Drainage layer

A Geonet drainage layer can be chosen if the HELP model shows it to be more effective than sand in controlling leachate production and is comparable in cost. It is assumed that the HELP model predicts a 75 to 80% reduction in leachate production. A Geotextile layer would be laid on either side of the Geonet drain to prevent clogging. A minimum slope of 3% would be provided to meet state requirements. 

Depending on the material type and construction method, the saturated hydraulic conductivities for these barrier layers are typically between 1 x 10-5 and 1 x 10-9 cm/s. In addition, conventional cover systems generally include additional layers, such as surface layers to prevent erosion protection layers to minimize freeze/thaw damage internal drainage layers and gas collection layers.6 22... 

Construction materials also vary. The most common synthetic materials are polypropylene (PP), polyester (PET), or polyethylene (PE). Because synthetic drainage layers are much thinner (<1 in.) than granular drainage layers (1 ft) and have similar design liquids capacity, their use in a landfill results in increased space for waste storage and disposal. This advantage translates into increased revenues for the owner/operator of a landfill. 

The main selection criteria for synthetic drainage materials are high hydraulic transmissivities, or in-plane flow rates, and chemical compatibility with the waste leachate. Discussion of the chemical compatibility of synthetic liners and drainage layers is given in a later section. 

Described in this section are the LDS requirements that apply to the secondary LCRS between the two liners in a landfill. These requirements focus on the drainage layer component of the LCRS. Figure 26.4 illustrates the location of an LDS in a double-lined landfill that meets these requirements. 

The minimum design standards for granular drainage layer materials require a minimum thickness of 1 ft and a minimum hydraulic conductivity of 1 cm/s. To meet this minimum hydraulic conductivity standard for granular drainage materials, the secondary LCRS, or LDS, must be constructed of clean gravels. 

The guidance16 recommends a three-layer cap design consisting of a vegetative top cover, a middle drainage layer, and a composite liner system composed of an FML over compacted low-permeability soil. The final cover is to be placed over each cell as it is completed. 

Surface drainage system capable of conducting runoff across the cap without rills and gullies Drainage layer design Thickness >1 ft... 

A 0.3-m-thick granular LDS drainage layer with a minimum hydraulic conductivity of 1 x 10 2 cm/s or a geosynthetic LDS drainage layer with a minimum hydraulic transmissivity of 3 x 10 3 m2/s. 

A 0.3-m-thick granular drainage layer with a minimum hydraulic conductivity of... 

U.S. EPA specifies that the minimum detection time for the leachate entering the LDS of an LDCR system is <24 h. Response time calculations are based on velocity in the geonet and/or granular soil drainage layer. Darcy s law is used to calculate flow velocity in the geonet, and a true velocity must be used for granular soil. 

Figure 26.31 shows a typical landfill profile designed to meet U.S. EPA s MTG requirements.38 The upper subprofile comprises the cap, or cover, and includes the required 2-ft vegetative top cover, 1-ft lateral drainage layer, and low-permeability cap of barrier soil (clay), which must be more than 2 ft thick. This three-tier system also includes an optional FMC and an optional gas control layer. The guidance requires a 40-mil thick FMC. 

The SWCR system is built on top of the FMC. The purpose of the SWCR system is to prevent infiltration of surface water into the landfill by containing and systematically removing any liquid that collects within it. Actual design levels of surface water infiltration into the drainage layer can be calculated using the water balance equation or the HELP model.36 37... 

Figure 26.33 shows the gravel filter and cobblestone components of the biotic barrier and their placement in the landfill system. The proposed 1-m thickness for a biotic barrier should effectively prevent penetration by all but the smallest insects. Note that the biotic barrier also serves as the surface water collection/drainage layer. Biotic barriers used in nuclear caps may be up to 14 ft thick... 

Erosion can seriously effect a landfill closure by disrupting the functioning of drainage layers and surface water and LCRSs. Heavy erosion could lead to the exposure of the waste itself. For this reason, it is important to predict the amount of erosion that will occur at a site and reinforce the facility accordingly. The Universal Soil Loss Equation shown below can be used to determine soil loss from water erosion5 ... 

The top layer of the detonation ground was excavated to a depth of 0.5m. The exeavated soil was temporarily stored on the site. A Bentonit seal was installed and a protective and a drainage layer were built over the seal. The excavated soil was incorporated into the slopes of the reconstructed ground (Figure 6). The new top layer of the detonation ground was made Irom... 

Many landfill operators have found that scrap tires can be used beneficially in the construction and operation of the landfill. Tire shreds are used in building leachate collection basins in new landfills and they can also be used as the drainage layer in landfill caps. Another application is mixing tire shreds with soil as daily cover material. 

All component layers—to provide sufficient thickness of clean materials Top soil—to support vegetation Sub-soil—to support vegetation Low permeability barrier layer—to prevent passage of water, gas or VOCs Buffer layer—to protect barrier layer Drainage layer/system—control drainage... 

MUU1-LAYERED MEDIUM WITH SUPPORT AND DRAINAGE LAYERS... 

Drainage layer for learhate Polyethylene film Clay... 

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