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J. H. KLEINFELDER & ASSOCIATES File No. 5-2599-1 <br /> March 3, 1983 <br /> P age 8 <br /> fill should be compacted at a moisture content at or near optimum. We recommend <br /> that samples of any proposed imported fill soil be submitted to the soils engineer for <br /> approval prior to being brought to the site. Engineered fill or structural backfill <br /> should be placed in thin horizontal layers a maximum of 8 inches in loose thickness, <br /> and compacted to a minimum of 90 percent of the maximum dry density as determined <br /> by the ASTM D-1557-70 test procedure. <br /> We recommend that structural and trench backfill be placed in the same manner <br /> as engineered fill. In our opinion, thench backfills may be jetted. However, if jetting <br /> is performed, sufficient time should be set aside to allow consolidation to occur. In <br /> addition, density tests should be taken to verify the recommended compaction. <br /> Gas-Control Systems <br /> The northerly half of Lot 1 has received fill with a very high organic content. <br /> At the time of the field explorations, this material was near saturation, and a strong <br /> odor was noted. These two conditions indicate that decomposition is in progress, and <br /> gasses are probably being generated. <br /> These gasses may build up locally within the fill. Although the fill is not <br /> uniform, gasses may move laterally through the fill. Since both gas buildup and <br /> distribution may be random, venting systems are recommended. <br /> It has been demonstrated in many studies that gasses generated by decomposition <br /> of refuse in a sanitary landfill can, under certain conditions, and in particular soil <br /> types, move a significant lateral distance from the landfill into adjacent soil. Gas <br /> moves in soils by means of molecular diffusion or by convective flow (pressure <br /> gradient). The driving force causing movement of gas from one location to another by <br /> means of molecular diffusion is a concentration gradient of the gas between the two <br /> locations. At sites that are surrounded by dry permeable soils, where little <br /> concentration gradient exists at the refuse-soil interphase, gasses will slowly move out <br />