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Geotechnical Engineering Evaluation NGA File No. 1322621 Clark Garage Slope February 4,2022 Everett,Washington Page 7 Protection of the site slope areas should be performed as required by the City of Everett. Specifically, we recommend that the site slopes not be disturbed or modified through placement of any fill or removal of the existing vegetation outside the proposed development area. We recommend that we be retained to review grading plans once plans are finalized. No additional material of any kind should be placed on either slope or be allowed to reach the slopes, such as excavation spoils, lawn clippings, and other yard waste, trash, and soil stockpiles. Vegetation should not be removed from the slopes. Replacement of vegetation should be performed in accordance with City of Everett code. Under no circumstances should water be allowed to concentrate on the slopes. The clearing of vegetation within the area of the proposed residential developments should not affect slope stability, provided the disturbed areas outside the building are revegetated as soon as practical and protected from erosion. In areas that are disturbed during or after construction, planting, hydro seeding, and/or straw mulching are effective ways to minimize erosion and allow vegetation to be re-established rapidly. 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. For all planned structures within the site we recommend a setback of at least 15 feet from the top of the steep east-facing slope on the eastern side of the property. Structures may setback as little as 10 feet from the top of slope if the down slope foundation lines are embedded a minimum of 3.5 feet below the finished ground surface to create a minimum effective setback of 15 feet. We also recommend that the planned garage addition be setback a minimum of 8 feet from the rockeries bordering the north and west sides of the site. In our opinion, an effective setback of 15 feet from the top of slope and a setback of 8 feet from the rockeries is adequate and should protect the new portions of the structure against potential failures on the slope during the life span of the addition. We should be retained to review final structure locations and to observe foundation excavations prior to placing forms. NELSON GEOTECHNICAL ASSOCIATES, INC.