Laserfiche WebLink
4735 Bayview Lane <br />Everet4 Washington Geotechnical Report <br />Conclusions and Recommendations <br />Landslide Hazards <br />The side walls of the Narbeck Creek drainage channel are tall and very steep with slopes measuring <br />1H:1 V to .75H:1 V (greater than 100 percent) and is on the order of 80 feet tall. Based on our <br />evaluation this slope area is generally stable from any large landslide events and has remained so <br />for many decades. The core of the slope is dense to very dense glacial till sediments comprised of <br />silty sand with gravel underlain by alternating layers of silt, clay, sand, gravel and sometime peat <br />of the Whidbey formation. Although the Whidbey formation is known to have stability issues in <br />certain locations, these sediments, in the absence of adverse ground water conditions, are typically <br />very stable under normal geologic conditions. Our evaluation did not observe any ground water <br />conditions on the property or the slope face. <br />However, as with all steep slopes the outer soil layer becomes loosened over time due to the action <br />of tree and plant roots, insects, general weathering of the soil and even burrowing animals. As <br />these soils loosen and are able to absorb more storm water, gravity oftentimes overcomes the <br />natural cohesion of the soil particles and surficial slope movement can occur. On slopes as steep <br />as are present on the side walls of the drainage channel this shallow slope movement, or surficial <br />landsliding, usually occurs when the outer layer is between 1 to 3 feet in thickness. <br />The slopes of the broad and shallow ravine that encompasses the area from the top of the steep <br />slope back westward to the existing house, are much gentler and do not exceed 25 percent slope <br />gradient. <br />Everett city code (Chapter 19.37) defines the subject very steep slope as an area of very high <br />landslide hazard area due to the slope gradient and the presence of the Whidbey formation that is <br />apparently exposed in the sidewalls of the ravine. This very high landslide hazard area extends <br />along both sides of the ravine for hundreds of feet north and south of the site. The other site slopes, <br />comprised of dense till or glacial drift soils with slopes less than 25 percent, should be classified <br />as a low landslide hazard area. <br />As per Chapter 19.37.80.B a buffer is to be established based on the information attained through <br />the geological assessment and shall be recommended by the qualified professional. Based on our <br />findings it is appropriate to establish a buffer from the very steep landslide hazard of 50 feet. This <br />buffer should only be for structures as planting trees and bushes in this area should be encouraged <br />as it currently is only vegetated with field grass. It is our opinion that no buffer needs to be <br />established for the other sloping portions of the site that are all less than 25 percent slope gradient <br />and are underlain by dense glacially consolidated sediments. The planned construction activities <br />will not alter the critical area and will not be located within any buffer area. Once the unpermitted <br />portions of the existing residence are removed the remodeled home will be nearly 150 feet away <br />from the very steep slope/landslide hazard area. <br />Gary A. Flowers, PLLC <br />5205 23`tl Avenue W Everett, Washington 98203 206-819-4304 <br />