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Geotechnical Engineering Evaluation Lyashenko Accessory Dwelling Unit Development Everett, Washington NGA File No. 1165620 April 10, 2020 Page 9 foundations should be poured "neat" against the native medium dense soils or compacted fill should be used as backfill against the front of the footing. We recommend that the upper one foot of soil be neglected when calculating the passive resistance. Deep Foundations Due to the presence of significant undocumented fill within the proposed development area and especially along the lower northeastern edge of the site, we alternatively recommend that the new ADU structure foundations could be supported on 4-inch pin piles to transfer foundation loads through the upper loose soils down to the underlying native competent bearing materials, interpreted to underlie the site at depth. Our explorations did not encounter any significant debris within the upper fill soils; however, there is potential that debris within the fill has the possibility to impede some of the piles. There should be contingencies in the budget and design for additional/relocated piles to replace piles that may be obstructed by debris. We also recommend that excavation equipment be available on site during pile installation so that shallow obstructions can be removed from the planned pile locations. We recommend that four -inch pipe piles be utilized and should be driven using a tractor -mounted hydraulic hammer, with an energy rating of at least 1,100 foot -lb. For this pile and hammer size, we recommend a design capacity of eight tons for each pile driven to refusal. The refusal criterion for this pile and hammer size is defined as less than one -inch of movement during 10 seconds of continuous driving at a rate of 550 blows per minute or higher. We recommend using galvanized schedule 40 pipe for the 4-inch pin piles. Maintaining these recommendations for minimum hammer size and refusal criteria is essential for obtaining successful piles. Final pile depths should be expected to vary and will depend on the depth to competent bearing soils. We recommend that the piles extend a minimum of five feet into the competent native soils, in addition to meeting the refusal criterion. Our explorations performed as a part of this evaluation encountered competent native soils at approximately 4.0 to 7.5 feet below the existing ground surface. The piles should be spaced a minimum of three feet apart to avoid a grouping effect on the piles. Due to the relatively small slenderness ratio of pin piles, maintaining pin pile confinement and lateral support is essential in preventing pile buckling. Pin piles should be suitably embedded into the reinforced concrete. The structural engineer should design the connections of the piles to the foundations. NELSON GEOTECWNICAL ASSOCIATES, INC.