4110 CLIFF DR 2022-01-26 - Laserfiche WebLink

Geotechnical Engineering Evaluation NGA File No. 1021318 Cliff Drive Short Plat Residence Development February 2, 2018 Everett, Washington Page 4 groundwater table. It is our opinion that the competent glacial outwash material interpreted to underlie the site has a low potential for liquefaction or amplification of ground motion. The glacial soils interpreted to form the core of the site slopes is considered stable with respect to deep- seated slope failures. All slopes have the potential for shallow sloughing failures during seismic events. Such events should not affect the planned structure provided the foundations are designed with the recommended embedment values and the slope and drainage systems are maintained as described in this report. Erosion Hazard The 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 Conservation Service (SCS) lists the site as Alderwood gravelly sandy loam, 15 to 30 percent slopes with a moderate erosion hazard. Based on our observations and the material encountered, we would interpret this site as having a low to moderate erosion hazard where the surficial soils are exposed. It is our opinion that the erosion hazard for site soils should be low in areas where the site is not disturbed. Landslide Hazard/Slope Stability The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions. The site is generally situated on a gentle to moderate west -facing slope that descends from the eastern property line to the western property line at gradients in the range of 7 to 14 degrees (12 to 25 percent). Approximately 100 feet to the west of the subject property a steep west -facing slope descends into a creek drainage ravine at gradients in the range of 23 to 40 degrees (42 to 84 percent). We did not observe evidence of significant slope instability within or in the immediate vicinity of the property during our investigation, such as deep-seated landsliding. We also did not observe groundwater seepage or recent signs of erosion or sloughing on the site slope at the time of our visit. The core of the slope is inferred to consist primarily of medium dense or better native glacial soils. Relatively shallow sloughing failures as well as surficial erosion are natural processes and should be expected on the steeper site slopes during extreme weather conditions. It is our opinion that while there is potential for erosion, soil creep, and shallow failures within the loose surficial soils on the site slopes, there is not a significant potential for deep-seated slope failures under current site conditions. Proper site grading and drainage as well as adequate foundation placement as recommended in this report should help maintain current stability conditions. NELSON GEOTECHNICAL ASSOCIATES, INC.