1901 W MUKILTEO BLVD Geotech Report 2022-10-25

Scriven Residence Improvements-REVISED NGA File No 974316 1901 West Mukilteo Boulevard November 29,2016 Everett,Washington Page 6 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 35 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, adequate setback distances and foundation placement as well as retaining wall construction as recommended in this report should help maintain current stability conditions. CONCLUSIONS AND RECOMMENDATIONS General Based on our recent explorations and observations of the current conditions in the vicinity of the top of slope area and review of previous explorations performed by others within the site, it is our opinion that the proposed grading activities between the top of slope and the existing residence along the proposed eastern residence addition are feasible from a geotechnical standpoint. We understand that the top of the slope will be lowered by a few feet. We recommend that the final grade of the yard area along the steep slope be graded back slightly to the south toward the residence and not toward the steep slope. All runoff generated from the hard surfaces and yard areas on this site should be directed to flow in the proposed drainage system. It is also our opinion that the soils that underlie the site and form the core of the site slope should be stable with respect to deep-seated earth movements, due to their inherent strength and slope geometry. However, shallow failures could occur on the slopes in the loose surficial soil and undocumented fill soils, especially during adverse weather or a significant seismic event. Proper erosion and drainage control measures as recommended in this report should reduce this potential. NELSON GEOTECHNICAL ASSOCIATES, INC.