4730 COLBY AVE SEPA 016 - 027 - YMCA Geotech Report 2025-04-14

October 25, 2015 YMCA - Everett L&A Job No. 15-089 Page 10 cut slopes or fill embankments should be seeded and vegetated as soon as possible for erosion protection and long-term stability, and should he covered with clear plastic sheets, as required, to protect them from erosion until the vegetation is fully established. RETAINING WALLS Cast-in-place retaining walls unrestrained at the top from lateral movement may be designed for active soil pressure. We recommend that an active soil pressure of 40 pcf EFD be used for the design of retaining walls with a level or descending backslope away from the walls. For walls with ascending backslope, an additional pressure of 0.75 pcf per degree of the backslope angle above the horizontal should be added to the above design pressures. To counter the above active pressure, a passive lateral soil pressure of 275 pcf EFD may be used. This passive pressure value is applicable only to walls with a level or ascending backslope away from the walls for a horizontal distance at least 1.5 times the wall height. To resist against sliding, the friction force between the footings and the subgrade soils may be calculated based on a coefficient of friction of 0.55. The above soil parameters are ultimate values based on a fully drained condition of the walls, and proper factors of safety should be applied in the design of the walls against sliding and overturning failures. A vertical drainage blanket, at least 12-inch-thick horizontally, consisting of clean, free- draining, pea gravel or washed gravel should be placed against the back of retaining walls to within 18 inches of the finish grade to prevent accumulation of groundwater behind and buildup of hydrostatic pressure against the walls. This drainage blanket fill should be compacted to a non-yielding state with a vibratory compactor. Structural fill should be used for the remaining wall backfill. The top 18 inches of the backfill may consist of compacted, LIU & ASSOCIATES, INC.