2702 59TH ST SW 2024-07-05 - Laserfiche WebLink

Geotechnical Engineering Evaluation Miller Property Slope Stabilization Everett, Washington NGA File No. 1333322 July 1, 2022 Page 10 CONCLUSIONS AND RECOMMENDATIONS General It is our opinion from a geotechnical standpoint that the top of the slide can be permanently stabilized against further landslide activity, provided that the geotechnical recommendations presented in this report are fully incorporated into project plans and followed during construction. Our explorations indicated that the upland southern yard areas along the top of slope are mantled by a relatively thick layer of fine-grained undocumented fill soils in a wet to saturated condition, which, coupled with additional recent slope modifications and heavy rainfall, were the main cause of the slide. In our opinion, stabilization of the landslide will require installation of a soldier pile wall with tieback anchors and drainage improvements. Two wall configurations are presented in this report, depending on the desired outcome and use of the property. These include wall placement along the alignment of the southern fence line (Option A) or wall placement immediately along the upper headscarp, just below the residence (Option B). Wall placement along the southern fence line would essentially restore the southern portion of the property to its former usable landscape and yard area but would require a relatively stronger wall and additional grading activities to reestablish previous yard elevation. Placement of the wall along the headscarp, closer to the residence will likely present a more economical option but would not protect the lower southern portion of the property. With this option, the southern yard areas could be lightly graded, and vegetation reestablished to reconstruct a yard area for recreational use with the understanding that further slope movement could severely impact this area in the future. All construction operations and drainage improvements planned as part of this project should be planned and completed in a manner that enhances the stability of the steep slopes, not reducing it. Major temporary cuts should be avoided but rather material removed should be performed in small sections and each section fully stabilized before further material removal is allowed. This should be supervised and approved by NGA on a full-time basis. Any excavation spoils generated during site improvements should not be stockpiled on site, but rather promptly hauled away. All current and future runoff generated within the site should be collected and routed to a permanent discharge location at the bottom of the slope or other approved existing drainage infrastructure. Under no circumstances should water be allowed to concentrate or flow uncontrolled over the slope. The vegetation cover on the slope should be evaluated for compatibility with desired slope stability conditions, and a vegetation management plan should be devised to enhance slope stability. NELSON GEOTECHNICAL ASSOCIATES, INC.