1343 MADRONA AVE Geotech Report 2022-10-25

Geotechnical Engineering Report Stark Residence Addition 1343 Madrona Avenue Everett, Washington December 8, 2014 RN File No. 2902-001A Page 4 loose, or disturbed soils, and bear on the underlying medium dense or firmer native soil, or on structural fill extending to these soils. Based on our site explorations, we anticipate these soils will generally be encountered at typical footing depths. Geologic Hazards Landslide Hazards: The core of the site is inferred to be composed of glacially overridden soils. We consider these soils to be of high strength and considered to be stable with regard to deep-seated slope failures. We observed surficial seepage at the bottom of the slope. We did not observe indications of shallow or deep-seated slope failures except for some possibly previously disturbed soil about 4.5 feet below the toe of the slope. There is a potential that the surficial soils on the steeper sections of the slope could slough over time. Any slough events are expected to be surficial, and are affected by surface water and man-made impacts. The risk of slough events can be minimized if proper drainage is installed, vegetation on the slope is maintained, and yard waste and other debris are kept off the slopes. We would expect if a slough event were to occur, it would be small in scale and relatively shallow. We did not observe any indication of recent sloughing on site. 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 (group classification), which are related to the underlying geologic soil units. We reviewed the Web Soil Survey by the Natural Resources Conservation Service (NRCS) to determine 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 (Unit 4). The corresponding geologic unit for these soils is till, which is in general agreement with the transitional bed soils encountered in our site explorations. The erosion hazard for the soil is listed as being severe for the moderately to steeply sloping conditions at the site. Seismic Hazard: It is our opinion based on our subsurface explorations that the Soil Profile in accordance with the 2012 International Building Code (IBC) is Site Class D with Seismic Design Category D. We used the US Geological Survey program "U.S. Seismic Design Maps Web Application." The design maps summary report for the 2012 IBC is included in this report as Appendix A. Additional seismic considerations include liquefaction potential and amplification of ground motions by soft soil deposits.The liquefaction potential is highest for loose sand with a high groundwater table. The underlying medium dense to very dense transitional beds are considered to have a low potential for liquefaction and amplification of ground motion. Slope Stability Analysis We analyzed global stability using a computer program by Rocscience known as Slide, version 6.0. Slide is a two-dimensional, limit-equilibrium, slope stability program for evaluating the safety factor or probability of failure, of circular or non-circular failure surfaces in soil or rock slopes. Slide analyzes the stability of slip surfaces using vertical slice limit equilibrium methods. Robinson Noble, Inc