4735 GLENHAVEN DR Geotech Report 2022-10-25

Geotechnical Engineering Evaluation NGA File No. 1113119 Dutton Foundation Stabilization and Slope Evaluation July 3,2019 I�� Everett,Washington Page 5 Landslide Hazard/Slope Stability The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions. Site slopes range in inclination from moderately to gently in the west,to steeply east facing in the east. Site slopes were measured to be up to 33 degrees (65 percent grade). We did not observe evidence of recent sloughing events or deep-seated landslides on the site slopes, or groundwater seepage emitting from the slope. It is our opinion that the core of the slope is stable and that the recommended pin piles should terminate in stable soils. Inclinations in the range of 33 degrees on the slope indicate internal strength and a high internal friction angle within the underlying soils. 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 site slopes, there is not a significant potential for deep-seated slope failures under current site conditions. CONCLUSIONS AND RECOMMENDATIONS General Based on our observations, our understanding of the existing foundation conditions, and the presence of loose fill soils underlying the foundations, it is our opinion that the settlement and distress to the residence is a direct result of on-going consolidation of loose undocumented fill soils that exist below the residence addition foundations. These soils will likely continue to experience settlement and subsidence under the current loads, and at an increased rate as a result of an earthquake or significant amount of rainfall, which could lead to more distress to the addition foundation and structure. To mitigate this condition and support the residence, the existing residence foundation should be underpinned by a deep foundation system,such as pin piles. We recommend that at a minimum, the exterior foundation lines of the residence addition on the southeastern side of the house be supported on deep foundations consisting of driven pin piles,to transfer the building loads through the loose undocumented fill down to the underlying competent native soils. Your structural engineer should consult with us to determine the extent of foundation areas to be underpinned, and overall layout of the underpinning system. Portions of the residence may be able to be re-leveled under the direction of the structural engineer and the contractor. We should review underpinning plans prior to implementation. NELSON GEOTECHNICAL ASSOCIATES,INC.