5704 9TH DR W 2025-02-28 - Laserfiche WebLink

L2COBALT Cobalt Geosciences,LLC G E 0 S C I E N C E S P.O.Box1792 North Bend,WA 98045 April 11, 2024 Reynaldo Santos Crisrey2(&comcast.net RE: Wall Design Proposed Wall Repair/Replacement 5704 9th Drive West Everett,Washington In accordance with your authorization, Cobalt Geosciences, LLC (Cobalt)has prepared this letter to discuss wall design information for a replacement retaining wall at the referenced site. We understand that an existing concrete block wall(similar to Ultra block in dimensions)recently failed near the top of a steep slope in the backyard area of the referenced property. The wall did not have geogrid reinforcement and was over 6 feet tall in places. We understand that you are interested in re-using the blocks on site to rebuild a properly designed wall in the same location and elevations as the previous wall. We visited the site in December 2023 to observe the soil conditions. In general, the soils are consistent with glacial till soils(silty-sand with gravel)used as fill behind the former wall.This part of Everett is underlain by glacially consolidated soils that are typically resistant to global instability. Geogrid reinforced walls are suitable to support yard areas and surcharges provided they are properly designed,constructed,and bear on suitable native soils(medium dense glacial till). Keyways and Benching Wall keyways should be excavated into medium dense or firmer native soils. This area appears to be underlain by Vashon Glacial Till,which is typically dense below a thin weathered zone of loose to medium dense soils. The areas behind the walls will consist of imported or on site soils used as structural fill where required, and cuts into medium dense to very dense glacial till where applicable. If overexcavation of loose soils is required,soils may be replaced with angular rock. We recommend a 2.5 foot embedment for the new block wall near the slope area with a 2 foot wide bench in front of the base of the wall. The bench may be constructed out of fill materials compacted to at least 95 percent of the modified proctor(ASTM D1557 Test Method). The area behind the wall is anticipated to be level and without surcharge loads. The keyways must be verified by the geotechnical engineer to confirm suitability. Two tier walls are suitable as long as the upper wall is set back a horizontal distance equal to or less than the exposed height of the lower wall. Block Walls The on site blocks are concrete blocks with dimensions of 2.5 feet by 2.5 feet by 5 feet long with interlocking features on top and bottom. We recommend a minimum 2.5 feet of embedment into medium dense native soils. All blocks should be placed to interlock. Upper tiers in existing level yard areas may have 12 inches of embedment. www.cobaltgeo.com (206)331-1097