4704 HARBOR LN 2018-01-02 MF Import

Geotechnical Engineering Evaluation Anderson Landslide Repairs 4704 I larbor Lane I Everett, Washington September 15, 2009 I NGA Pile No. 816909 Page 9 However, the overlying loose surlicial materials on the slope including all of the landslide debris across the entire slope have a serious potential for shallow sloughing failures during seismic events. Rcconunendations presented in this report for permanent slope stabilization, drainage improvements, and erosion control systems should greatly reduce this potential. Erosion Hazard The erosion hazard criteria used for determination of affected areas includes 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, Washington, by the Soil Conservation Service (SCS) was reviewed to detennine the erosion hazard of the on -site soils. The site surface soils were classified using the SCS classification system as Alderwood-Everett gravelly sandy loam, 25 to 70 percent slopes. This unit is listed as having a high erosion hazard for areas where the soils arc exposed. It is our opinion that the Ierosion hazard for the entire slope should be considered high due to the recent and past landsliding events. However, in areas where appropriate erosion control and vegetation systems can be implemented and maintained, the erosion hazard for the site soils should be considered low to moderate. I_ Landslide Hazard/Slope Stability The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions. A steep north -facing slope with a gradient of up to approximately 40 di•grees (84 percent) with a height of approximately 100 to 120 feet is located below the existing residence and pool. We observed evidence of signif icant past and recent land movement and sloughing along the entire slope. Soil subsidence was noted along the top of the slope. An older landslide scarp was observed to the west of the recent slide, lowards the western property line. The exposed scarp of the recent landslide is approximately 70 feet in length and up to approximately 30 feet wide. We observed a second, larger scarp and slump to the west of the recent slide. We also observed minor slope failures along the middle portion of Ilse slope to the west and cast of the recent slide, and on the property to the cast of the site. These failures were located in the vicinity of the sand and silt contact and groundwater seepage on the slope. It was difficult to closely examine these failures due to the amount of large tree debris that has NELSON GEOTECHNICAL ASSOCIATES, INC. IS