4704 HARBOR LN 2018-01-02 MF Import

Geotechnical Engineering Evaluation Anderson Landslide Repairs 1 4704 1larbor Lane 1 Everett, Washington September 15, 2009 NGA Pile No. 816909 Page 10 been left on the slope from the tree cutting. The remainder of the slope remains covered with vegetation and we did not observe any other areas of significant erosion on the slope. However, the eastern side of the upper slope adjacent to the pool appears to be marginally stable and could experience slope movement in the near future. Our explorations indicate that the core of the steep slope consists primarily of medium dense/stiff to very dense/hard sand, silty, and silty sand with surficial undocumented fill in and around the pool area and along the top of' slope. Inclinations of up to 40 degrees on the slope indicate high strength and internal friction angle within the underlying soils. Relatively shallow to more significant slides should be expected to continue to occur near the lop of these slopes, as has already occurred, if conditions are not promptly improved. The core and lower portion of the slope are composed of competent material that is considered stable. proper site grading and drainage, as well as stabilization techniques as recommended in this report should help improve current stability conditions for the entire slope and drastically lessen the risk to the house and swimming pool areas, as well as the entire backyard. LABORATORY ANALYSIS We perfornaed eight grain -size analyses tests and one direct shear test on selected soil samples obtained front our geotechnical explorations. The results of the sieve analyses are presented as Figures B-1 through 13-8 in Appendix B. A Direct Shear test was also conducted on one soil sample to evaluate the strength envelope of the upper site soils, and to aid in slope stability analysis. "I'he results of this test are presented as Figure B-9 in Appendix B. SLOPE STABILITY ANALYSIS The slope was also analyzed for stability along Cross Sections A -A', B-B', and C-C' using the computer program WinStabl, produced by the University of Wisconsin. WinStabl is a two-dimensional, limit equilibrium slope stability program that generates random potential failure surfaces and determines their corresponding factors of safety with respect to failure. The factor of safety is defined as the ratio of the internal soil strength divided by the gravity driving forces that cause the failure. By generating a large number of random surfaces, a potential failure surface with the minimum factor of safety can be obtained. NELSON GEOTECHNICAL ASSOCIATES, INC. t�.