417 TAMARACK AVE Geotech Report 2025-08-19

September 23,2022 Page 3 of io Geotechnical Evaluation Conclusions and Recommendations General The site is underlain by fill and at depth by weathered and unweathered glacial till which becomes denser with depth. The proposed residential structure may be supported on a shallow foundation system bearing on medium dense or firmer native soils or on structural fill placed on the native soils. Local overexcavation or recompaction of loose weathered native soils may be necessary depending on the proposed elevations and locations of the new footings. Overexcavation of any undocumented fill is required below footing areas extending outward at a 1H:1V envelope from all footing edges. Widespread infiltration is not feasible due to the presence of widespread fill and a restrictive layer at shallow depths. We anticipate that perforated or direct connection to City or County infrastructure will be utilized. We can provide additional recommendations once a civil plan has been prepared. Other shallow systems could be feasible depending on their locations and elevations. Systems other than dispersion devices should not be situated in or above undocumented fill. 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 18 inches. Deeper excavations will be necessary below large trees and in any areas underlain by undocumented fill. We note fill to 3 feet below grade in some places. The native soils consist of silty-sand with gravel. Most of the native 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. Temporary Excavations Based on our understanding of the project,we anticipate that the grading could include local cuts on the order of approximately 3 feet or less for foundation and most of the utility placement. Any deeper temporary excavations should be sloped no steeper than 1.5H:IV(Horizontal:Vertical) in loose native soils and fill, 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 www.cobaltgeo.com (206)331-1097