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 6 12C� northeast-facing slope well to the north of the existing residence. We also observed some minor seepage emitting from the slope face along the northeastern property line. Relatively shallow failures as well as surficial erosion are natural processes and could occur on the steeper slopes within the site during extreme weather or as a result of seismic activity. This has occurred in the past and should be expected to occur again. It is our opinion that while there is a strong potential for erosion, soil creep, and shallow failures within the undocumented fill and loose surficial soils on the steeper slopes within the site, the potential for deep-seated slope failure under current site conditions is low. Proper addition and deck foundation support and embedment, and site grading and drainage as recommended in this report should help maintain and/or improve current stability conditions. CONCLUSIONS AND RECOMMENDATIONS General It is our opinion that the planned basement addition and deck expansion are feasible from a geotechnical standpoint, provided that our recommendations are incorporated into the design and construction of this project. Our explorations within the vicinity of the proposed basement addition and deck area encountered approximately 7.5 to 10.0 feet of loose/soft undocumented fill soils underlain by medium dense/stiff or better native glacial soils. It is our opinion that the undocumented fill soils are considered unsuitable for foundation support of the planned basement addition and deck supports. Due to the close proximity of the proposed basement addition and deck foundations to the top of the steep ravine slopes and the potentially large amount of fill that would need to be removed to reach the competent native soil in planned foundation areas, we recommend that the basement addition and deck foundations be supported on pin piles extending through the fill and terminating within the underlying competent native glacial soils. We recommend that at a minimum the foundation elements for the basement addition and the northern portion of the deck closest to the top of slope area be supported on 4-inch driven steel pin piles. Due to some access constraints within the site for the pile driving equipment, the upper southern foundations for the deck may need to be supported on 2-inch pin piles that are driven with smaller hand- held equipment. The steep north- and south-facing slopes are considered stable with respect to deep-seated failures. However, there is a strong potential for shallow sloughing and erosion events to occur on the slopes within the loose surficial and undocumented fill soils, as has happened in the past. During periods of extended rainfall and/or as a result of seismic activity, shallow slough-type failures may originate on the slope and travel down slope towards the ravine areas. NELSON GEOTECHNICAL ASSOCIATES, INC.