1020 NORTH PARK DR 2022-02-23 - Laserfiche WebLink

Geotechnical Engineering Evaluation NGA File No.961916 Mason Residence Addition and Deck August 24,2016 Everett,Washington Page 7 25 The proposed residence 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 development on the slope and allow for normal slope recession and shallow failures during a reasonable life span of the structure. This is further discussed in the Structure Setback subsection of this report. If slab-on-grades are planned for this project, we recommend that at a minimum, the slab subgrade be over-excavated by a minimum of two feet and the over-excavation backfilled with 2-inch crushed rock. Soft or yielding areas exposed at the bottom of the slab excavation should be further over-excavated and replaced with 2-inch crushed rock. This method should reduce the potential for future differential settlement and cracking of the slab; however, some settlement, differential settlement, and minor slab cracking should still be expected over time, as some of the undocumented fill would remain underneath the slab. Additional steel reinforcement in the slab and incorporating ample cold joints may reduce the effects of settlement. If settlement or slab distress of any kind cannot be tolerated, the slab-on-grade should be designed as structural slab and supported on pin piles. This is discussed further in the Slab-on- Grade subsection of this report. We also observed numerous drain pipes within the steep slope area along the northeastern property line below the residence. We were unable to determine the extent of these drains during our site visit, but we anticipate that these drains may be associated with the residence roof and footing drains or the neighboring residence drains. We recommend that all residence downspouts and yard drains be investigated to understand where they are directed and verify they are functioning as intended. At a minimum, we recommend that all residence downspouts and yard drains be tightlined and directed to discharge at the bottom of the slopes or to a suitable discharge location within the roadway above. This is discussed further in the Site Drainage subsection of this report. NELSON GEOTECHNICAL ASSOCIATES, INC.