5228 EAST DR Geotech Report 2022-10-25

COBALT GEOTECHNICAL INVESTIGATION G E o S C I E N C E S EVERETT,WASHINGTON December 5,2019 7.0 DISCUSSION 7.1.1 General The site is underlain by weathered and unweathered glacial till. The proposed residential buildings may .:be supported.on shallow foundation systems bearing on medium dense or firmer native soils and/or structural fill placed on suitable native soils. Local overexcavation of fill and/or loose soils may be necessary below proposed foundation elements. 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 Iayer. We recommend direct connection of new runoff collection devices to City stormwater infrastructure. 8.o Recommendations 8.1.1 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. There are large roots locally within the property that may need to be removed during clearing. The native soils consist of silty-sand with gravel. These soils may be used as structural fill provided they achieve compaction requirements'and arewithin 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. 6 PO Box 82243 Kenmore,WA 98028 cobaltgeoClgmail.com 206-331-1097