215 DORN AVE Geotech Report 2022-10-25

Geotechnical Engineering Evaluation NGA File No. 1031018 Multi-Family Residential Development April 19,2018 Everett,Washington Page 5 CONCLUSIONS AND RECOMMENDATIONS General It is our opinion from a geotechnical standpoint that the site is compatible with the planned development. Our explorations indicated that the site was underlain by a surficial layer of topsoil and undocumented fill, an intermediate layer of silty fine to medium sand, and competent glacial till soils at relatively shallow depths. The native glacial soils should provide adequate support for foundation,slab,and pavement loads. We recommend that the new structure be designed utilizing shallow foundations. Footings should extend through any loose soil,and be founded on the underlying medium dense or better native soil,or structural fill extending to these soils. The competent soil should typically be encountered approximately one to three feet below the existing surface throughout the site,based on our explorations. However,localized areas of undocumented fill may be present in unexplored areas of the site. This condition, if encountered, would require deeper excavations in foundation,slab,and pavement areas to remove the unsuitable soils. We also performed on-site infiltration testing based on the 2014 WSDOE Stormwater Management Manual for Western Washington. Our explorations generally encountered fine to coarse sand with gravel and silt that we interpreted as native glacial soils. Two small pilot infiltration tests (small PITs) were conducted at locations in the vicinity of proposed stormwater infiltration facilities at the direction of the civil engineer. Based on our on-site testing, it is our opinion stormwater infiltration is reasonably feasible within this site. This is further discussed in the Stormwater Infiltration subsection of this report. The surficial soils encountered on this site are considered moisture-sensitive and may 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 rock spalls to protect exposed subgrades and construction traffic areas. Some of the native on-site soils may be suitable for use as structural fill depending on the moisture content of the soil during construction. NGA should be retained to determine if site soils can be used as structural fill during construction. Erosion Control The erosion hazard for the on-site soils is interpreted to be slight to moderate for exposed soils,but actual erosion potential will be dependent on how the site is graded and how water is allowed to concentrate. 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. Disturbed areas should be planted as soon as practical and the vegetation should be maintained until it is established. The erosion potential of areas unstripped of vegetation should be low. NELSON GEOTECHNICAL ASSOCIATES, INC. 88