4704 HARBOR LN 2018-01-02 MF Import

Anderson Slide Shoring Plan Comments Everett, Washington October 28, 2009 NGA Pile No. 816909 Page 2 the upper loose material from time to time which caused this material to travel down the slope under gravitational forces, as the soils lost their shear strength. The recent slide was caused by water leakage that saturated the upper soils. What the wall does is retain a portion of this material, and allow for the removal of the remainder of the loose material, as part of the planned grading below the wall, thus eliminating the source of the slides. Once the wall is installed and the loose soil below the wall is removed, the remainder of the slope would be covcrcd with erosion control material and vegetation as described in the report. Of course, the downed trees, slide debris piles, and unstable material would be removed prior to placing the erosion control elements. This is standard practice and we have successfully implemented erosion control systems on similar slopes in the nearby vicinity. This work would be completed under our supervision, and once the work is implemented, we would issue a final letter stating that the erosion control systems have been successfully completed. The other issue that seems to be a major point of contention is the final drainage picture of this site. At the time our report was prepared, our understanding was that all runoff generated on this site would be lightlined into the existing IIDPG pipe that currently conveys water to the bottom of the slope. This is still the most prudent and natural way to control runoff and prevent it from reaching the slope. I lowever, even if runoll" from the Anderson property is allowed to shect0ow towards the slope, or even if another waterline breakage would to occur in the future, water flow should not cause future slides. The wall will act as a large dispersal system and will allow any runoff reaching the wall to infiltratc/disperse uniformly over the slope below the wall. This mechanism, coupled with the recommended erosion systems below the wall, should prevent any significant future landslides. We should note that the slope continues onto the neighboring property to the north. it is crucial that the recommended erosion control systems cover all exposed sloping ground, including neighboring property, for the system to function properly. We should also note that steep slopes in this area, including the slope in question, will always be prone to minor erosion/sloughing events, especially during extreme weather. This is it natural process and should be expected from time to time. In closing, we have designed and supervised the successful installation of numerous slide stabilization systems in the Puget Sound area and within the City of Everett. There are two examples of similar projects only a few hundred feet down the road, on the bluff -side of Mukilteo Boulevard. We are more NELSON GEOTECHNICAL ASSOCIATES, INC. Z/S