TM 5-814-7Figure 6-10. Final cover details.6-23CANCELLED
6-24sure covers is provided in EPA SW-867. More detaileddesign criteria and procedures are provided in EPA600/2-79-165 and EPA SW-873.(3) The low permeability layer includes aminimum 2-foot-thick soil liner and a syntheticmembrane at least 20 mil thick. General design,selection, and construction procedures for both syntheticand clay liners are provided in paragraph 6-2.(a) soils suitable for the lower liner are nativeclay materials, or soils blended with clay, bentonite, orother additives, which can exhibit, when placed on a firmbase, a recompacted permeability of 41 x 10-7 cm/sec.The soil liner should generally fall into the CL/CH UnifiedSoil Classification System, with not less than 50 percentby weight passing a No. 200 sieve (U.S. Standard), aliquid limit between 35 and 60, and a plasticity indexabove the "A" Line in the plasticity chart of the USCS.Any additive which increases the soil’s vulnerability tocracking by settlement or excessive shrinkage should beavoided.(b) Achievable field densities for cover soil linersare generally less than for base liners, because waste fillareas provide a softer, more flexible constructionsubgrade. The designer should obtain laboratory tests ofthe permeability of representative soil liner samplesremolded to achievable field densities at moisturecontents greater than optimum to establish constructionprocedures for low in-place permeability of the soil liner.(c) The designer should specify moistureconditioning requirements, the thickness of soil layers forcompaction, the type and weight of equipment, and thenumber of equipment passes required to achieve therequired density/permeability and avoid flexural crackingduring placement. The constructed soil liner shall beprotected from drying until placement of the syntheticmembrane. Spraying with water or application of anemulsion to prevent drying may be necessary.(d) The EPA guidance documents specify asynthetic liner at least 20 mil thick; demonstration of theliner’s compatibility with the waste or leachate is notrequired in this case, because the liner is not expected tobe in contact with waste or leachate. Nevertheless, linerselection should be based upon its resistance to thewaste present and to degradation, as well as its ability toundergo deflection due to settlement without cracking ortearing.(e) The synthetic liner must be protected bothabove and below by a layer of material no coarser thansand. Sands should be classified as either SW or SP bythe USCS, with less than 5 percent passing the No. 100sieve. In addition, sands which act as filters must meetfilter graduation requirements, such as those shown inchapter 5 of TM 5-820-2. The synthetic liner can beplaced directly on the soil liner with ade-TM 5-814-7quate protection, provided the upper 6 inches is nocoarser than sand and free of rock, fractured stone,debris, cobbles, rubbish, and roots. A drainage layerselected to meet the requirement for bedding materialcan be used above the liner.(f) Where surface slopes are 3:1 or steeper,geotextile fabrics are recommended for placement overthe synthetic liner. Heavy geotextile fabrics >, 12 oz/ydare increasingly being used in combination with flexiblemembrane liners in hazardous waste units to protect themembranes from puncture and abrasion. If geotextilesare used to protect synthetic membranes, it is importantthat they, like the synthetic membranes, be tested forcompatibility with hazardous waste. However, manysuch fabrics are made of polypropylene or polyestermaterials and may have compatibility characteristicssimilar to those exhibited by liners of the same materials.(g) Care must be taken to avoid any penetrationof the liner. Where inlets or outlets are required (e.g., foran impoundment), inflow/outflow piping should bedesigned to go over the top whenever possible. Energydissipaters may be needed at the pipe inlet/outlets.Where penetrations cannot be avoided, precautionsmust be taken to ensure an adequate seal between theliner and any unavoidable penetration. In such cases,flange-type connections should be considered. EPASW-870 outlines procedures for sealing between theliner and any penetration.(h) EPA requires that the liner must also beprotected from damage by sudden changes in slope; toprevent liners from freezing, they must be locatedentirely below the frost line. Procedures are provided inEPA 600/2-79-165.(4) The drainage layer must be at least 12inches thick, exhibit a permeability of >1 x 10-3 cm/sec,and be able to carry infiltrating waters to the sides of thecover for discharge.(a) The designer should carefully evaluate thedrainage layer for its ability to carry waters for discharge,and the need for a synthetic fabric filter or gradedgranular layer to prevent plugging due to infiltration ofsoils from the vegetated soil cover layer. Measuresshould be considered to preclude piping of the drainagelayer at discharge areas.(b) Selection of a clean sand (SP), whichexhibits the required permeability and is able to carry thevolume of infiltrating water, will not only satisfy thebedding requirements for the synthetic liner, but mayalso eliminate the need for a granular layer to preventplugging; nevertheless, a synthetic fabric filter should beconsidered to ensure the long-term effectiveness of thedrainage layer.(c) Although the EPA guidance documentsindicate drainage collection devices are not necessary, aperforated drainage collection pipe to intercept andCANCELLED
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