4704 HARBOR LN 2018-01-02 MF Import

Geotechnical Engineering Evaluation Anderson Landslide Repairs I 4704 harbor Lane Evercti, Washington September 15, 2009 I NGA Pile No. 816909 Page 14 Pernianent stabilization of the site slope and prevention of further movement will require a retaining wall and some slope re -grading and protection from erosion. We have recommended a soldier pile wall, up to 14 feet in exposed height, for stabilizing the upper portion of the slope. This wall should extend from the eastern property line, across the slide area, and be tenninated near the western property line, as indicated on the Site Plan. Tic -backs will be needed within the tallest portions of the shoring wall to provide additional load -carrying capacity and reduce loading on the lower portions of the slope. This is discussed further in the Tie -Backs subsection of this report. A schematic detail showing the shoring and design loads are presented as Figures D-1 and D-2 in Appendix D. Groundwater was not encountered while drilling the explorations, but groundwater was observed emanating from file slope at several locations. In July, temporary water seepage was observed emanating front of the • lope within the slide scarp approximately 15 feet below the top or slope. ']'his seepage was attributed to the water leakage that we concluded as the cause of the slide. Pennanent water seepage was observed approximately 50 to 60 feet below the top of slope. We do not believe that the installation of the recommended wall will change the hydrologic make-up of the area since the majority d the water currently emitting front the slope will be well below the bottom of the wall, and therefore the wall should not change the overall drainage picture of the slope. Also, vegetation will be restored on the slope which should improve the evaportranspiration capabilities of the slope. The slopes below and in the vicinity of the wall will also need to be permanently stabilized. For this, we recommend that loose material left below the wall be removed and the resulting slope covered with erosion control material and property vegetation. This is further discusses in the Slope Improvements and Drainage subsection of this report. All grading operations and drainage improvements planned as pan of this development should be planned and completed in a matter that enhances the stability of the steep slope, not reduces it. Excavation spoils should not be stockpiled on or near the slope, nor should they be allowed to encroach the top of the slope. Also, nutoff generated within the site should be collected and routed into a permanent discharge system and not be allowed to flow over the slopes. Future vegetation management on the slope should be the subject of it specific evaluation and a plan approved by the City of Everett. The slopes and wall should be monitored on an on -going basis, especially during the wet season, for any signs of instability, and NELSON GEOTECHNICAL ASSOCIATES, INC. 20