1209 MADRONA AVE 2016-10-05 - Laserfiche WebLink

Geotechnical Engineering Evaluation NGA File No. 870113 Packer Property March 11,2013 Retaining Walls and Steep Slope Evaluation Page 7 Everett,Washington Landslide Hazard The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions. A steep northwest-facing slope with gradients in the range of approximately 20 to 60 degrees (36 to 173 percent) is located on the northwestern portion of the property. The core of the slope is inferred to consist primarily of medium dense/stiff or better native silty sand and silt deposits. Inclinations of up to 60 degrees on the slope indicate high strength and internal friction angle within the underlying soils. Relatively shallow sloughing failures as well as surficial erosion are natural processes and should be expected on this slope during extreme weather conditions as has been observed recently and in the past. It is our opinion that there is significant potential for erosion, soil creep, and shallow failures within the loose surficial soils on the slope. There is also a significant potential for the existing lower wall to fail under the current site conditions if conditions are not improved as recommended in this report. Proper slope management, along with new retaining wall installation and slope and drainage improvements as recommended in this report should improve the site stability conditions. CONCLUSIONS AND RECOMMENDA'T'IONS General In our opinion, the existing tiered retaining walls appear to generally be in a stable condition and have experienced little movement throughout the years, with the exception of a localized section of one the walls which experienced failure. We have provided repair recommendations for this failing wall in the Failing Wall Repairs subsection of this report later in this report. It is our opinion that the recent sliding activity was not caused by or influenced by the presence of the block walls on the slope. The recent slope activity is rather a natural occurrence that was directly related to the steepness of the slopes, the makeup of the surficial soils, and recent heavy rainfall. It is also our opinion that the recent slides may have been partially influenced by the lack of proper drainage and suitable vegetation on the neighboring property. The slope on the neighboring property is covered with blackberry bushes,which do not provide adequate root support for the surficial soils. NELSON GEOTECHNICAL ASSOCIATES, INC.