1617 EDGEMOOR LN 2016-11-10 - Laserfiche WebLink

McKinnon Residence Retaining Wall NGA File No 910614 1617 Edgemoor Lane December 2, 2014 Everett, Washington Page 5 undocumented fill on the slope have the potential for shallow sloughing failures during seismic events. Such events should not directly affect the proposed wall provided the wall is designed and implemented as described in this letter. Landslide Hazard/Slope Stability The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions. Steep northeast- to east -facing slopes with gradients in the range of 28 to 38 degrees (53 to 78 percent) are located below the existing residence. We did not observe significant evidence of past deep- seated slopes instability or major erosion or sloughing events on this slope during our site visit. We also did not observe indications of seepage on the slopes during our visit. Our explorations and observations indicate that the core of the steep slope consists primarily of dense glacial soils. Inclinations up to 38 degrees on the slope indicate high strength and internal friction angle within the underlying soils. Relatively shallow failures as well as surficial erosion are natural processes and should be expected on these slopes especially within the loose surficial soils and the undocumented fill soils on the slopes, as has occurred in the vicinity of the existing landscaping wall. It is our opinion that while there is potential for on -going sloughing, soil creep, and shallow failures within the loose surficial and undocumented fill soils on the steep slope, there is not a significant potential for deep-seated rotational slope failures under current site conditions. Proper site grading and drainage as well as retaining wall construction as recommended in this letter should help maintain current stability conditions. CONCLUSIONS AND RECOMMENDATIONS General Based on our explorations and observations of the current conditions in the vicinity of the top of slope area, it is our opinion that the proposed installation of a pin pile cantilever retaining wall along the top of the steep slope is feasible from a geotechnical standpoint. Due to the presence of loose, undocumented fill soils within the yard and steep slope within the vicinity of the top of the steep slope, it is also our opinion that these soils may experience settlement and failures potentially impacting the back yard area, especially during a seismic event if the proposed retaining wall is not constructed. In our opinion, construction of the proposed retaining wall should aid in stabilizing the top of slope area along the yard NELSON GEOTECHNICAL ASSOCIATES, INC.