1907 W MUKILTEO BLVD 2021-11-15 - Laserfiche WebLink

• Pearson Property Slope Stabilization NGA File No 945216 1907 West Mukilteo Boulevard February 19,2016 Everett,Washington Page 5 The medium dense or better native glacial soils interpreted to form the core of the site slope are considered stable with respect to deep-seated slope failures. However, the loose surficial materials and 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 our recommendations as described in this report are followed. Landslide Hazard/Slope Stability The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions. A steep west-facing slope with gradients up to 38 degrees (78 percent) is located below the existing residence. We did not observe significant evidence of past deep-seated slopes instability, or major erosion or sloughing events, on the slope directly below the residence during our site visit. However, indications of settlement and erosion were observed within the upper surficial soils along the top of the steep slope. We also observed evidence of surficial instability on the neighboring property to the west. Some groundwater seepage was observed emitting from near the toe of the steep slope. Our explorations and observations indicate that the core of the steep slope consists primarily of medium dense or better 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. There is evidence that such events have taken place on the steep slopes in the vicinity of this site. 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 report 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 soldier 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 continued settlement and failures potentially impacting the back yard area and residence, especially during a seismic event if a retaining wall is not constructed. In our NELSON GEOTECHNICAL ASSOCIATES, INC.