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October 25, 2015 <br /> YMCA - Everett <br /> L&A Job No. 15-089 <br /> Page 19 <br /> ESTIMATED INITIAL INFILTRATION RATES, Ksat <br /> Sample Test D10 D60 D90 nines a Ksat Ksat <br /> No. Pit No. Size Size Size cm/sec in/hour <br /> 1 TP-2 0.1820 1.39 14.74 0.048 0.0324 11.5 <br /> 2 TP-4 0.1649 0.94 15.78 0.068 0.0257 9.1 <br /> 3 TP-7 0.1383 0.56 11.57 0.066 0.0257 9.1 <br /> 4 TP-10 0.1746 0.45 1.36 0.038 0.0468 16.6 <br /> a log10(Ksat) _ -1.57 + 1.90(D10) + 0.015(D60) - 0.013(D90)—2.08(ffines) <br /> Recommended Design Infiltration Rate <br /> The design infiltration rates of the soil samples were determined by applying a factor of <br /> safety of 4.0 to the Ksat values in the table above, and are presented in the above table. For <br /> design of infiltration facilities to be used for onsite stormwater disposal for the subject <br /> project, we recommend an infiltration rate not to exceed 4.5 iph be used in disposing <br /> stormwater by infiltration into the targeted advance outwash deposit at and in the immediate <br /> vicinity of the test pits from which the soil samples were obtained. <br /> Infiltration Facilities Construction <br /> If infiltration trenches/galleries are used for stormwater disposal, they should be set back at <br /> least 5 feet from property lines and 10 feet from nearby building footing foundations or <br /> utility trenches. Also, the bottom of infiltration trenches should be at least a foot lower than <br /> adjacent footing foundations and utility trenches. The soil unit at bottom of infiltration <br /> trenches should be verified by a geotechnical engineer. The stability of the trench cut bank <br /> should also be verified by a geotechnical engineer during excavation. <br /> LIU & ASSOCIATES, INC. <br />