518 51ST ST SW 2023-06-23 - Laserfiche WebLink

• I April 6,2021 Page 2 of 7 Limited Geotechnical Evaluation The glacial till consists of a nonsorted mixture of silt, sand, clay, gravel, and cobbles. These deposits typically become dense within the upper 15 feet, below a weathered zone. The outwash includes fine to medium grained sands that become denser with depth. Soil & Groundwater Conditions As part of our evaluation,we excavated one hand boring to 6 feet below grade. The hand boring encountered approximately 6 inches of topsoil and vegetation underlain by approximately 5 feet of loose, silty-fine to medium grained sand trace to some gravel (Fill?/Weathered Till?). This layer was underlain by medium dense,silty-fine to medium grained sand with gravel(Glacial Till) which continued to the termination depth of the hand boring. Groundwater was not encountered in the hand boring. Geologic Hazards Most critical area ordinances designate slopes with magnitudes greater than about 40 percent and vertical relief of at least io feet as potentially geologically hazardous (steep slope/landslide hazards). We anticipate that the southern portion of the site and areas further south consist of steep slope and erosion hazard areas. The following are code items with our comments underlined after the relevant sections. A. Designation. The following geologically hazardous areas shall not be altered except as otherwise provided by this chapter: 1. Landslide hazard areas: a. Those areas defined as high and very high/severe risk of landslide hazard in the Dames and Moore Methodology for the Inventory, Classification and Designation of Geologically Hazardous Areas,City of Everett,Washington:July 1,1991,or as revised through best available science: (1) Very high/severe: slopes greater than fifteen percent in the Qtb, Qw, and Qls geologic units; and slopes greater than fifteen percent with uncontrolled fill. (2) High: slopes greater than forty percent in all other geologic units (not Qtb, Qw, and Qls or uncontrolled fill). b. Those areas defined as medium risk of landslide hazard in the Dames and Moore Methodology for Inventory, Classification and Designation of Geologically Hazardous Areas, City of Everett,Washington:July 1, 1991,or as revised through best available science,when combined with springs or seeps,immature vegetation,and/or no vegetation: (1) Slopes less than fifteen percent for Qtb,Qw,and Qls geologic units and uncontrolled fill. (2) Slopes of twenty-five percent to forty percent in all other geologic units. c. Any area with all three of the following characteristics: (1) Slopes greater than fifteen percent;and (2) Hillsides intersecting geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment or bedrock;and (3) Springs,ground water seepage,or saturated soils. d. Any area which has shown movement during the Holocene epoch (from ten thousand years ago to the present)or which is underlain or covered by mass wastage debris of that epoch. www.cobaltgeo.com (206)331-1097