5816 25TH AVE W Geotech Report 2025-06-25

Kierstein Residence Retaining Wall Everett, Washington Geotechnical Engineering Report provided to us, the sloping area where the planned retaining wall replacement project will be located is greater than 40 percent. Observations were made of the slopes at the time of our subsurface exploration in April 2019. We observed that both of the existing segmental block retaining walls exhibit indications of past landslide movements. Portions of both walls are out of alignment from their presumed original straight and near vertical arrangement. The lowermost western wall exhibits a greater amount of deflection than the uppermost eastern wall. AESI observed a tension crack in the lawn area about 5 to 10 feet east of the upper wall during a prior site visit. Localized hummocky topography and slight bulging of the ground surface were observed directly down slope of the lowermost wall. Additionally, one of the evergreen trees situated within a few feet of the toe of the lowermost wall appeared to be slightly back rotated. We understand that the lower wall failed in early 2022. No groundwater seepage was observed at the time of our field studies. Slope Stability Assessment Based on the observed surface and subsurface conditions at the site as described above, it is AESI's opinion that the landslide movements that have affected the existing segmental block retaining walls likely occurred as a result of elevated groundwater within the existing loose fill and weathered native soils that are present behind the walls and below the base of the existing walls. The existing walls appear to have been founded on loose subgrade soils and without subsurface drainage to collect subsurface groundwater. The replacement retaining wall will need to be founded on suitable foundation bearing soils, have surface water controls to limit surface water infiltration behind the new wall, and subsurface wall drainage to prevent a rise in groundwater in the wall backfill as described in this report. It is AESI's opinion that the new wall designed and constructed in conformance with our recommendations will present a low risk of damage to the subject site under either static or seismic conditions. EROSION HAZARDS AND MITIGATIONS Project plans should include implementation of temporary erosion controls in accordance with local standards of practice. Control methods should include limiting earthwork to seasonally drier periods, typically April 1 to October 31, use of perimeter silt fences, and straw mulch in exposed areas. Removal of existing vegetation should be limited to those areas that are required to construct the project, and new landscaping and vegetation with equivalent erosion mitigation potential should be established as soon as possible after grading is complete. During construction, surface water should be collected as close as possible to the source to minimize silt entrainment that could require treatment or detention prior to discharge. Timely implementation of permanent drainage control measures should also be a part of the project plans, and will help reduce erosion and generation of silty surface water onsite. July 1, 2022 ASSOCIATED EARTH SCIENCES,INC. SASlih-20180614EO01-002 Page 5