202 ALDER ST Geotech Report 2025-07-30

Limited Geotechnical Engineering Study View Ridge Elementary School Portable Classroom Installation Everett, Washington and Downspout Infiltration Feasibility The silty interbeds observed in the upper portion of the Vashon advance outwash will likely restrict infiltration rates if encountered in the base of the trench. To mitigate the potential reduced infiltration rates, we recommend that the base of the downspout infiltration trench be excavated through the fill and situated a minimum of 3 feet into the Vashon advance outwash and set approximately 7 feet below the ground surface. Infiltration Facility Design and Construction Recommendations We have the following comments and recommendations: • The infiltration facility base must be excavated through any encountered existing fill and unsuitable soils and the base of the infiltration facility should be embedded a minimum of 3 feet into the existing outwash. In our opinion, downspout infiltration facilities completed at this depth will provide adequate separation between the bottom of the infiltration trench and the seasonal high groundwater table while penetrating through the silty interbeds observed in the upper portion of the native advance outwash. • Overflow Path: We recommend an overflow path be specified such that runoff above the facility's design capacity does not cause flooding of a building or emergency access, erosion, or downstream sedimentation. • Subgrade Preparation: We recommend constructing infiltration trenches with an excavator with a toothed bucket that will scarify the subgrade without tracking over it. If the excavation of the overlying fill or finer-grained soils occurs to a greater depth than the design subgrade, the fill or unsuitable sediments should be removed and replaced with an engineered drainage aggregate. We are available on request to assist in specification of an appropriate drainage aggregate. • Temporary Excavation and Sequencing: Careful consideration should be paid to sequencing, excavation techniques, staging of backfill materials,and backfill installation methods to limit the potential for caving of the infiltration trench sidewalls. • Unsupported temporary slope angles should be limited to inclinations of 1.5H:1V (Horizontal:Vertical) or flatter. If excavation sidewalls are not stable sidewall caving will contaminate infiltration receptor soils, requiring additional work to restore caved and contaminated areas.Slip box shoring or similar contractor means and methods are likely to be needed to construct the downspout infiltration facilities as planned. July 12,2023 ASSOCIATED EARTH SCIENCES,INC. ecr/Id-20100255E004-002 Page 9