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amec <br /> foster <br /> wheeler <br /> Footing and Stemwall Backfill: To provide erosion protection and lateral load resistance, we 1 <br /> recommend all footing excavations be backfilled and compacted on both sides of the footings and <br /> stemwalls after the concrete has cured. The excavations should be backfilled with structural fill and <br /> compacted to a density of at least 90 percent (based on ASTM D-1557). <br /> Lateral Resistance: Footings and stemwalls that have been properly backfilled as described above will <br /> resist lateral movements by means of passive earth pressure and base friction. We recommend using <br /> the following design values, which incorporate static and seismic safety factors of at least 1.5 and 1.1, <br /> respectively. <br /> Design Parameter Allowable Value I <br /> Static passive pressure 300 pcf <br /> Seismic passive pressure 400 pcf <br /> Base friction coefficient 0.4 <br /> Note: pcf=pounds per cubic foot <br /> Base friction can be combined with the respective passive pressure to resist static and seismic loads. <br /> 4.3 SLAB-ON-GRADE FLOORS ' <br /> In our opinion, soil-supported slab-on-grade floors can be used in the proposed buildings if the <br /> subgrades are properly prepared. We offer the following comments and recommendations concerning <br /> 111 <br /> slab-on-grade floors. Figure 3 displays these recommendations. <br /> Floor Subbase: In order to prevent differential settlement and cracking due to the lacustrine silt I <br /> subgrade, we recommend a subbase of imported sand and gravel be placed above the subgrade <br /> soils. The subbase should be at least 8 inches thick and consist of"Gravel Borrow" per <br /> 111 <br /> WSDOT Standard Specification 9.03.14. During wet weather work we recommend a geotextile <br /> separation fabric be placed between the lacustrine silt subgrade and gravel borrow subbase. <br /> Capillary Break: To reduce the upward wicking of groundwater beneath the floor slab, we recommend <br /> a capillary break be placed over the subbase. This capillary break should consist of at least 4 inches <br /> of pea gravel or other clean, uniform gravel, such as "Gravel Backfill for Drains" per WSDOT Standard <br /> Specification 9-03.12(4). <br /> Vapor Barrier: We recommend a vapor barrier at least 10 mil thick be placed directly between the <br /> capillary break and the floor slab to prevent moisture from migrating upward through the slab. During ' <br /> Amec Foster Wheeler <br /> 12 Project No.6-917-18116-0 <br /> \\Sea-fs1\WordProc\_Projects\18000s\18116 Everett School District\Geotech Report\Revised\2017_10-19_Everett Middle School Geotech_Sx.docx <br />