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Pacific Ridge — DRH, LLC ES-8528.01 <br /> July 18, 2022 Page 12 <br /> Stormwater Vault Design <br /> According to the referenced feasibility plans, stormwater management designs may incorporate <br /> one or more stormwater vaults. Vault foundations should be supported on competent native soil <br /> or crushed rock placed on competent native soil. Final storm vault designs must incorporate <br /> adequate buffer space from property boundaries and critical area buffers such that temporary <br /> excavations to construct the vault structure can be successfully completed. Perimeter drains <br /> should be installed around the vault footings and conveyed to an approved discharge point. The <br /> presence of perched groundwater seepage should be anticipated during excavation activities for <br /> the vault, which may dictate temporary slope inclinations required for the vault excavation (as <br /> described in the Excavations and Slopes section of this report). <br /> The following parameters can be used for stormwater vault design: <br /> • Allowable soil bearing capacity (dense native soil) 5,000 psf <br /> • Active earth pressure (unrestrained) 35 pcf <br /> • Active earth pressure (unrestrained, hydrostatic) 80 pcf <br /> • At-rest earth pressure (restrained) 55 pcf <br /> • At-rest earth pressure (restrained, hydrostatic) 100 pcf <br /> • Coefficient of friction 0.40 <br /> • Passive earth pressure 300 pcf <br /> • Seismic surcharge 8H* <br /> Where H equals the retained height. <br /> The passive earth pressure and coefficient of friction values include a safety factor of 1.5. The <br /> vault walls should be backfilled with free-draining material or suitable common earth if a sheet <br /> drain material is used. The upper one foot of the vault backfill can consist of a less permeable <br /> soil, if desired. A perforated drainpipe should be placed along the base of the vault walls and <br /> connected to an approved discharge location. If the elevation of the vault bottom is such that <br /> gravity flow to an outlet is not possible, the portion of the vault below the drain should be designed <br /> to include hydrostatic pressure. Design values accounting for hydrostatic pressure are included <br /> above. <br /> ESNW should observe grading operations for the vault and the subgrade conditions prior to <br /> concrete forming and pouring to confirm conditions are as anticipated, and to provide <br /> supplemental recommendations as necessary. Additionally, ESNW should be contacted to <br /> review final vault designs to confirm that appropriate geotechnical parameters have been <br /> incorporated. <br /> Earth Solutions NW, LLC <br />