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, ' <br /> DAVID L. NELSON AND ASSOCIATES, INC. <br /> Consulting Enginccring Geolr,gy <br /> November 27, 1991 <br /> Project No. 10591 <br /> Puge 4 <br />, <br /> deFendent on the ability of excess pore pressures to dissipate. <br /> That is, if the deposit is relatively thin with a short drainage <br /> path, the pore pressures can dissipate fairly rapidly. <br /> We did not find a significant thickness of saturated silty sand <br /> soils that would be create a significant risk to the structure <br /> area due to liquefaction. The fill areas encountered are <br /> recommended for removal and/or backsloping and drainage to aid in <br /> long-term stability. We recommended that the residence <br /> foundations extend through any loose surficial or fill� soils down <br /> to dense soils for bearing support. Therefore the potential <br /> influence of liquefaction or hillside instability on struciures <br /> due to these soil types is low. The same reasoning may be <br /> applied to the attenuation of ground motion. <br /> Erosion Control <br /> Although this type of hazard is not specifically referred to in <br /> the ordinance, we commonly conduct our evaluations to include <br /> erosion hazards . The erosion hazard criteria used for <br /> determination of affected areas include soil type, slope <br /> gradient, vegetation cover ar.3 ground water conditions . The <br /> erosion sensitivity is relah_ecl 'to vegetative cover and the <br /> specific surface soils types (gro�ip classification) , which are <br /> related to the underlying geologi;: �oa.1/rock units. The <br /> following table is outlines erosion hazard for the on site soils <br /> that have been stripped of vegetaticn. The surface soils found <br /> within the site have been classified in accordance with the Soil <br /> Consezvation Service (SCS) classification system and have been <br /> grouped with respect to the corresponding geologic unit. <br /> Geologic Unit Soil Sensitivity At Slope 0-15� 16-39� >40� <br /> QvR Everett or Med High Hign <br /> (Glacial Outwash) Indianola <br /> Qi Kitsap Lo;•r Med High <br /> (Whidbey Fmn) <br /> Therefore, the soils on the steep slope are considered to have a <br /> high erosion potential when the vegetation is stripped. <br /> Landslide Hazard (Unstable Soils Hazards Areas) <br /> The on site Whidbey soils and loose to medium dense sand is <br /> considered stable with respect to deep seated failures a�d <br /> landslides . The loose to medium dense sand on the steep slope <br /> should be considered marginally stable with respect to shallow <br /> failures and sloughing. These conditions will be either <br /> //fc I <br />� <br />