1033 51ST PL SW Geotech Report 2022-10-28

in permeability due to biological activity and accumulation of fines. The recommended correction factors to be applied to the"short-term"rate measured in the tests are summarized as follows: • • Correction-factor"F,;,,,ng accounts for uncertainties in testing methods. A correction factor of 0.5 is required by DOE for the small-scale PIT. • Correction factor Fv,,abtaa accounts for site subsurface variability and the number of locations tested. We recommend a correction factor F,a,;,brnaof 0.6. • Correction factor F.accounts for reduction in infiltration rates over the long term due to siltation and bio-buildup. We recommend a correction factor of 0.9. The total correction factor to be applied is obtained by multiplying the individual correction factors. A cumulative correction factor of 0.27 should be applied to the measured infiltration rates. Table 3 summarizes the Infiltration test results along with the correction factor. Table 3.Soil Infiltration Rate Analysis' Averaged Short-Term Long-Term Design Infiltration.. Soil Type Infiltration Rate Infiltration Rate' Location (inches per hour) (inches per hour) TP-1 Silty Sand with gravel - Fill 6.0 1.6 TP-2 Silty Sand with gravel Fill 3.7 1.0 1. Based on the recommended combined correction factor of 0.27 in accordance with DOE(2014). As seen in the test results, the infiltration rate measured in the PITs varies between test locations. -• Weanticipate the.underlying glacial till will have a lower infiltration rate than the overlying material. Considering the variation in the underlying soil, the measured infiltration rates, and the time of the year the tests were performed,we recommend using a long-term design rate of 0.8 inch per hour for the project. . 7.5.2 Soil Suitability for Treatment A CEC of at least 5 meq per 100 grams and a minimum organic content of 1.0 percent are required for treatment. The soil encountered in TP-5 at a depth of 2.0 feet BGS meets the CEC and organic contentcriterla. Soil amendment is not anticipated for the proposed development. 7.5.3 Groundwater Separation We anticipate the depth of permeable pavement/bioretention areas will be approximately • 1.5:feet BGS DOE'requires a minimum-of 1 foot of separation between the bottom of permeable pavement/bioretention areas and the local groundwater table or restrictive layer. We anticipate that groundwater is below a depth of 10 feet BGS based on our explorations. The base of infiltrative LID elements is assumed to be approximately 3 feet. Adequate separation between the base of infiltrative LID elements and groundwater exists. ragnFSif.NZ a • NWHome-1-01:101618