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Geotechnical Engineering Evaluation NGA File No. 961916 Mason Residence Addition and Deck August 24,2016 Everett,Washington Page 9 f'25 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. Protection of the setback and steep slope areas should be performed as required by the City of Everett. Specifically, we recommend that the setback area and top of slope not be disturbed or modified through placement of any fill or removal of the existing vegetation. No additional material of any kind should be placed on either slope or be allowed to reach the slope, such as excavation spoils, lawn clippings, and other yard waste, trash, and soil stockpiles. Vegetation should not be removed from the slope. Replacement of vegetation should be performed in accordance with City of Everett code. Any proposed development within the slope setback area, other than light decks or patios, should be the subject of a specific geotechnical evaluation. Under no circumstances should water be allowed to concentrate on the slope. Structure Setbacks Uncertainties related to building along steep slopes are typically addressed by the use of building setbacks. The purpose of the setback is to establish a"buffer zone" between the structure and the top of the slope so that ample room is allowed for normal slope recession during a reasonable life span of the structure. In a general sense, the greater the setback distance, the lower the risk of slope failures impacting the structure. From a geological standpoint, the setback dimension is based on the slope's physical characteristics, such as slope height, surface angle, material composition, and hydrology. Other factors such as historical slope activity, rate of regression, and the type and desired life span of the development are important considerations as well. The proposed residence basement addition is planned to be as close as approximately 25 to 30 feet from the top of the existing steep northeast-facing ravine slope. The proposed deck expansion is planned to be as close as 10 to 15 feet from the top of the steep northeast-facing ravine slope. We recommend that the proposed addition foundations and deck foundations within the lower portion of the deck area along the top of the steep slope be supported on 4-inch driven steel pin piles extending through the upper undocumented fills and be embedded into the competent native glacial soils at depth. We anticipate that the pin piles will extend a minimum of 15 to 20 feet below the existing ground surface within the vicinity of the proposed addition and deck expansion. It is our opinion that the proposed addition and deck foundation setback along with supporting the basement addition and deck foundations closest to the top of slope on 4-inch driven pin piles embedded into the competent native soils should be adequate for this project. The proposed setback and deep foundation elements should limit the impacts of the proposed NELSON GEOTECHNICAL ASSOCIATES, INC.