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Geotechnical Engineering Report Mixed -Use Everett 10521-191h Ave. SE Everett, Washington March 23, 2016 RN File No. 3109-001A Page 7 rest earth pressure can be calculated based on equivalent fluid density. Equivalent fluid densities for active and at -rest earth pressure of 35 pounds per cubic foot (pcf) and 55 pcf, respectively, may be used for design for a level backslope. These values assume that the on - site soils or imported granular fill are used for backfill, and that the wall backfill is drained. The preceding values do not include the effects of surcharges, such as those due to foundation loads or other surface loads. Surcharge effects should be considered where appropriate. Seismic lateral loads are a function of the site location, soil strength parameters and the peak horizontal ground acceleration (PGA) for a given return period. We used the US Geological Survey program "2008 PSH Deaggregation on NEHRP" to compute the PGA for the site. The 3- D histogram is included in Appendix A. The PGA for the active condition was reduced by one- half, referenced in Geotechnical Earthquake Engineering by Steve L Kramer, 1996. When considering seismic conditions, the above drained active and at -rest values should be increased as shown in the table below. H represents the wall height. Seismic Surcharge Loads Probability of Exceedance (Recurrence Interval) 20% in 50 years (1 in 224 year) 10% in 50 years 0 in 475 year) Acceleration (g) 0.26 0.39 Active > 0.001 H 5H 8H Non -Yielding 5H 10H The above lateral pressures may be resisted by friction at the base of the wall and passive resistance against the foundation. A coefficient of friction of 0.5 may be used to determine the base friction in the native glacial soils. An equivalent fluid density of 300 pcf may be used for passive resistance design. To achieve this value of passive pressure, the foundations should be poured "neat" against the native dense soils, or compacted fill should be used as backfill against the front of the footing, and the soil in front of the wall should extend a horizontal distance at least equal to three times the foundation depth. A resistance factor of 0.67 has been applied to the passive pressure to account for required movements to generate these pressures. All wall backfill should be well compacted. Care should be taken to prevent the buildup of excess lateral soil pressures due to overcompaction of the wall backfill. Hand compaction equipment of a small roller should be used directly adjacent to the wall. Slabs -On -Grade Slab -on -grade areas should be prepared as recommended in the Site Preparation and Grading subsection. Slabs should be supported on medium dense or firmer native soils, or on structural fill extending to these soils. Where moisture control is a concern, we recommend that slabs be underlain by 6 inches of pea gravel for use as a capillary break. A suitable vapor barrier, such as Robinson Noble, Inc