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 7 Hydrologic Conditions We observed areas of seepage on the slope, primzrily at the contact of the sandier advance outwash soils with the underlying relatively impertncable fine-grained soils of the Transitional beds. We also observed indications of this seepage within the slopes of the neighboring properties to the cast and west. Commonly, groundwater seepage exists above low permeability deposits, such as the Transitional beds encountered on the site, or within sandier layers of the overlying dense glacial till. We would consider seepage overlying the glacial till to be perched groundwater flow. Perched groundwater is usually shallower and less chronic than the secp,,ge observed lower in the slope. A perched groundwater condition could develop on this site and daylight on the slope during wetter times of the year. Perched water does not represent a regional groundwater "table" within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of rainfall. We would expect the amount of perched groundwater to decrease during drier times of the year and increase during wetter periods. This seasonal perched water may have led to previous failures near the top of the 'ape. The most recent slide was also caused by perched water that originated from the waterline breakage. During the normally wetter periods of the year, we would expect perched groundwater above the till within the near -surface weathered strata and the finer grained zones within the Transitional bed deposits. Normally, where the ground surface may slope down toward the ravine slope, this near -surface water can outcrop, and then flow down the slope face. This could become more apparent during periods of extended precipitation, or because of other drainage issues. This type of seepage and perched water is usually seasonal and tends to decline markedly during the drier months of the year. Groundwater seepage was encountered on the ravine slopes within Iland Augers 2 and 3 at depths of 3.5 and 1.5 feet below the existing ground surface, respectively, tVe also observed groundwater seepage on the surface of the slope at the contact of the samlicr portion of the slope and the underlying silt during our August visits. It is our opinion that the groundwater encountered in our hand augers and observed on the slope during our August visits is part of it year-round groundwater seepage zone that feeds the near -by Merrill & Ring Creek drainage regime. We do not anticipate that this groundwater would fluctuate significantly through the year within this location of the slopes. NELSON GEOTECHNICAL ASSOCIATES, INC. 13