1023 100TH ST SW Geotech Report 2022-10-25

COBALT GEOTECHNICAL INVESTIGATION GEosciENcEs EVERETT, WASHINGTON December 3,2019 Infiltration of stormwater runoff is not feasible at this site. The property is underlain by dense to very dense glacial till which is nearly impermeable and acts as a restrictive layer. Permeable pavements or rain gardens may be used if necessary. However, both types of systems should be situated at least 12 inches above the unweathered glacial till. If necessary, detention systems with overflow to City stormwater infrastructure may be utilized. We can provide additional recommendations once a design has been.prepared. 8.0 Recommendations S.1.1 Site Preparation Trees, shrubs and other vegetation should be removed prior to stripping of surficial organic-rich soil and fill. Based on observations from the site investigation program, it is anticipated that the stripping depth will be 6 to 12 inches. Deeper excavations will be necessary below large trees, existing foundation areas, and in any areas underlain by undocumented fill materials. The native soils consist of silty-sand with gravel. These soils may be used as structural fill provided they achieve compaction requirements and are within 3 percent of the optimum moisture. Some of these soils may only be suitable for use as fill during the summer months, as they will be above the optimum moisture levels in their current state. These soils are variably moisture sensitive and may degrade during periods of wet weather and under equipment traffic. 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 4 feet or less for foundation and utility placement.Any deeper excavations should. be sloped no steeper than 1.5H:1V in loose soils, 1H:1V in medium dense native soils and 3/4H:1V in dense to very dense native 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. 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 10 feet of the top of any temporary cut slope. 4 PO Box 82243 Kenmore,WA 98028 obaltgeo(Qgmail.com 206-331-1097