4704 HARBOR LN 2018-01-02 MF Import

Geotechnical Engineering Evaluation Anderson Landslide Repairs I 4704 Harbor lane Everett, Washington September 15, 2009 NGA File No. 816909 Page 11 'fhe subsurface soil deposits encountered on the site have various characteristics and densities depending on their origin and history since deposition. friction angles, cohesion values, and densities of these soils are impacted by their composition, their degree of over consolidation, and any subsequent disturbance by past glacial action or earth movements. We have used information gathered from our field explorations, laboratory tests, and observations, and have assigned soil properties to the analyzed soil layers to reasonably reflect their engineering properties. 'fhe results of the stability analyses arc presented as Figures C-1 through C-12 in Appendix C. Our slope stability analyses indicated that the majority of upper portion of the slope is marginally stable under static conditions and unstable when subjected to a design seismic event. Our analyses also indicated that the Iowa slope and core of the slope are composed of competent materials that are stable. This is consistent with our site observations. We have also analyzed slope stability with a soldier pile retaining wall installed at the top of the slope and extending down into the core of the slope. These analyses indicated a stable slope under both static and seismic loading. AERIAL PHOTOGRAPHY REVIEW We reviewed two pairs of stereoscopic aerial photographs in an attempt to evaluate the slope conditions on the site over time. 1'he stereoscopic pairs provide an illusion of a 3-dimensional image from 2- dimensional photographs. The photos we reviewed were dates August 11, 1961 and August 31, 2003. 'Phis technique is commonly used to compare terrain characteristics in relation to time, over a wide area and at the specific site. This can provide a historic assessment of activity within and nearby the site, including whatever modifications have been made on the ground. Our review and interpretation was primarily intended to c- • rine historic slope activity that may have occurred within or near the site. In the 1961 photos, the :lie and neighboring properties had not yet been developed. The slopes within the vicinity of the site appear to be well vegetated with dense evergreen and deciduous trees with little to no open areas observed on the slope. We did not observe any indications of significant landslide activity on the slopes in the vicinity of the site at this time. NELSON GEOTECHNICAL ASSOCIATES, INC. h