8102 EVERGREEN WAY MALL KIOSK EVERETT 4 CORNERS - MAIL KIOSK Geotech Report 2025-08-22

DevCo, Inc. April 1, 2019 Infiltration Feasibility ES-6516 Page 10 As indicated in the Subsurface section of this report, native soils encountered during our fieldwork were characterized primarily as dense to very dense glacial till deposits. Based upon the results of USDA textural analyses performed on representative soil samples, native soils are also classified as gravelly sandy loam, gravelly course sandy loam, and very gravelly loam. Fines contents were about 30 to 47 percent within the sandy loam and about 50 percent within the gravelly loam, at the tested locations. Given the appreciable fines contents and dense to very dense in -situ condition of the till, it is our opinion full-scale infiltration is not feasible from a geotechnical standpoint. Small scale infiltration devices incorporating overflow may be feasible and can be further evaluated by ESNW, if requested. Preliminary Detention Vault Design Although unspecified at this time, we presume a detention vault will be used as the primary means of stormwater management. Based on our experience with similar projects, we assume grade cuts of 10 or more feet will be necessary to achieve the subgrade elevation of the vault foundation. Based on our field observations, grade cuts for the vault are likely to expose dense to very dense, undisturbed glacial till. The vault foundation should be supported directly on competent native soils. Should overexcavation(s) be necessary at the vault foundation subgrade, quarry spalls should be used to restore grades. The final vault design must incorporate adequate buffer space from property boundaries such that temporary excavations to construct the vault structure may be successfully completed. Perimeter drains should be installed around the vault and conveyed to an approved discharge point. Perched groundwater seepage should be anticipated within the vault excavation; however, buoyancy is not expected to influence the vault structure. The following preliminary design parameters may be used for the vault: • Allowable soil bearing capacity • Active earth pressure (unrestrained) • Active earth pressure (unrestrained, hydrostatic) • At -rest earth pressure (restrained) • At -rest earth pressure (restrained, hydrostatic) • Coefficient of friction • Passive earth pressure • Seismic surcharge 5,000 psf (dense native soil) 35 pcf 80 pcf 50 pcf 95 pcf 0.40 350 pcf 6H psf* * Where H equals the retained height (in feet) Earth Solutions NW, LLC