11625 AIRPORT RD SONRISE CHRISTIAN CENTER Geotech Report 2022-10-25

I GEOTECHNICAL EVALUATION EVERETT,WASHINGTON February 6,2018 Water table elevations often fluctuate over time. The groundwater level will depend on a variety of factors that may include seasonal precipitation, irrigation, land use, climatic conditions and soil permeability. Water levels at the time of the field investigation may be different from those encountered during the construction phase of the project. 6.o Geologic Hazards 6.1 Erosion Hazard The Natural Resources Conservation Services(NRCS)maps for Snohomish County indicate that the site is underlain by Everett very gravelly sandy loam (o to 8 percent slopes). These soils have a "Moderate" erosion potential in a disturbed state. It is our opinion that soil erosion potential at this project site can be reduced through landscaping and surface water runoff control. Typically erosion of exposed soils will be most noticeable during periods of rainfall and may be controlled by the use of normal temporary erosion control measures, such as silt fences,hay bales,mulching,control ditches and diversion trenches. The typical wet weather season,with regard to site grading,is from October 3ist to April 1st. Erosion control measures should be in place before the onset of wet weather. 7.o DISCUSSION 7.1.i General It is our opinion that new pavements may be supported by the underlying weathered and unweathered glacial till and/or structural fill. The native soils are moisture sensitive and will degrade if earthwork occurs during the wet season. If the native soils will be used as structural fill to create a level parking lot and/or to backfill any detention vaults, we recommend that the work take place during the dry grading season (generally June through September). Some drying of the near surface native soils should be expected. Due to high groundwater levels,a shallow depth to a restrictive layer, and a general low permeability,the near surface soils are not suitable for infiltration of stormwater runoff. Runoff should be managed through other shallow devices and/or a combination of detention and flow control with overflow to an approved location. 8.o Recommendations 8.1.i Site Preparation Trees, shrubs and other vegetation should be removed prior to stripping of surficial organic-rich soil. Based on observations from the site investigation program, it is anticipated that the stripping depth will range from 6 to 18 inches. Deeper excavations will be necessary to remove unsuitable fill materials and below large trees where roots will likely extend to greater depths. 3 PO Box 82243 Kenmore,WA 98028 (206)331-1097 Cobaltgeo(@gmail.com