1209 MADRONA AVE 2016-11-17 - Laserfiche WebLink

Geotechnical Engineering Evaluation Packer �'roperty Retaining t�'ali and Steep Slope Evaluation E�jerert, R'ashingtan NGA File No. 876113 March 11, 2013 Sun�niary - Page 2 VF�e 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 eaisting retaining u�alis as u�ell. Based on our explarations and obsenfations of tl�e retaining wall construction, we have concluded that the retaining ��-alls are relatively stable in their current conditions. In our opinion, the retaining walis appear to have performed favorably throughout the years and appear to be functioning as intended. Due to tlie location of the existing retaining walls and the steepness of the slope, removal and reconstruction of the retaining walls would Iikely cause more stability issues to the sieep slope area than leaving the u�alis in their curreni condition. However, due to the proximity of che recent landslide and known stability issues ��ithin the steep slope below the retaining walls, we recommend tl�e use of a"light" retaining wall to protect ihe steep slope area directly belou� the lower retainin� ��alls within the recent landslide area, alang with sozne erosian control and drainage improvements to be utilized above and belau� the new reiaining «�all. Also, the surficial stal�ility within the remainder af the steep slope should be improved through the use of erosion control systems and deep-rooting ve�etation where soil is exposed. VJe have provided recommendations for design and instaFlation of a new pipe pile wall with timber laggin� and tieback anchors to stabilize the ravine. The anchor system could consist of either grauted tiehacks or mechanically instailed anchors. We have also provided recommendations far impravements on the siope to lessen the potential for future sliding. As inentioned above, the recent landslide included a portion of your naighbor's property to the west. We therefore strongiy recommend, if possible, that you wark with yaur neighbor on addressing the stabi?ity issues on both properties since these issues can additianally affect your property and the prop�sed impravements, if re�airs on the neighboz-ing property are not perf'onned. Slope stability improvements such as erosion control and drainage systems would need to be eonducted on the neighbor's side of the slide to help prevent future events from impacting the recamrnended repairs on your property and to im�rove overall slape stability within tlle two properties. w'e reconlmend that NGA be retained to review fina] repair plans prior to construction, and to provide manitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recoznmendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate u�hether or not earthwork activities comply with contract plans and specifications. We appreciate the opportunity to pravide service to you on this project. Please contact us if you have any questions regarding this report or require further info�nation. 5incereiy, �ELSOtV GEOTECHNfCAL ASSOCIAT'ES, ll�'C. Khaled M. Shawish, PE Pt-incipal Four Copies Subinitted