7505 SEAWAY BLVD Geotech Report 2022-10-25

November 11, 2016 HWA Project No. 2014-117-21 replaced with"Structural Backfill" or be suitably compacted. Additionally, undocumented fill, saturated soil, soil significantly disturbed by construction methods, and otherwise unsuitable subgrade material may also need to be removed and replaced with"Structural Backfill". Where CSBC and CSTC are used,the materials should meet the gradation requirements specified in Section 9-03.9(3) of the WSDOT Standard Specifications (WSDOT, 2016). Structural backfill and compaction considerations are discussed in Sections 4.8.3 and 4.8.5, respectively. 4.5 SEAT WALLS We understand that several seat walls will be constructed as part of the project. Most will be short, about 20 inches tall and 14 inches wide, while the wall along the southern property line (PUD property border)will be taller and will require deeper foundations. All foundations for the seat walls should bear at least 18 inches below final ground surface. If the ground surface is different on either side of the seat walls (unbalanced), they should be designed considering lateral earth pressures. If backfill on both sides of the wall is at the same level, these considerations will not be necessary. Assuming the walls are free to deflect slightly, they should be designed for lateral earth pressures equivalent to a fluid weighing 35 pcf for static conditions and 55 pcf for seismic conditions. For design purposes, an allowable passive earth pressure equivalent to a fluid weighing 300 pcf may be assumed for properly compacted fill placed against the sides of the foundations. An allowable bearing capacity of 3,000 psf should be used for foundation design. 4.5.1 Seat Wall Subgrade Preparation Subgrade preparation is important to limit differential settlement of the wall and maintain global stability. All organic material should be removed from beneath the entire footprint of the seat walls prior to construction. Loose or soft soil, defined as being penetrable more than 1 foot with a %2-inch diameter rod pushed in under a 150 lb load(T-probe pushed in by hand), should be removed and replaced with"Structural Backfill" or be suitably compacted. Where unbalanced lateral earth pressures exist, a 6-inch-diameter perforated drain pipe should be installed behind the base of the walls so that hydrostatic pressures do not develop. The drain pipe should be sloped to drain and routed to an appropriate discharge location. We recommend an HWA geotechnical engineer, or their representative,be present during construction to verify the assumptions made for the foundations of the walls are met. The depth and extent of excavation will be directed by the geotechnical engineer on site. 2014-117 Final Geotechnical Report 10 HWA GEOSCIENCES INC.