5109 CHINOOK DR 2021 Plans 2024-06-11

Bradley Retaining Wall Page 4 of 4 May20,2022 Everett,Washington RGI Project No.2021-684-1 Evaluation and Conclusion RGI's geotechnical engineer performed a site reconnaissance to evaluate the stability of the site slope. During our field observations, we did not find any signs such as rotational failures, tension cracks or exposed slope surfaces indicating previous major landslide activities on the slope surface. No seeps or springs were observed on the slope face through most of the property. Review of the geology on Washington Geologic Information Portal indicates that the native soil in the project area is mapped as glacial till which is very dense silty sand with gravel and compacted by continental glaciers. RGI's geotechnical engineer probed the retaining wall foundation subgrade and found that the retaining walls are supported on very dense native soil which is consistent with geologic mapping. Based on our observation, the existing slope, retaining walls, and rockery are in stable condition. Therefore, RGI believes that most of the retaining walls are properly built and don't need to be repaired. Recommendations Typically, a Keystone wall over 4 feet in height needs to be reinforced with Geogrid to keep it stable. Most of the retaining walls are less than 4 feet in height and don't need to have Geogrid reinforcement. However, the northern portion of retaining wall (about 25 feet) are over 4 feet in height due to the grade drop. RGI recommends that Geogrids, Mirafi 5XT or approved equivalent, be installed behind the retaining walls that over 4 feet in height every 2 feet vertically. The length of the Geogrid should be same as the wall height. The retaining walls less than 4 feet in height can be backfilled with permeable soil to the top of the retaining wall For more detailed information about the Keystone retaining wall construction, please see the attached Figures 1 to 3. We trust the information presented is sufficient for your current needs. If you have any questions regarding this letter report or require additional information, please call us at (425) 415-0551. Sincerely yours, THE RILEY GROUP, INC. gUNQ? • WASbrI, , a ss r" a° 4 NAL '' Ricky R. Wang, Ph.D., PE Principal Engineer Attachments: Figure 1. Keystone Wall Location Figure 2. Keystone Wall Sections Figure 3. Keystone Wall Specifications Keystone Wall Design Calculations II 1111 I i_ RILEYGROUP