3121 TULALIP AVE 2026-02-05 - Laserfiche WebLink

Geotechnical Engineering Evaluation NGA File No. 959616 Watson Residence Addition July 6,2016 Everett, Washington Page 5 Landslide Hazard/Slope Stability The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions. A steep northwest-facing slope with a gradient ranging from 30 to 35 degrees (58 to 70 percent) is located to the northeast of the existing residence. We did not observe signs of recent movement on the site slopes or obvious settlement of the existing residence foundation. Relatively shallow failures as well as surficial erosion are natural processes that could continue to occur on the steep slope. It is our opinion that while there is potential for erosion, soil creep, and shallow failures within the loose surficial and undocumented fill soils on the steep slope, the potential for deep- seated slope failures under current site conditions is low due to the competent glacial outwash soils interpreted to make up the slope. Proper site grading, setbacks and drainage as recommended in this letter should help maintain current stability conditions. CONCLUSIONS AND RECOMMENDATIONS General It is our opinion from a geotechnical standpoint that the site is compatible with the planned addition on the southern side of the residence within the relatively level grass yard area. In our opinion the planned earthwork associated with the additions should be minor and should not have adverse effects on the steep slope if conducted according to the recommendations presented in this letter. Our explorations within the proposed addition and deck area generally encountered medium dense or better native glacial soils at shallow depths. In our opinion, the native glacial soils should provide adequate support for the planned addition foundation, as well as the deck supports. We recommend that the footings be extended through any loose soil or undocumented fill and be embedded a minimum of one foot into the medium dense or better native soil underlying the site at depth. The medium dense or better soil should typically be encountered approximately one to two feet below the existing ground surface, based on our explorations. We understand that the new addition and deck will be setback approximately 50 and 43 feet from the top of the steep slope. In our opinion,these setbacks from the top of the steep slope are adequate. The recommended foundation embedment of at least one foot into the competent native soils along with the proposed structure setback should allow for normal slope recession during a reasonable life span of the structures provided that the site slope is not disturbed during or after construction. Setback and foundation details related to the planned improvements and additions are further discussed in the Structure Setback and Foundations subsection of this letter. NELSON GEOTECHNICAL ASSOCIATES, INC.