4934 27TH AVE W 2017-03-10 - Laserfiche WebLink

G E® l E C H 13256 Northeast 20th Street,Suite 16 Bellevue,Washington 98005 CONSULTANTS, INC. (425)747-5618 FAX(425)747-8561 November 9, 2010 JN 10292 Bob Collett 5809— 135th Place Southwest Edmonds, Washington 98026 via email rcollett@msn.com Subject: Construction of New Deck Foundations Existing Residence 4934—27th Avenue West Everett, Washington Dear Mr. Collett: This letter presents our geotechnical observations and conclusions related to the foundations for the new deck to be built on the north side of the existing house. We have visited the subject site to meet with your architect, Tom Hagge, and to observe the existing conditions in the area of the proposed deck. The existing house is undergoing a substantial remodel. As a part of this work, a new elevated deck will be constructed along the north side of the house. This deck will be supported on isolated wood posts, and new foundations will have to be constructed. The foundations on the western portion of the deck will be located within 5 to 10 feet of a steep slope that declines down toward the northwest. From our observations, the western portion of the level area to the north of the house was created by filling over the original sloping ground when the house was initially constructed. This fill extends down to a 5-to 6-foot masonry retaining wall, below which appears to be the original sloped ground. This masonry wall is old, but it has visibly rotated outward over time, indicating that it may be inadequately reinforced and/or supported on loose soils. We observed no indications of recent instability on the steep slopes to the north and west of the existing house. During our visit, we conducted two hand-augered test borings along the proposed northern extent of the new deck. These explorations found loose, gravelly, silty sand fill extending to a depth of approximately 7 feet. This fill soil is consistent in composition to the native glacial till that is often found in the area, and it may have originated from the excavation for the existing house's basement. Beneath the fill was organics and topsoil extending to the maximum 8-foot depth that we could explore. From our previous work in the general vicinity, we expect that the old topsoil is underlain by weathered, gravelly silty sand and then glacial till. Conclusions and Recommendations The area to the north of the western portion of the existing house is comprised of fill soils that were placed over the original sloping ground when the residence was constructed. There were no visible indications of recent instability in this area, and the existing masonry wall may be lending some stabilizing effects to the fill. Even so, with any steep slope there is a potential for at least shallow soil movement as the near-surface soils become looser over time from natural weathering. Use of small-diameter pipe piles to support the new deck in the fill area would prevent the construction from adversely impacting slope stability, and provide protection for the deck foundations in the