1915 W MUKILTEO BLVD 2018-01-02 MF Import

Geotechnical Engineering Evaluation and Slide Repairs 1915 Mukilteo Boulevard July 26, 2006 NGA File No. 743206 Page 7 more elaborate stabilization methods. Based on this understanding, we recommend the use of a light retaining wall to protect the top of the slope within the areas that show the most damage (namely the portion of the slope along the northern side of the residence), along with erosion control and drainage systems to be utilized above and below the retaining wall. Within the areas of the slope that have not been severely impacted by the slide, but are still at significant risk of future movement (the area northeast and east of the residence), we recommend a combination of drainage improvements and erosion control measures to reduce the potential for future sliding. We recommend that the slope be monitored on an ongoing basis after the recommended improvements have been installed to assess the future performance of these areas and the potential need for additional repairs. It should be understood that the recommended improvements are intended to reduce the rate and potential for future slope movements, but are not intended to entirely eliminate this potential. As mentioned above, a complete elimination of such potential would require implementing cost -prohibitive measures. It is our opinion, from a geotechnical standpoint, that installing the planned wall along the top of the slope along with performing surficial improvements on the slope below the wall should help reduce the rate of slope recession and sliding and erosion activities on the slope. This measure, however, should not be considered a permanent slope stabilization approach. Slope erosion and sloughing is a chronic condition in this area due to the nature of the material, site location, and the steepness of the slope. We have provided recommendations for design and installation of a pipe pile wall with timber lagging and tie- backs to protect the top of the slope. We have also provided recommendations for improvements on the slope below the wall to lessen the potential for sliding on the slope. Even with these measures, the slope should be expected to continue to erode, especially during extreme weather conditions, and the wall may ultimately need to be repaired/replaced. All grading operations and drainage improvements planned as part of this project should be planned and completed in a matter that enhances the stability of the steep slope, not reduces it. Any excavation spoils generated during site improvements should not be stockpiled near the slope or be allowed to encroach on the slope. Also, runoff generated within the site should be collected and routed to a permanent discharge location at the bottom of the slope and not be allowed to flow over the slope. Under no circumstances should water be allowed to concentrate or `.low uncontrollably over the slope. The vegetation cover on NELSON GEOTECHNICAL ASSOCIATES, INC. I Dd7