3301 SEAWAY BLVD Geotech Report 2022-10-25

January 13,2016 Project No. T-7342 c. Any area with all three of the following characteristics: i. Slopes greater than 15 percent. ii. Hillsides intersecting geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment or bedrock. iii. Springs,groundwater seepage,or saturated soils. d. Any area which has shown movement during the Holocene epoch(from 10,000 years ago to the present)or which is underlain or covered by mass wastage debris of that epoch. e. Any area potentially unstable as a result of rapid stream incision, stream bank erosion, or undercutting by wave action. f. Areas of historic failures, including areas of unstable, old and recent landslides or landslide debris within a head scarp, and areas exhibiting geomorphological features indicative of past slope failure, such as hummocky ground, slumps,earthflows,mudflows,etc. g. Any area with a slope of 40 percent or steeper and with a vertical relief of 15 or more feet, except those manmade slopes created under the design and inspection of a geotechnical professional, or slopes composed of consolidated rock. h. Areas that are at risk of landslide due to high seismic hazard. i. Areas that are at risk of landslides or mass movement due to severe erosion hazards.." The City of Everett Landslide Hazard, Critical Areas Map 2, dated March 2006, shows the eastern site margin is within or adjacent a medium landslide hazard area. Existing slope gradients throughout the site vary from 7 to 100 percent. Therefore,the site would be classified as a landslide hazard by the City of Everett. Section 37.080.C.3 of the EMC allows for development within geologic hazard areas provided the development will decrease the risk for the hazard. Based on the grading plan provided, it is our opinion that the final development will reduce the potential for a landslide at this site. Specifically,the grading plan removes the 100 percent slopes that are located along the southern property boundary and replaces them with retaining walls and open flat space.The other slopes are re-graded and are generally incorporated into the flat building areas. The only slope that will remain in its current condition is the south-southeast slope. This slope has a gradient between 20 and 60 percent. We did not observe indications of deep-seated instability or persistent groundwater seepage on or below this slope face, and much of the slope supports growth of relatively-straight mature coniferous trees. The proposed buildings are located outside of the 50-foot buffer that will be required for the landslide hazard, however, the proposed detention pond is located directly adjacent the steep slope. Based on review of the proposed plans, the detention pond will primarily be constructed by excavating below current site grades with some fill placed in the northeast corner where the existing slope has a gradient of approximately 30 percent. The pond will be lined to prevent stormwater from infiltrating through the soils and onto the slope. Equilibrium indicates that by removing heavier soil and replacing it with lighter water the overall stability of the slope will increase. Therefore, it is our opinion that the pond can be constructed as designed without impacting the steep slope to the east. Page No. 6