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Everett Housing Authority April 22. 2020 Page 5 <br />The design infiltration rate is determined by applying correction factors to the measured infiltration rate. <br />The correction factors account for uncertainties in site variability and number of tests, testing method, and <br />long-term reductions in permeability due to plugging from biological activity and accumulation of fines. <br />Table 111-3.3.1 of the 2014 SWMMWW describes the correction factors to be used to estimate design <br />infiltration rates. <br />Table 111-3.3.1 Correction Factors to be Used With In -Situ Saturated <br />Hvdraulic Conductivitv Measurements to Estimate Design Rates <br />Issue <br />Partial Correction Facto <br />Site variability and number of locations tested <br />JCFv = 0.33 to 1.0 <br />Test Method <br />. Large-scale PIT <br />. Small-scale PIT <br />. Other small-scale (e.g. Double ring, falling head) <br />. Grain Size Method <br />. CFt = 0.75 <br />. = 0.50 <br />. = 0.40 <br />= 0.40 <br />Degree of influent control to prevent siltation and bio-buildup <br />CF = 0.9 <br />' Total Correction Factor. CFT = CFv x CFt x CFm <br />■ CFv accounts for site variability and number of locations tested. A CFv of 0.7 was used for the analyses. <br />■ CFt is determined based on the test method. The CFt for small-scale PIT testing is 0.50. <br />■ CFm accounts for the degree of influent control to prevent siltation and bio-buildup and is 0.9. <br />The total correction factor CFT for the PITs is 0.32. When applied to the measured infiltration rates of 3.89 <br />and 0.23 inches per hour, the design infiltration rates for PIT-1 and PIT-2 are 1.2 and 0.1, respectively. <br />' INFILTRATION RATE RECOMMENDATIONS <br />' Based on in -situ field infiltration testing within the footprint of proposed infiltration facilities, the design <br />infiltration rates calculated for PIT-1 and PIT-2 are 1.2 and 0.1 inches per hour, respectively. However, on <br />the basis that the depth of fill at PIT-1 is significantly deeper than observed anywhere else on site, the <br />presence of groundwater mounding on top of the glacial till, and the degree of variability indicated, we <br />recommend using a maximum design infiltration rate of 0.5 inches per hour for Facility 1. <br />REFERENCES <br />Washington State Department of Ecology, 2014, "2014 Stormwater Management Manual for Western <br />Washington." <br />F� <br />1 <br />GEOENGINEERS� <br />1 <br />