4704 HARBOR LN 2018-01-02 MF Import

' Geotechnical Engineering Evaluation Anderson landslide Repairs ' 4704 Harbor Lane Everett, Washington September 15, 2009 NGA pile No. 816909 ' Page 8 rIn addition, we also observed seeping water on the slope just below the top of the slope during our visit in 1 early July. We have not observed this condition since. Based on the soil-, encountered in the explorations and observed on the slope, it is our opinion that this seepage was likely the remnants of the water that was discharged onto the slope front the waterline breakage. This water likely travelled through the upper sandier soil horizon and daylighted on the slope along the top of the low permeability layer. Vegetation on the ravine slopes generally has an impact on the amount and effects of groundwater seepage through seasonal evapotranspiration. During summer months, vegetation evapotranspirates significantly more water than during the wet, winter season. This is particularly the case with deciduous vegetation than with evergreen vegetation. We noted that the vegetation that had been located within the slopes that had been cut were it mixture of deciduous trees and evergreen trees; however, the mixed underbrush mostly still remains. Within the slide area, all vegetation has been removed by the moving earth. Some reduction in the amount of evapotranspiration should be expected due to the removal of the I deciduous and evergreen bees, and particularly within the exposed soil slide area. Replacement of selected evergreen vegetation within the influence of the seepage zones, and onto the exposed soil areas, will aid in reducing long -terns seepage and increase site area evapotranspiration. However, we do not I_ anticipate significant long-term increases of water seepage near the toe of the slope as a result of vegeu,tion disruption. This is due to the fact that this seepage is derived from a relatively large recharge area Ih,,, extends beyond [his site. SENSITIVE AREA EVALUATION Seismic Hazard We reviewed the 2003 International Building Code (IBC) for seismic site classification for this project. Since medium dense/stiff to dense/hard sand, silt, and silty sand deposits generally underlying the site, the site conditions best fit the IBC description for Site Class D. Hazards associated with seismic activity include liquefaction potential and amplification of ground motion by soil deposits. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the groundwater table. The medium dense/stiff to dense/hard sand, silt, and silty sand deposits interpreted to underlie the site have a 10\v potential for liquefaction or amplification of ground motion. NELSON GEOTECHNICAL ASSOCIATES, INC. f-f