4320 THOMSON AVE 2023-08-10 - Laserfiche WebLink

GeoTest Services, Inc. 4320 Thomson Avenue, Everett, WA April 12, 2019 Project No.19-0050 Bare earth imagery was obtained through the Puget Sound LIDAR Consortium, DNR Geologic Information Portal, and the Snohomish County PDS Map Portal websites. CONCLUSIONS AND RECOMMENDATIONS Ra�P� 2the evaluation ofthe data collected during this investigation the subsurface conditions at the site are suitable for the proposed develor�ment provided the recommendations contained herein are incorporated into the project design. According to the City of Everett Municipal Code 19.37.080 (Geologically Hazardous Areas), Landslide HazardAreasare 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 7, 7997. This methodology identifies several distinct definitions for landslide hazard areas. The proposed residence will be situated at the base of an approximately 40-foot tall slope with an average inclination of 47 percent. Due to the grade, height, and geologic setting, this slope would satisfy the City's definition of a Landslide Hazard Area. Furthermore, this slope would qualify as a "high" risk slope per the City of Everett due to its grade and geologic setting. Additionally, due to the presence of slopes of 25 to 40 percent that are comprised of Advance Outwash (Qva), the site slopes are considered to be a high Erosion Hazard per Sections 19.37.Go0 of tIi Everett IvlurllClpal Code. Landslide Hazards and Slope Stability As discussed previously, the existing (and proposed) residence is situated at the toe of a steep slope that extends onto the adjacent western property. Based on the soils encountered during our subsurface explorations, it appears that the core of the slope consists of dense Advance Outwash. GTS would also expect that dense Till is present to the west (upslope) of the subject property. We did not observe any noticeable signs of either shallow or deep-seated instability on the site slope during our visits to the site. Large scale global instability, consisting of deep-seated rotational failures, can extend down into the subsurface to substantial depths. These failures typically leave geomorphic evidence of their existence on the slope. Typical indicators are recessional and sometimes nested head scarps, tension cracks, sag ponds, seepage zones, hummocky ground surface and slump blocks. Obvious visual indications of large scale global slope instability, such as those referenced above, were not observed at the subject property. As such, it is our opinion that there is a low risk of large scale landslides occurring and impacting the planned development site. It should be noted that no amount of engineering can completely mitigate or prevent slope instability. In addition, please keep in mind that the Pacific Northwest is seismically active and it is difficult to predict how the slope(s) at the subject site may 0