1209 MADRONA AVE 2016-10-05 - Laserfiche WebLink

Geotechnical Engineering Evaluation NGA File No. 870113 Packer Property March 11,2013 Retaining Wall and Steep Slope Evaluation Summary-Page 2 Everett,Washington We performed four hand auger explorations within the retaining walls and slide area to observe the subsurface soil conditions. We also mapped the steep slope below the existing retaining walls as well. Based on our explorations and observations of the retaining wall construction,we have concluded that the retaining walls are relatively stable in their current conditions. In our opinion, the retaining walls appear to have performed favorably throughout the years and appear to be functioning as intended. Due to the location of the existing retaining walls and the steepness of the slope, removal and reconstruction of the retaining walls would likely cause more stability issues to the steep slope area than leaving the walls in their current condition. However, due to the proximity of the recent landslide and known stability issues within the steep slope below the retaining walls, we recommend the use of a "light" retaining wall to protect the steep slope area directly below the lower retaining walls within the recent landslide area,along with some erosion control and drainage improvements to be utilized above and below the new retaining wall. Also,the surficial stability within the remainder of the steep slope should be improved through the use of erosion control systems and deep-rooting vegetation where soil is exposed. We have provided recommendations for design and installation of a new pipe pile wall with timber lagging and tieback anchors to stabilize the ravine. The anchor system could consist of either grouted tiebacks or mechanically installed anchors. We have also provided recommendations for improvements on the slope to lessen the potential for future sliding. As mentioned above, the recent landslide included a portion of your neighbor's property to the west. We therefore strongly recommend, if possible, that you work with your neighbor on addressing the stability issues on both properties since these issues can additionally affect your property and the proposed improvements, if repairs on the neighboring property are not performed. Slope stability improvements such as erosion control and drainage systems would need to be conducted on the neighbor's side of the slide to help prevent future events from impacting the recommended repairs on your property and to improve overall slope stability within the two properties. We recommend that NGA be retained to review final repair plans prior to construction, and to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not earthwork activities comply with contract plans and specifications. We appreciate the opportunity to provide service to you on this project. Please contact us if you have any questions regarding this report or require further information. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, WC. Khaled A Shawish,PE Principal Four Copies Submitted