9620 19TH AVE SE Geotech Report 2024-03-28

Pacific Ridge — DRH, LLC ES-7829.01 August 26, 2021 Page 13 If buildings will incorporate crawl spaces rather than slab-on-grade, in our opinion, a crawl space drain system will provide adequate drainage in lieu of perimeter footing drains. The crawl space drain must provide positive drainage to an appropriate outlet. Infiltration Evaluation The existing fill is not conducive to infiltration. Additionally, the underlying dense, cemented, and unweathered glacial till soils (hardpan) generally exhibit very poor soil infiltration characteristics. In our opinion, the use of small and large-scale infiltration systems for stormwater control is not recommended and should be considered infeasible. Stormwater Vault Design We understand a stormwater detention vault is proposed for site stormwater management. Based on the currently proposed vault location and elevations, we anticipate several feet of existing fill will be present below the vault subgrade. Vault foundations should be supported on at least two feet of new structural fill; compaction of the existing soil can be evaluated during construction. Additional remedial measures may be necessary for the vault subgrade depending on the conditions of the exposed existing fill. If the vault location or elevation changes and the vault will be supported on native soil, a higher allowable bearing capacity can be considered. Final storm vault designs must incorporate adequate buffer space from property boundaries such that temporary excavations to construct the vault structure can be successfully completed. Perimeter drains should be installed around the vault and conveyed to an approved discharge point. The presence of perched groundwater seepage should be anticipated during excavation activities for the vault. The following parameters can be used for stormwater vault design: • Allowable soil bearing capacity (structural fill) 2,500 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 Earth Solutions NW, LLC