4704 HARBOR LN 2018-01-02 MF Import

Geotechnical Engineering Evaluation Anderson Landslide Repairs ' 1704 Harbor lane Everett, Washington September 15, 2009 ' NGA Pile No. 816909 Page 13 Other contributing factors may include the presence of undocumented fill and dc' ris near the top of the slope from past grading operations and human activity. In our opinion, the recent tree removal from the slope was not the cause of this slide, although this action has rendered the slope more prone to more frequent and serious activity ifconditions are not improved. Tree removal on steep slopes can lend the slope more prone to shallow sloughing and erosion -type failures. These events usually take place during the rainy season after the trees have been removed, as the surficial soils are exposed to the elements. The trees were removed in early June at the beginning of a long period of dry weather. The slide took place in late Jnne. No significant precipitation was experienced leading up to the slide. On the other hand, large-scale slides, such ;i .'!.. o... .'iat took place on this property, are usually caused by hydrostatic pressure build-up and soil sawmtion • tenuning from a subsurface water flow. In this case, the substantial water leak that saturated the slope likely resulted in the loss of the soil shear strength and the slide, rather than the earlier removal of the trees. It is also our opinion that slope movement has occurred in the past and will likely continue to occur in the future and possibly impact the residence and surrounding properties, if adequate measures of stabilizing the slope are not implemented immediately. The landslide is still fairly active and conditions could worsen during the upcoming rainy season if the slide is not promptly repaired. It is very important to temporarily secure the slope as soon as possible until a pernianent solution can be implemented. As recommended in our initial correspondence, the pool and the jacuzzi have been emptied. We recommend that the trees that are lying on the slope be cut into smaller pieces and be removed from the slope. Care should be taken while removing the tress as to not cause further disturbance to the slope. We also recommend that all of the downspouts and footing drains for the residence be located and tightlined into a temporary system down to the bottom of the slope, to prevent water from flowing onto or over the slope. The upper portion of the slope should be covered with plastic and the plastic secured with sandbags to protect the exposed soil from the elements and further erosion until the permanent fix is implemented. All existing drains, including the exposed IiDPE pile should be investigated and repaired if needed. We also recommend that the perforated drain surrounded with washed rock and filter fabric that was found along the top of the slope be removed. NELSON GEOTECHNICAL ASSOCIATES, INC. 19