1125 80TH ST SW 2016-01-01 MF Import

Cornerstone Columbia Dei opment Co. W-6984 5 July 1990 Page 5 If during stripping, areas of soft or pumping subgrade or deeper organic rich soils are encountered, these areas should be overexcavated and backfilled with "structural fill' as recommended below. Areas excavated to design or subgrade elevations or areas which are to receive structural fill should be prerolled with a vibratory roller to a firm, non- yielding condition if they are near their optimum moisture level. We recommend that prerolling be accomplished such that the upper 12 inches of the exposed non-organic soil is compacted to a density of at least 90 percent of the laboratory maximum, using the modified Proctor method (ASTM:D-1557) as the density standard. If the subgrade is more than 2 percent above the optimum moisture content, adequate compaction of the site soils may be difficult or impossible to achieve. Prerolling of the silty subgrade soils may actually initiate site disturbance in wet weather conditions. Under such conditions, it may be necessary to scarify the upper layer of wet subgrade soils, allow them to dry and then recompact. If this is not practical, it may be necessary to remove the wet soils and replace with free-draining import fill. Depending on final grades and site conditions during construction, at the discretion of the geotechnical engineer, it may be acceptable to strip the ground surface as much as possible and place fill directly on the native ground surface without prerolling. If grading must be initiated when areas of the silty site soils are more than about 2 percent wetter than optimum, those areas to receive fill should be blanketed with a clean, coarse gravel, crushed rock, quarry spalls or equivalent non-moisture sensitive material as described above to provide a working surface for the subsequent structural fill placement. The contractor should be made aware of the fact that the silty site soils are highly susceptible to disturbance when they are only slightly wetter than optimum. Therefore, the contractor would need to perform final stripping with care in order to minimize site disturbance. Additionally, the contractor should avoid vehicular traffic above prepared subgrade areas, especia'ly in wet weather or wet site conditions. 4.2 Structural Fill All fill placed beneath the building areas and walkways, as well as utility trench backfill, should be placed in accordance with the following recommendations for structural fill. Prior to placement of the structural fill, the surfaces to receive fill should be prepared as previously recornmended by prerolling and compaction. Structural fill should be placed in lifts not exceeding 8 inches in loose thickness. Individual lits should be mechanically