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plan of operation for the placement of solid wastes must be prepared. Various operational methods have been developed, primarily on the basis of field experience. The methods to fill dry areas are substantially different from those used to fill wet areas.

4.2.7.1 Conventional Methods for Dry Areas. The principal methods used for landfilling dry areas may be classified as (1) area, (2) trench, and (3) depression. (See Figures 4-2-7A, B, and C.) In addition to these methods, which usually are used for unprocessed municipal solid wastes, landfilling using milled (shredded or compressed and baled) solid wastes is also discussed.

4.2.7.2 Area Method. The area method is used when the terrain is unsuitable for the excavation of trenches in which to place the solid wastes. Operationally (see Figure 4-2-7A) the wastes are unloaded and spread in long, narrow strips on the surface of the land in a series of layers that vary in depth from 16 to 30 in. Each layer is Each layer is compacted as the filling progresses during the course of the day until the thickness of the compacted wastes reaches a height varying from 6 to 10 ft. At that time, and at the end of each day's operation, a 6- to 12-in. layer of cover material is placed over the completed fill. The cover material must be hauled in by truck or earthmoving equipment from adjacent land or from borrow-pit areas.

4.2.7.3 The filling operation usually is started by building an earthen levee against which wastes are placed in thin layers and compacted. The length of the unloading area varies with the site conditions and the size of the operation. The width over which the wastes are compacted varies from 8 to 20 ft, again depending on the terrain. A completed lift, including the cover material, is called a cell (see Figure 4-2-7A). Successive lifts are placed on top of one another until the final grade called for in the ultimate development plan is reached. The length of the unloading area used each day shall be such that the final height of the fill is reached at the end of each day's operation.

4.2.7.4 If a small amount of usable cover material is available at the disposal site, the ramp variation of the area method is often used (see Figure 4-2-7B). In this method, solid wastes are placed and compacted as described for the area method and are partially or wholly covered with earth scraped from the base of the ramp. Additional soil must be hauled in, as in the area method. Because of increasing costs and the problems associated with obtaining usable cover material, the use of the ramp method must be based on a detailed economic feasibility study.

4.2.7.5 Balefill Method. Operation is similar to the area method except refuse is compressed and baled then stacked in the area prior to covering.

4.2.7.6 Trench Method. The trench method of landfilling is ideally suited to areas where an adequate depth of cover material is available at the site and where the water table is not near the surface. Typically, as shown in Figure 4-2-7C, solid wastes are placed in trenches varying from 100 to 400 ft in length, 3 to 6 ft in depth, and 15 to 25 ft in width. To start the process, a portion of the trench is dug and the dirt is stockpiled to form an

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embankment behind the first trench. Wastes are then placed in the trench, spread into thin layers (usually 18 to 24 in.), and compacted. The operation continues until the desired height is reached. The length of trench used each day shall be such that the final height of fill is reached at the end of each day's operation. The length also shall be sufficient to avoid costly delays for collection vehicles waiting to unload. Cover material is obtained by excavating an adjacent trench or continuing the trench that is being filled. The trench method, however, is not readily amenable to the proposed requirements for installation of liners and leachate collection and treatment systems.

4.2.7.7 Depression Method. At locations where natural or artificial depressions exist, it is often possible to use them effectively for landfilling operations. Canyons, ravines, dry borrow pits, and quarries have all been used for this purpose. The techniques to place and compact solid wastes in depression landfills vary with the geometry of the site, the characteristics of the cover material, the hydrology and geology of the site, and the access to the site.

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4.2.7.8 If a canyon floor is reasonably flat, the first fill in a canyon site may be carried out using the trench method operation discussed previously. Once filling in the flat area has been completed, filling starts at the head end of the canyon and ends at the mouth. An important consideration is that since the canyons and ravines are formed by water erosion, landfilling may involve a water course. This practice prevents the accumulation of water behind the landfill. Wastes usually are deposited on the canyon floor and from there are pushed up against the canyon face at a slope of about 2 to 1. In this way, a high degree of compaction can be achieved. Compacted densities as high as 1200 lb/yd3 have been reported. Even higher densities have been recorded in the lower portions of the landfill as the height of the fill increases.

