1915 W MUKILTEO BLVD 2018-01-02 MF Import

Geotechnical Engineering Evaluation and Slide Repairs 1915 Mukilteo Boulevard July 26, 2006 NGA File No. 743206 Page 6 foot tall face of exposed soils near the top of the slope towards the western property line. The area of sloughed soil was about 30 feet wide, 35 feet tall and exposed sand with silt material. We observed a second, smaller scarp located near the northeastern portion of the top of the slope. The scarp was approximately six feet tall and was covered by sloughed material and grass. The majority of the slope remained covered with vegetation and we did not observe any other areas of significant erosion of the slope. Our explorations indicate that the core of the steep slope consists primarily of medium dense to very dense sand. Inclinations of up to 40 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, as has already occurred. It is our opinion that while there is potential for erosion, soil creep, and shallow failures within the loose surficial soils on the steep slope, there is not a significant potential for deep-seated slope failure under current site conditions. Proper site grading and drainage as well as stabilization techniques as recommended in this report should help improve current stability conditions. CONCLUSIONS AND RECOMMENDATIONS General Based on our observations and subsurface explorations, it is our opinion that the slope movement that was experienced at this site was due to saturation of loose material that was placed along the top of the slope in the past. It appears that during the heavy rainfall of the past winter, these loose soils became over- saturated and in turn experienced a significant reduction in their shear strength, which led to the slide. Although, it is our opinion that the slide only affected surficial loose soils rather than the core of the slope, It is likely that the slope movement will continue and will eventually impact the backyard and the residence, if adequate measures for stabilizing the affected areas are not implemented. Based on our explorations and site observations, it is our opinion that permanent stabilization of the site slope and prevention of further movement would require massive retaining walls and/or major slope buttressing and regrading. Based on our discussions with the property owners, we understand that they desire to improve the slope stability conditions using more surficial and less costly measures. Their goal is to reduce the potential and rate of future slope movement to lessen the adverse impacts on their backyard and residence, without incurring the major expenses that would otherwise be associated with the NELSON GEOTECHNICAL ASSOCIATES, INC. �9a