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J. H. KLEINFELDER & ASSOCIATES File No. 5-2599-1 <br /> March 3, 1983 <br /> P age 9 <br /> of the fill by molecular diffusion. <br /> The rate of movement of methane gas by diffusion dispersion is slower through <br /> soils with fine particles than in coarse-grained material. The movement of gasses in a <br /> lateral direction from the landfill to adjacent soils can be a phenomenon of great <br /> concern where methane gas is involved. Adequate measures must be taken to dissipate <br /> the gas and prevent such subsurface movement. Fires and explosions have resulted <br /> from accumulation of methane gas in confined spaces of structures on or adjacent to <br /> landfills after the gas became trapped. The pertinent factors affecting gas production <br /> and gas control are the percentage of organic material, the permeability and thickness <br /> of soil cover, temperature variations, density, and moisture content. Control efforts <br /> will also be affected by factors such as the permeability of the soils underlying and <br /> adjacent to, the landfill, and subsidence. <br /> Pleasures which may be taken to prevent the movement of gasses into adjacent <br /> soils include physical barriers and ventilation devices. A physical barrier involves the <br /> placement of an impervious membrane between the landfill and the adjacent soils. The <br /> ventilation system for controlling gas movement involves the release and venting of <br /> gasses from beneath or in the vicinity of buildings. When gasses can escape through <br /> vent pipes or surface gravel trenches situated on or around the landfill, the gradient <br /> normally responsible for the movement can be lessened. <br /> We propose a trench backfilled with clean gravel as the main portion of the gas <br /> collection system. We recommend the trench be lined with a construction fabric, such <br /> as a Phillips product, Supae 4PC or equivalent. We suggest the trench be placed in <br /> an east-west direction at approximately the center of Lot 1. The system should run <br /> the full width of Lot 1 to within approximately 50 feet of the river. It should be at <br /> least 20 feet deep, or to the depth of the landfill, whichever is shallower, and at <br /> least 18 inches wide. This system should help collect gas migrating from the north <br /> onto the building sites. <br /> o <br />