1528 GRAND AVE Geotech Report 2022-10-25

GeoTest Services, Inc. Report Date 7/17/2019 Anderson Residence- Everett, Washington GTS Project No.19-0384 at the subject property. The western foundation wall of the existing residence is approximately 50 feet from the crest of the slope; the proposed addition will advance the residence footprint about 10 feet closer to the top of the slope for an overall setback of approximately 40 feet. Numerous other dwellings along the west side of Grand Avenue were observed to be about this same distance from the crest of the slope. The Snohomish County Online Property Information (SCOPI) portal indicates that the existing residence on the subject property was constructed in 1926, while nearby houses also situated close to the slope were built at least as early as 1905. Signs of large-scale earth movements' were not identified during site reconnaissance, historic aerial photography, or bare- earth LIDAR imagery from the Washington LIDAR portal. For these conditions, it is our opinion that the proposed 40-foot setback from the top of the slope is appropriate for the planned residential improvement, and no further landslide mitigation should be required. The proposed carport will extend west from habitable portions of the residence. Current plans suggest that the carport will be slightly over 10 horizontal feet from the existing retaining wall/top-of-slope area. Given that the carport is a lower use structure and that GTS anticipates relatively light structural loads from any posts or piers used to construct the carport, it is our opinion that a minimum 10-foot top-of- slope setback can be justified for this structure. Although the proposed carport is unlikely to have any significant impacts to the slope, reasonable mitigations can be incorporated into the construction of the carport to mitigate potential impacts. Items like deepened posts or piers to transfer structural loads deeper into the slope, not structurally connecting the carport to the residence, or providing shim spaces or threaded bolts that will allow re-leveling of critical carport elements should settlement occur would all be suitable mitigations to accompany the proposed 10- foot top-of-slope setback required to construct the carport. It should be noted that no amount of engineering can completely mitigate or prevent slope instability. The Pacific Northwest is seismically active and it is difficult to predict how the slope(s) at the subject site may behave during a large earthquake. GTS's review did not find evidence of historic, large scale slope failures on or below this project site. Although large scale failures are feasible,the more likely method of slope failure would be via small scale "skin"slides. Small scale landslides are typically failures of variable widths and shallow depths. Although these types of failures can be visually impressive, in most instances these failures only have limited impacts to the slope. Horizontal, top-of-slope setbacks and vegetation maintenance is commonly used to mitigate small scale landslide potential. Mitigation is intended to make the risk posed 4