4.2.7.9 Pit and quarry landfill sites are always lower than the surrounding terrain, so control of surface drainage is often the critical factor in the development of such sites. Also, borrow pits and quarries usually do not have adequate soil or geological properties for landfilling because they display high permeability and fracturing. As with canyon sites, pit and quarry sites are filled in multiple lifts, and the method of operation is essentially the same. A key to the successful use of pits or quarries is the availability of adequate cover material to cover the individual lifts as they are completed and to provide a final cover over the entire landfill when the final height is reached. Because of settlement, it is usually desirable to fill pit and quarry sites to a level slightly above that of the surrounding terrain. The depression method is also not readily amenable to liners and leachate collection system.

4.2.7.10 Evaluation of Seepage Potential (Tchobanoglous, Theisen, and Eliassen 1977). Core samples must be obtained to evaluate the seepage potential of a site that is being considered for a landfill. Sufficient borings should be made so that the stratigraphic formations under the proposed site can be established from the surface to (and including) the upper portions of the bedrock or other confining layers. At the same time, the depth to the surface water table should be determined along with the piezometric water levels in any bedrock or confined aquifers that may be found.

4.2.7.11 The resulting information is then used to (1) determine the general direction of groundwater movement under the site, (2) determine whether any unconsolidated or bedrock aquifers are in direct hydraulic connection with the landfill, and (3) estimate the vertical seepage that might occur under the landfill site.

4.2.7.12 Drainage and Seepage Control Facilities. In addition to the seepage analysis, it is also necessary to develop an overall drainage plan for the area that shows the location of storm drains, culverts, ditches, and subsurface drains as the filling operation proceeds. In some cases it may also be necessary to install seepage control facilities.

4.2.7.13 To ensure the rapid removal of rainfall from the completed landfill and to avoid the formation of puddles, the final cover should have a slope of about 1%. Where relatively impervious cover material such as clay is used, lesser slope values may be feasible. The theoretical amount of water that could enter the landfill per unit area in a 24-h period for various cover

materials is given in Table 4-2-7A, assuming that (1) the cover material is saturated, (2) a thin layer of water is maintained on the surface, and (3) there is no resistance to flow below the cover layer.

TABLE 4-2-7A

Theoretical Volume of Water that Could Enter Completed
Landfill Through 1 ft2 of Various Cover Materials in 1 Day

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4.2.7.14 Clearly, these data are only theoretical values, but they can be used in assessing the worst possible situation. In actual practice the amount of water entering the landfill will depend on local hydrological conditions, the characteristics of the cover material, the final slope of the cover, and whether vegetation has been planted.

4.2.7.15 Among the methods to control the seepage into and out of landfills are (1) the use of impermeable cover materials, (2) the interception of high groundwater before it reaches the fill, (3) equalization of the water levels within and outside the landfill, and (4) the use of an impervious layer of clay material or other sealants.

4.2.7.16 Conventional Methods for Wet Areas. Recently, because of concern over the possibility of groundwater contamination by leachate and gases from landfills and the development of odors, the direct filling of wet areas is no longer considered acceptable. Installation personnel need to consult with the state agency before considering disposal in wet areas because it may be illegal. If wet areas are to be used as landfill sites, special provisions must be made to contain or eliminate the movement of leachate and gases from completed cells. Usually this is accomplished by first draining the site and then lining the bottom with a clay liner or other appropriate sealants. If a clay liner is used, it is important to continue operation of the drainage facility until the site is filled in order to avoid the creation of uplift pressures that could cause the liner to rupture from heaving.

4.2.7.17 Equipment

4.2.7.18 A wide variety of equipment is available from which to select the proper type and size needed for an efficient operation. The size, type, and amount of equipment required at a sanitary landfill depends largely

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