1209 MADRONA AVE 2016-10-05 - Laserfiche WebLink

Geotechnical Engineering Evaluation NGA File No.870113 Packer Property March 11,2013 Retaining Walls and Steep Slope Evaluation Page 8 Everett;Washington Even though the walls appear to be stable, it is also our opinion that the walls were not constructed in accordance with current engineering standards or with the manufacture's recommendations for this type of wall. Similar engineered wall systems of this height and orientation are typically constructed with larger, heavier blocks interlocked with fiberglass pins,anchor geogrid reinforced backfill. To improve the existing walls to meet engineering standards would likely include removing the walls and creating significant cuts and fills on the sloping area. It is our opinion,that removal of the walls and re-grading of the slope are unfeasible due to the limited access conditions and would likely cause additional stability issues than if the walls are left in their current state. These walls have performed well since their installation but could experience some settlement and movement in the future, as has been experienced in the eastern section of one the walls. It is our opinion, however, that these walls should be left as is to reduce adverse impacts to the slope. If fixture wall distress were to occur, we recommend that we be retained to evaluate these occurrences on a case-by-case basis to provide specific recommendations for repairs to the walls. We recommend that drainage for the walls be maintained, and the condition of the walls be observed throughout the years for any changes. It is our opinion that the recent landslide and stability issues observed below the walls could significantly impact the existing wail system if conditions are not improved below the walls. Based on our observations of the current conditions in the vicinity of the recent landslide, it is our opinion that the landslide was likely due to weak surficial soils on the steep slope on both properties becoming saturated during the recent relatively wet winter and in turn experienced a reduction in their shear strength leading to the landslide. The material that slid off the neighbor's steep slope emanated from a considerably higher vertical elevation than the Packer property below. Although, it is our opinion that the observed slide only affected the loose surficial soils rather than the core of the slope, it is likely that the slope movement will continue and could eventually impact the walls if adequate measures for stabilizing the affected areas on both properties are not implemented as recommended in this report. Based on our explorations and site observations, we recommend the use of a "light" retaining wall to provide permanent support for the steep slope immediately below the lower western retaining wall in the approximate area of the recent landslide, along with erosion control and drainage systems to be utilized above and below the retaining wall. Also, the surficial stability within the remainder of the steep slope should be improved through the use of erosion control systems. We recommend that the new retaining wall be located to stabilize the slide area to the western property line and extend eastward past the slide to NELSON GEOTECHNICAL ASSOCIATES, INC.