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' Geotechnical Engineering Report SSGC <br /> Hardeson Road Retail <br /> Everett,Washington <br /> ' SSGC Project No. 18072 <br /> September 18,2018 <br /> Liquefaction <br /> Soil liquefaction is a condition where loose, typically granular soils located below the <br /> groundwater surface lose strength during ground shaking, and is often associated with <br /> earthquakes. The risk of liquefaction at this site is considered low as native soils consist of <br /> overall dense glacial till. <br /> Infiltration Characteristics <br /> Infiltration to control stormwater will be difficult at this site due to the presence of glacial till at shallow <br /> depths. Assessment of infiltration was completed per the 2016 King County Surface Water Design <br /> Manual (and 2014 DOE Stormwater Management Manual for Western Washington). Two small-scale <br /> ' Pilot Infiltration Tests (PIT) were completed in the upper 2 feet of soils on the site. Results of the <br /> infiltration tests are provided in Table 3. <br /> Table 3. Infiltration Rates <br /> Corrected <br /> Test Site and Field Infiltration Rate Infiltration <br /> Correction <br /> Depth(in) Soil Type (inlhr) Rate Factors* <br /> ' (in/hr) (CFviCFt/CFm) <br /> i Alluvium/weathered <br /> PIT-I,26 in till 3.0 0.70 (0.5/0.5/0.9) <br /> *Correction Factors from the 2014 DOE Stormwater Management Manual for Western Washington <br /> The field and corrected infiltration rate is considered appropriate for the soil tested based on other test <br /> results in similar soils. However,this rate more likely demonstrates horizontal flow through upper native <br /> soils as denser glacial till below will restrict vertical infiltration. Conventional infiltration facilities <br /> (trenches or ponds) are not considered feasible at this site. Bioswales, pervious pavements, or other <br /> shallow dispersion systems could be considered in the upper native soils where natural grades direct water <br /> to the north, if allowed by the City. A long-term rate of 0.70 inches is recommended for infiltration in the <br /> ' upper native soils to a depth of 3 feet or shallower. <br /> Cation Exchange Capacity (CEC) and organic content test were completed on a sample from the base of <br /> the PIT sites.Test results are summarized in the table below. <br /> 1 <br /> 9 <br /> 1 <br />