2514 RAINIER AVE Geotech Report 2022-10-25

_ zit I amec K foster I wheeler Curtain Drains: To preclude development of hydrostatic pressure behind the backfilled retaining wall, we recommend a curtain drain be placed behind the walls. This curtain drain should consist of pea gravel, washed rock, or some other clean, uniform, well-rounded gravel, extending outward a minimum of 12 inches from the wall and extending upward from the footing drain to within about 12 inches of the ground surface. The curtain drain should connect to a 4-inch-diameter perforated drain pipe behind the heel of the wall, and the drain pipe should discharge away from the wall. I Backfill Soil: Ideally, all retaining wall backfill placed behind the curtain drain would consist of clean, free-draining, granular material, such as "Gravel Backfill for Walls," per WSDOT Standard 111 Specification 9-03.12(2). Alternatively, on-site granular soils could be used as backfill if they are placed at a moisture content near optimum for compaction. I Backfill Compaction: Because soil compactors place significant lateral pressures on retaining walls, I we recommend only small, hand-operated compaction equipment be used within 3 feet of a backfilled wall. In addition, all backfill should be compacted to a density as close as possible to 90 percent of the maximum dry density (based on ASTM D-1557); a greater degree of compaction closely behind the I wall would increase the lateral earth pressure, whereas a lesser degree of compaction might lead to excessive post-construction settlements. Applied Loads: Overturning and sliding loads applied to retaining walls can be classified as static pressures and surcharge pressures. We offer the following specific values for design purposes: I ► Static Pressures: Yielding (cantilever) retaining walls should be designed to withstand an appropriate active lateral earth pressure, whereas non-yielding (restrained) walls should be designed to withstand an appropriate at-rest lateral earth pressure. These pressures act over the entire back of the wall and vary with the backslope inclination. Assuming a level backslope, we recommend using active and at-rest pressures of 35 pcf and 55 pcf, respectively. ► Surcharge Pressures: Static lateral earth pressures acting on a retaining wall should be I increased to account for surcharge loadings resulting from any traffic, construction equipment, material stockpiles, or structures located within a horizontal distance equal to the wall height. For simplicity, a traffic surcharge can be modeled as a uniform horizontal pressure of 75 psf acting against the upper 6 feet of the wall. I ► Seismic Pressures: Static lateral earth pressures acting on a retaining wall should be increased to account for seismic loadings. These pressures act over the entire back of the wall and vary with the backslope inclination, the seismic acceleration, and the wall height. For short site walls that are allowed some deformation, we recommend the seismic loading be modeled as uniform active pressure of 6H psf, assuming a level backslope and the exposed wall height is H. Amec Foster Wheeler I 14 Project No.6-917-18116-0 \\Sea-fs1\WordProc\_Projects\18000s\18116 Everett School District\Geotech Report\Revised\2017_10-19_Everett Middle School Geotech_Sx.docx