1329 MADRONA AVE Geotech Report 2022-10-25

Subsurface Exploration,Geologic Hazard,and Janowicz Property Geotechnical Engineering Report Everett,Washington Geologic Hazards and Mitigations II. GEOLOGIC HAZARDS AND MITIGATIONS • The following discussion of potential geologic hazards is based on the geologic, slope, and shallow groundwater conditions,as observed and discussed herein. 5.0 LANDSLIDE HAZARDS AND MITIGATION The City of Everett identifies landslide hazard areas based on certain geologic, topographic, hydrologic, and historic conditions. The sloping terrain and landslide deposits (Qls) found on the subject project meet the criteria for a very high/severe landslide hazard area as defined in Everett Municipal Code (EMC) 19.37.080.1. The definition is stated as "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." It should be noted that the steep slope to the northwest of the site is located on adjacent private property (the railroad right-of-way) and therefore our ability to comment on the slope hazard here is limited to review of existing geologic mapping,our visual observations from the project site, and our nearby borings. However, it is likely that the steep slope was created • during the development of the railway corridor. The condition of the slope along the northwest side of the property is typical of slopes in the Puget Sound. There is evidence of small-scale erosion and surface slope movement in the area. Due to weathering and/or groundwater seepage, the lower slopes are inherently unstable and commonly exhibit surface failures. Comparing the existing lower slope with the surrounding property may indicate that the top of the slope could migrate back to an inclination similar to the slopes on the adjacent properties to the north and south. In order to mitigate landslide hazards and improve slope stability to allow construction on the subject lot within the landslide hazard area,we offer the following recommendations: • Deep foundations should be used to carry building loads through landslide deposits to competent bearing stratum below as determined by a geotechnical engineer. • All site development should be set back a minimum distance from the top of steep slope not less than twice the height of the steep slope, about 50 feet. This setback area should not be loaded with such loads as material stockpiles and heavy equipment. • All excavations deeper than 4 feet made in colluvium or landslide deposits should be shored during construction. Permanent cuts should be retained by retaining walls or basement walls founded on suitable bearing stratum and designed to withstand lateral pressures imposed by colluvium and landslide deposits. May10,2018 ASSOCIATED EARTH SCIENCES,INC. NS/1d-170649E002-2-ProJects120170649lKE1WP Page 6