3815 SHORE AVE 2020-05-20 - Laserfiche WebLink

Geologically Hazardous Areas: Geotechnical Letter Kennedy Single Family Residence 3815 Shore Avenue March 23, 2018 RN File No, 3279-001A Page Geologic Hazards General:The City of Everett Municipal Code (EMC) section 19.37.080 designates the site as a geologic sensitive area for landslide and erosion hazards. The site-specific landslide, erosion and seismic hazards are discussed below. Landslide Hazards: The'steep slope on site is mapped by the City's Landslide Hazard: Critical Areas Map 2 as being within a very high landslide hazard area. 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 did not observe any indications of deep-seated slope failures. We observed nearby conifer trees on lots to the north and south which did not exhibit any trunk curving that would indicate past slope movement We measured the grade of the floor of the cellar-like structure adjacent to the slope on site and found the floor to dip approximately 1 degree towards the slope, indicating very little movement over the:estimated 70+ year existence of the structure. Additionally, the.BNSF right of way installed at the toe of the slope has protected the toe from erosion due to waves for decades, preserving the current geometry of the slope and decreasing the risk of large slope failure events. In our opinion, the proposed site development will not create any additional impacts to the. landslide hazard risk relative to the existing conditions. The new single family residence will maintain the approximately 70 foot setback from the top of slope. The planned setback satisfies the requirement per subsection B.2 of EMC 19.37.080 and IBC Figure 1808.7.1 that the setback be at least 40 feet from the top of slope, The plans show stormwater from the residence and driveway, along with groundwater collected in footing drains, will be collected and tightlined down the slope, which will improve the hydrologic condition of the slope. Combined, we anticipate these measures will either not affect or decrease landslide'`hazard risk at the site and adjacent properties relative to the current conditions on site. Erosion Hazard: The steep slope on site is mapped by the City's Erosion Hazard: Critical Areas Map 3 as being within a high erosion hazard area. Factors affecting erosion hazard include soil type, slope gradient, vegetation cover, and groundwater conditions. We did observe evidence of shallow surficial slope erosion in the form of scarps on the steep slope on the northwest side of the site. The grass-covered lawn currently extends to the top of slope, and becomes hummocky within'approximately 5 feet of top of slope. On the slope, the surficial soils on the steeper sections of the slope appear to be slowly sloughing based on sediment accumulated behind the recent BNSF debris wall. These slough events are expected to continue to be surficial, and are affected by surface water and man-made impacts. The risk of continued, slough events can be minimized if proper drainage is installed, vegetation on the slope is maintained or improved, and yard waste and other debris are kept off the slopes, We expect if a slough event were to occur, it would be small in scale and relatively shallow. Based on the exposure of a vent routed horizontally out of the cellar-like structure, we estimate surficial sloughing has eroded the crest of the steep slope back approximately 1 foot at that location since the structure was constructed. Areas like the grass-covered lawn with little vegetation to secure the sandy soils at the top of slope are likely to continue to erode at a higher rate. Robinson Noble, Inc.