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Pacific Ridge — DRH, LLC ES-7852.01 March 15, 2022 Page 12 The following parameters can be used for stormwater vault design: • Allowable soil bearing capacity (dense native soil) 5,000 psf • Active earth pressure (unrestrained) 35 pcf • Active earth pressure (unrestrained, hydrostatic) 80 pcf • At-rest earth pressure (restrained) 55 pcf • At-rest earth pressure (restrained, hydrostatic) 100 pcf • Coefficient of friction 0.40 • Passive earth pressure 300 pcf • Seismic Surcharge 8H* * Where H equals the retained height. The vault walls should be backfilled with free-draining material or suitable sheet drainage that extends along the height of the walls; backfill with free-draining material may be prudent along the east side of the vault where the existing wetland area will be cutoff. The upper one foot of the wall backfill can consist of a less permeable soil, if desired. A perforated drain pipe should be placed along the base of the wall and connected to an approved discharge location. If the elevation of the vault bottom is such that gravity flow to an outlet is not possible, the portion of the vault below the drain should be designed to include hydrostatic pressure. Design values accounting for hydrostatic pressure are included above. The above passive earth pressure and coefficient of friction values include a safety factor of 1.5. ESNW should observe grading operations for the vault and the subgrade conditions prior to concrete forming and pouring to confirm conditions are as anticipated, and to provide supplemental recommendations as necessary. Additionally, ESNW should be contacted to review final vault designs to confirm that appropriate geotechnical parameters have been incorporated. Vault backfill should be placed and compacted in accordance with the recommendations provided in the Structural Fill section of this report. Earth Solutions NW, LLC