600 54TH ST SW I Geotech Report 2025-01-29

GEOTECHNICAL INVESTIGATION EVERETT,WASHINGTON March 2,2018 The native soils below the vegetation and topsoil(and fill)consist of glacial till and outwash-like materials. These materials are generally considered suitable for use as structural fill provided they are within 3 percent of the optimum moisture content. It should be noted that glacial till soil materials are typically suitable for structural fill during the summer months only if they can be dried to optimum moisture levels. The sandy outwash materials may require some drying;however,the fines content was generally low,indicating suitability during a majority of weather conditions. Imported structural fill should consist of a sand and gravel mixture with a maximum grain size of 3 inches and less than 5 percent fines(material passing the U.S.Standard No.200 Sieve). Structural fill should be placed in maximum lift thicknesses of 12 inches and should be compacted to a minimum of 95 percent of the modified proctor maximum dry density,as determined by the ASTM D 1557 test method. 8.1.2 Temporary Excavations Based on our understanding of the project,we anticipate that the grading could include local cuts on the order of approximately io feet or less for detention vault construction. The location of any stormwater vault has not been finalized;therefore,the temporary excavation angles cannot be fully determined. For reference,excavations should be sloped no steeper than 3/4H:1V(Horizontal:Vertical)in dense to very dense glacial till(where present),1H:1V in medium dense to dense native soils,and 1.5H:1V in fill soils. If an excavation is subject to heavy vibration or surcharge loads,we recommend that the excavations be sloped no steeper than 2H:1V,where room permits. We can provide specific temporary slope angles once the locations of any excavations have been determined. Temporary cuts should be in accordance with the Washington Administrative Code(WAC)Part N, Excavation,Trenching,and Shoring. Temporary slopes should be visually inspected daily by a qualified person during construction activities and the inspections should be documented in daily reports. The contractor is responsible for maintaining the stability of the temporary cut slopes and reducing slope erosion during construction. Temporary cut slopes should be covered with visqueen to help reduce erosion during wet weather,and the slopes should be closely monitored until the permanent retaining systems or slope configurations are complete. Materials should not be stored or equipment operated within io feet of the top of any temporary cut slope. Soil conditions may not be completely known from the geotechnical investigation. In the case of temporary cuts,the existing soil conditions may not be completely revealed until the excavation work exposes the soil. Typically,as excavation work progresses the maximum inclination of temporary slopes will need to be re-evaluated by the geotechnical engineer so that supplemental recommendations can be made. Soil and groundwater conditions can be highly variable. Scheduling for soil work will need to be adjustable,to deal with unanticipated conditions,so that the project can proceed and required deadlines can be met. If any variations or undesirable conditions are encountered during construction,we should be notified so that supplemental recommendations can be made. If room constraints or groundwater conditions do not permit temporary slopes to be cut to the maximum angles allowed by the WAC,temporary shoring systems may be required. The contractor should be responsible for developing temporary shoring systems,if needed. We recommend that Cobalt Geosciences and the project structural engineer review temporary shoring designs prior to installation,to verify the suitability of the proposed systems. 7