4920 24TH AVE W Geotech Report 2023-06-22

GeoTest Services, Inc. October 8, 2021 2405 Hillside Lane, Everett,WA Project No.21-0845 siommi Temporary unsupported excavations in loose to medium dense fill encountered at the project site are classified as a Type C soil according to WAC 296-155-66401 and may be sloped as steep as 1.5H: 1V (Horizontal: Vertical). Native Transitional Beds can be classified as a Type B soil and may be sloped as steep as 1H: 1V. All soils encountered are classified as Type C soil in the presence of groundwater seepage and may be sloped as steep as 1.5H: 1V. Flatter slopes or temporary shoring may be required in areas where groundwater flow is present and unstable conditions develop. Temporary slopes and excavations should be protected as soon as possible using appropriate methods to prevent erosion from occurring during periods of wet weather. GeoTest recommends that permanent cut or fill slopes be designed for inclinations of 2H: 1V or flatter. Permanent cuts or fills used in earth slopes intended to hold water should be 3H: 1V or flatter. No ponds should be constructed on or near steep slopes.All permanent slopes should be vegetated or otherwise protected to limit the potential for erosion as soon as practical after construction. Stormwater Infiltration Potential The near-surface fill soils observed in the explorations will not be suitable for stormwater infiltration, as we do not have any information as to how the soils were placed and compacted. The native soils observed underlying the site contain elevated silt contents and are expected to exhibit low permeability characteristics. Glacial deposits such as Transitional Beds are generally widely regarded as "hydraulic restriction layers" as defined by the Stormwater Management Manual for Western Washington (SWMMWW) and are used as construction materials in stormwater pond liners and in berm construction where low-permeability soils are required as part of the design.The SWMMWW is the stormwater document currently adopted by the City of Everett, per EMC Chapter 14.28.020. Further, GeoTest anticipates the presence of perched groundwater seepage atop low permeability, Transitional Beds during the wet season and after an extended period of heavy precipitation. It is GeoTest's opinion that the proposed development area,which appears to be underlain in its entirety by loose fill and silty Transitional Beds, is unsuitable for the conventional infiltration of stormwater. Stormwater management strategies that include elements of Low Impact Development (LID) may be feasible but should have a fundamental expectation that native soils underlying facilities will consist of low permeability materials. In addition, infiltration may have negative Impacts to the slope. For these reasons, infiltration appears to be infeasible on this project. 16