2702 59TH ST SW 2022-06-21 - Laserfiche WebLink

Memorandum to City of Everett February 28, 2022 Page 3 are often unstable on steep slopes and coastal bluffs where seepage flows in the cleaner sand layers and emerges on the slope face.Groundwater tends to infiltrate down and through the more permeable sand layers and becomes perched on the silt layers. Landslides are typically initiated as trapped water emerges on the slope/bluff face and undermines the overlying more permeable units. Episodic landslides are common where these beds are exposed along ravines and bluffs. CONCLUSIONS We reviewed hillshade imagery and topographic maps developed from available LiDAR data from 2017,2006, 2005, and 2003 obtained from the Washington Department of Natural Resources (https://lidarportal.dnr. wa.gov/).We identified a pre-existing landslide on the slope below the two properties as outlined in Figures 1 and 2.There was not a significant difference observed in the topography derived from the 2005 and the 2017 LiDAR (The 2003 LiDAR did not have sufficient resolution to compare to 2005). The 1990 black and white Google Earth photo shows the entire area pre-development to be heavily vegetated and there is no apparent change in vegetation post-development in the 2002 aerial photo, Based on this,the topographic morphology, and our experience mapping other landslides in the region we do not believe this landslide was generated by the development and was an older pre-existing landslide that has been reactivated as described below. As discussed previously, significant debris was observed within the upper portion of the landslide as shown in Figures 1 and 2. It also appears that fill may have been placed at the head of the slide during original site development, and a small amount of additional fill may have also been recently placed during recent landscaping. In our opinion the landslide appears to be the reactivation of an older landslide from the result of a combination of saturation of the surface and subsurface soils from precipitation, removal of trees and vegetation and site drainage routed to the head of the slide,combined with the mass of debris and fill that was placed at the head of the slide.Based on review of the LiDAR data,the pre-existing slide is at least 17 years old(observed in 2005 data). It appears that approximately 2 to 3 feet of additional fill was placed at the top of the slope in recent years based on observations of the head scarp and LiDAR data. Continued instability of the slide area should be anticipated depending on the depth of the landslide debris and continued potential for rainfall and runoff to enter the upper part of the slide mass. Subsurface explorations (borings), laboratory analyses, and slope stability modeling should be completed to characterize the landslide mass and develop long-term mitigation/repair measures (to be completed by others). We recommend the following short-term measures be considered by property owners at the site: • Do not place debris (vegetation, clippings, wood) and fill or other surface loads on the slope or within at least 25 feet of the crest of the slope.A greater setback distance may be appropriate based on findings in subsurface explorations and the geotechnical study. ■ Direct all surface water away from the slope and route all roof and hard-surface drains to the storm drain system away from the slope,or tightiine these to the toe of the slope where necessary. ■ Do not use the irrigation system unless an engineered slope repair and drainage system can be developed that prevents discharge into the slide mass. GeoEngineers,Inc. File No.0661-133.00