4626 HARBOR LN 2018-01-02 MF Import

l Geotechnical Engineering Evaluation NGA Fi.e No.853512 Fox Residence Underpinning and Retaining wall Design August 3, 2012 Everett, Washington Page 4 undocumented fill that supports the walkway anti a portion of the residet.cc is loose and has a high potential for subsidence in an earthquake. The risks will be reduced after the recommendations in this report arc inmplemcnted. Erosion Hazard 1-he criteria used for determination of the erosion hazard for affected areas include soil type, slope gradient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to vegetative cover and the specific surface soil types, which are related to the underlying geologic soil units. The Soil Survey of Snohomish County Area. Washineton, by the Soil Conservation Service (SCS) was reviewed to determne the erosion hazard of the on -site soils. The surface soils for this site were mapped as Alderwood-Everett gravelly sandy loams, 25 to 70 percent slopes. The erosion hazard for this material is listed as moderate to high. It is our opinion that the erosion hazard for site soils should be low in areas where the site is not disturbed. CONCLUSIONS AND [RECOMMENDATIONS General it is our opinion from a geotechnical standpoint that the issues related to fill settlement with this site can be mitigated. it is also our opinion that the northern and western portions of the garage were constructed on loose fill material. Our explorations indicate that loose fill is located below the foundation lines, garage slab, and underneath the walkway directly adjacent to the northern and western sides of the garage. We recommend that the foundations on the norther and western side of the garage be underpinned using 2-inch pin piles. We provide recommendations for pin pile installation in ule Foundation improvements subsection of this report. The structural engineer will determine the number and actual layout of the pin piles. The pin piles will likely be installed on the outside of the foundations. The garage slab may also need to be supported on pin piles or a portion of the garage slab can be removed along with associated fill to allow for placement of structural fill or a new slab. This can be evaluated during final design. It is our opinion that a Compac Keystone block wall constructed on the western side of the house to treat: a new walkwaylpatio area should provide permanent support for a walkway on that side of the residence. The existing concrete walkway and underlying fill would be removed as part of wall conctruuion. We recommend that all of the existing undocumented fill be removed and replaced with compacted structural fill with geogrid reinforcement behind the wall to prevent future settling. The underlying medium dense NELSON GEOTECHNICAL ASSOCIATES, !,'VC.