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 foundation bearing pads may be assumed to have an allowable base friction coefficient of 0.30 for concrete foundation elements cast against compacted crushed rock or structural fill. Foundation bearing pads cast neat against on-site soils without the use of forms, or bearing pads backfilled with structural fill compacted to 95 percent of the modified Proctor maximum dry density, may be designed using an allowable passive resistance of 250 pounds per cubic foot (pcf) expressed as an equivalent fluid. Both of these are allowable values. Structural design of the buildings should follow 2018 International Building Code (IBC)standards using Site Class"D" as defined in Table 20.3-1 of American Society of Civil Engineers(ASCE) 7-16 Minimum Design Loads for Buildings and Other Structures. Infiltration Feasibility Our assessment of infiltration feasibility includes general surface characterization, subsurface exploration, and grain-size testing. It is our understanding that the City of Everett has adopted the 2019 Washington State Department of Ecology Stormwater Management Manual for Western Washington (Ecology Manual),with additional stormwater requirements established by the Director, found in Chapter 4 of the Design and Construction Standards and Specifications (DCSS). The feasibility of stormwater infiltration depends upon the presence of a suitable native soil receptor of sufficient thickness, extent, permeability, and vertical separation from groundwater. Based on the subsurface conditions encountered during our exploration, it is our opinion that downspout infiltration and other shallow infiltration best management practices (BMPs), are feasible based on Volume 5, Chapter 4, BMP T5.10A of the Ecology Manual. This section states that downspout infiltration is feasible if the depth to seasonal high water table, hardpan, or other low-permeable layer is 3 feet or more from the proposed final grade,and 1 foot from the facility subgrade, and if the infiltration system can be designed to meet the minimum design criteria identified in BMP T5.10A. The stormwater infiltration design should rely on the existing native advance outwash sediments below existing fill. The following sections present downspout infiltration system design and construction recommendations. Grain-Size Based Design of Downspout Infiltration Systems The Ecology Manual (BMP T5.10A) provides a prescriptive roof downspout design procedure based on grain-size testing of the infiltration receptor sediments. We recommend that the downspout infiltration trench lengths use design procedures consistent with BMP T5.10A and a receptor soil gradation of "Fine sand, loamy sand." July 12,2023 ASSOCIATED EARTH SCIENCES, INC. BcY/Id-20100255E004-002 Page 8