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Geotechnical Engineering Evaluation NGA File No. 1165620 Lyashenko Accessory Dwelling Unit Development April 10, 2020 Everett, Washington Page 5 CONCLUSIONS AND RECOMMENDATIONS General It is our opinion that the planned site development is feasible from a geotechnical standpoint. Our explorations indicated that the site is generally underlain by competent native glacial bearing soils at relatively shallow depths. The native glacial soils encountered at depth should provide adequate support for foundation, slab, and pavement loads. We recommend that the planned structures be designed utilizing shallow foundations. Footings should extend through any loose soil or undocumented fill soils and be founded on the underlying medium dense or better native glacial bearing soils, or structural fill extending to these soils. The medium dense or better native glacial bearing soils should typically be encountered approximately 4.0 to 7.5 feet below the existing ground surface, based on our explorations. We should note that localized areas of deeper unsuitable soils and/or undocumented fill could be encountered at this site. This condition would require additional excavations in foundation, slab, and pavement areas to remove the unsuitable soils. Alternatively, to limit excavation and grading work, especially if the proposed ADU is to be situated within the lower northeastern corner of the site, foundations could be supported on 4-inch pin piles to advance structure foundations through upper undocumented fill soils down to competent native. This option is further discussed in the Deep Foundations subsection of the report. Based on the results of our infiltration testing and the presence of groundwater seepage within our explorations, it is our opinion that the onsite native soils are not suitable for stormwater infiltration. However, shallow, low -impact design infiltration systems and/or dispersion systems may be feasible at specific areas within the site depending on final layout as determined by the civil engineer. This is further discussed in the Site Drainage section of this report. 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 not stripped of vegetation should be low. NELSON GEOTECHNICAL ASSOCIATES, INC.