4704 HARBOR LN 2018-01-02 MF Import

' Geotechnical Engineering I -valuation Anderson Landslide Repairs ' 4704 Harbor Lane Everett, Washington September 15, 2009 NGA pile No. 316909 Page 12 In the 2003 photos, we observed that the neighborhood along Harbor Lane had been mostly developed. Numerous residences had been constructed along the top of the steep slope near the top of the ravine. The general vegetation conditions on the slopes within the vicinity of the site appear to be similar to the 1961 photos. however, the amount of vegetation along the slope within the site appears to have been drastically reduced. We also observed two open areas on the slope below the residence that appear to have minimal vegetation and trend from the middle portion of the slope down to the creek area. The slope below the residence appears to have experienced some movement and failures as evident by the reduced amount of' vegetation and open areas along this portion of the slope. Since the slope has re - vegetated somewhat, the landslide activity appears to be infrequent. Based on our review of the two sets of stereoscopic air photos, we are of the opinion that the slope in the vicinity of the site has experienced some significant activity sometime from 1961 to 2003. It appears that a majority of the activity on the slope is infrequent to relatively recent and may be partially related to grading and other human activity along the slopes within the site which has occurred since 1961 to the present. CONCLUSIONS AND RECOMMENDATIONS General Based on our observations and subsurface explorations, it is our opinion that the recent slope movement has affected a relatively localized area on the slope directly below the residence. It is our opinion that the recent slide was directly related to the water leakage on the other side of the residence. It is our understanding that in the upwards of 250,000 gallons of water leaked out of a broken irrigation line over a two-day period, just prior to the slide. We did not see evidence that this water flowed over the slope. However, during our initial visit to the site on July 1, 2009 shortly after the slide, water was still flowing from the slope within the slide area roughly 15 feet below the top of the slope. The water leakage and the slide are roughly in line. It seems the leaking water travelled through the upper sandy material found in the vicinity of the residence and eventually made its way onto the slope below the residence, saturating the upper portion of the slope. The over -saturation of the upper soils and subsequent loss of their shear strength was likely the direct cause of the slide. NELSON GEOTECHNICAL ASSOCIATES, INC. 16