designs to accommodate the difference in the engineering properties of TDA: density, shear strength, compressibility, specification (Type A or Type B), and hydraulic conductivity. TDA is lighter and more permeable than gravel and rock, and has a higher bulk density (so less TDA is needed to occupy the same volume as rock or gravel). It also produces less dust (compared to gravel).
Although the methods are similar, care should be taken when placing TDA. Steel wire protruding from TDA has punctured rubber tires on loaders and other machinery. When spreading TDA, track-mounted equipment should be used including dozers, loaders, or blade-equipped steel wheel compactors (see installation).
Examples of TDA use in landfill include:
Drainage layer in a leachate collection system: TDA is incorporated as a component of the leachate collection system to provide drainage, limit freezing on overlying clay barriers in cold climates and protect the liner system from damage during construction and operation. When excavating trenches for the leach field, the TDA serves as a media for the treatment of wastewater, so the material should be free of any contamination or debris that might interfere with treatment or negatively impact the surrounding soil or groundwater. A separation barrier is generally used in leach field construction between the aggregate media and the soil backfill. The barrier prevents the migration of fines into the aggregate, which could reduce the hydraulic conductivity of the media and result in plugging of the system. Treated paper is often used as the barrier for conventional rock aggregate systems; since the protruding wires in TDA could rip this paper, geotextile fabric is a good alternative.
Leachate recirculation trenches: TDA can be used as backfill around leachate recirculation trenches, typically replacing conventional materials such as granular soil. Leachate recirculation trenches are used to reintroduce collected leachate back into waste. Rock aggregate (natural or crushed gravel) is the traditional medium used for septic system leach fields. However, the practice of gravel mining from rivers is increasingly limited due to environmental degradation associated with the practice. Environmental controls on rock quarries have increased the operational costs of these facilities, and in some cases, they have ceased operation. The net result is that rock aggregate has become expensive and difficult to obtain in many locations. TDA has numerous benefits that make it an excellent alternative to rock aggregate for drainage applications. TDA has higher permeability than traditional drain rock aggregate and a substantially lower unit weight. TDA has a higher surface area per unit volume, which increases the area for biofilm development and subsequent biological treatment of the drainage water. Rather than causing potential environmental problems by mining or producing rock aggregate, producing TDA reuses a waste product and keeps used tires out of landfills. Because of these benefits, TDA may perform better than rock aggregate and in many locations may be less expensive to use.
Gas collection trenches: TDA used in this application replaces granular soil as the bedding material for gas extraction pipes in the collection trenches. These trenches are usually located beyond the footprint of the landfill to control the lateral migration of landfill gas.
Gas collection layer: TDA layers provide good drainage and help protect clay and geomembrane liners in landfill gas collection systems.
Drainage layers in landfill covers: TDA can be used as a drainage layer in landfill covers. In this application, the TDA is typically enclosed in geotextile to decrease the number of fines in the TDA layer and maintain TDA’s high permeability.