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1 ' <br /> ' amec <br /> foster <br /> wheeler <br /> subsequent casting of the concrete slab, the contractor should exercise care to avoid puncturing this <br /> vapor barrier. <br /> ( Vertical Deflections: Soil-supported slab-on-grade floors can deflect downward when vertical loads <br /> are applied due to elastic compression of the subgrade. In our opinion, a subgrade reaction modulus <br /> of 200 pounds per cubic inch can be used to estimate these deflections. <br /> ' 4.4 FOUNDATION DRAINS <br /> The building should be provided with permanent drainage systems to minimize the risk of future <br /> moisture problems. We offer the following recommendations and comments for drainage design and <br /> construction. Figure 3 displays these recommendations. <br /> ' Perimeter Drains: We recommend the new building structures be encircled with a perimeter drain <br /> system to collect possible seepage water. This drain should consist of a 4-inch-diameter perforated <br /> rigid pipe within an envelope of pea gravel or washed rock, extending at least 6 inches on all sides of <br /> 111 the pipe. The gravel envelope should be wrapped with filter fabric to reduce the migration of fines from <br /> the surrounding soils. Ideally, the drain invert would be installed no more than 8 inches above or <br /> below the base of the perimeter footings. <br /> Runoff Water: Roof-runoff and surface-runoff water should not be allowed to flow into the foundation <br /> ' drainage systems. Instead, these sources should flow into separate tightline pipes and be routed <br /> away from the buildings to an appropriate location. In addition, final site grades should slope <br /> downward away from each building so that runoff water will flow by gravity to suitable collection <br /> points, rather than ponding near the buildings. Ideally, the area surrounding the buildings would be <br /> capped with concrete, asphalt, or low-permeability (silty) soils to minimize surface-water infiltration <br /> next to the footings. <br /> 4.5 BACKFILLED WALLS <br /> We offer the following recommendations for relatively short walls supporting grade changes at the <br /> site. Underground vaults could also be designed as backfilled walls. <br /> Footing Depths: For frost and erosion protection, concrete retaining wall footings should bear at least <br /> 18 inches below the adjacent ground surface. However, greater depths might be necessary to <br /> develop adequate passive resistance and/or bearing resistance in certain cases. Flexible gravity <br /> ' walls, such as gabions and modular block walls, should be embedded at least 8 inches below final <br /> grades. <br /> 111 <br /> Amec Foster Wheeler <br /> Project No.6-917-18116-0 13 <br /> 1 \\Sea-fs1\WordProc\_Projects\18000s\18116 Everett School District\Geotech Report\Revised\2017_10-19_Everett Middle School Geotech_Sx.docx <br />