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Proposed View Ridge Subsurface Exploration, Geologic Hazards, Infiltration Potential, Elementary School Replacement and Preliminary Geotechnical Engineering Report Everett, Washington Preliminary Design Recommendations drainage conditions where site grades will slope towards pavement edges or building areas, since surface water and perched seepage might be directed towards these structures. If site grades will slope toward pavement edges or structures, it would be appropriate to include cutoff drains in project plans. We should be allowed to offer situation-specific recommendations for these drains once a grading plan has been finalized. For planning purposes, cutoff drains should be located outside of the building perimeter(s) and pavement edges, and should extend to depths sufficient to achieve a minimum 1 foot of penetration into the underlying advance outwash. The cutoff drains should be at least 18 inches wide, should be graded to drain by gravity to a suitable discharge location, and should be provided with a perforated pipe surrounded by washed drain rock. For the planned south driveway, the drain for the expected south retaining wall may serve as an effective cutoff drain, depending on final design. 12.0 FLOOR SUPPORT Floor slabs can be supported on suitable native sediments, or on structural fill placed above suitable native sediments. Floor slabs should be cast atop a minimum of 4 inches of clean, washed, crushed rock or pea gravel to act as a capillary break. Areas of subgrade that are disturbed (loosened) during construction should be compacted to a non-yielding condition prior to placement of capillary break material. Floor slabs should also be protected from dampness by an impervious moisture barrier at least 10 mils thick. The moisture barrier should be placed between the capillary break material and the concrete slab. 13.0 FOUNDATION WALLS All backfill behind foundation walls or around foundation units should be placed as per our recommendations for structural fill and as described in this section of the report. Horizontally backfilled walls, which are free to yield laterally at least 0.1 percent of their height, may be designed to resist active lateral earth pressure represented by an equivalent fluid equal to 35 pounds per cubic foot (pcf). Fully restrained, horizontally backfilled, rigid walls that cannot yield should be designed for an equivalent fluid of 50 pcf. Walls with sloping backfill up to a maximum gradient of 2H:1V should be designed using an equivalent fluid of 55 pcf for yielding conditions or 75 pcf for fully restrained conditions. If parking areas are adjacent to walls, a surcharge equivalent to 2 feet of soil should be added to the wall height in determining lateral design forces. As required by the 2009 IBC, retaining wall design should include a seismic surcharge pressure in addition to the equivalent fluid pressures presented above. Considering the site September 22, 2010 ASSOCIATED EARTH SCIENCES, INC. DWG/tb-KE100255A2-Projects120100255IKElWP Page 19