10723 19TH AVE SE Geotech Report 2023-04-07

Geotechnical Engineering Evaluation NGA File No. 1014317 Zerzura Townhome Development November 9,2017 Everett,Washington Page 5 We also performed on-site infiltration testing based on the 2012 WSDOE Stormwater Management Manual for Western Washington, as amended in 2014. Our explorations generally encountered silty fine to medium sand with gravel that we interpreted as native glacial till soils at depth. One small-scale pilot infiltration test (small PIT) was conducted within the property. Based on our on-site testing, it is our opinion that the native glacial till soils encountered within the property are not conducive to traditional infiltration systems. However, it is our opinion that shallow low-impact design infiltration systems in the form of pervious pavements, bio swales, or rain gardens may be feasible at specific areas within the site depending on the final layout. This should be further discussed during final design with the civil engineer. Due to the medium dense to very dense native glacial soils encountered within our explorations, we recommend that any shallow infiltration systems installed on the site have an overflow component directed to an approved discharge point. This is further discussed in the Stormwater Infiltration subsection of this report. The soils encountered on this site are considered moisture-sensitive, and will disturb easily when wet. We recommend that construction take place during the drier summer months, if possible. If construction is to take place during wet weather, the soils may disturb and additional expenses and delays may be expected due to the wet conditions. Additional expenses could include the need for placing a blanket of rock spalls to protect exposed subgrades and construction traffic areas. Some of the non-organic on-site soils could be used as structural fill, provided they could be compacted to specifications. This will depend greatly on the moisture content of the soils at the time of construction. NGA should be retained to determine if the on-site soils can be used as structural fill material during construction. Erosion Control The erosion hazard for the on-site soils is listed as slight to moderate. Best Management Practices (BMPs) should be used to control erosion. Areas disturbed during construction should be protected from erosion. Erosion control measures may include diverting surface water away from the stripped or disturbed areas. Silt fences and/or straw bales should be erected to prevent muddy water from leaving the site or flowing down the sloping ground surface. Stockpiles should be covered with plastic sheeting during wet weather. Disturbed areas should be planted as soon as practical and the vegetation should be maintained until it is established. The erosion potential for areas not stripped of vegetation should be low. Site Preparation and Grading After erosion control measures are implemented, site preparation should consist of removing loose soils, topsoil, and any undocumented fill from foundations, slab, and pavement areas, to expose medium dense or better native soils at depth. The stripped soil should be removed from the site or stockpiled for later use as a landscaping fill. Based on our observations,we anticipate native, medium dense or better soil to NELSON GEOTECHNICAL ASSOCIATES,INC.