3101 CEDAR ST STORM DRAINAGE DESIGN 2025-07-25

MAGNUSSON KLEMENCIC L_ ASSOCIATES ■ - SPECIAL STUDIES OR REPORTS Existing soil types are described in depth in the project geotechnical investigation prepared by GeoEngineers titled "Geotechnical Design Services, Central Utility Plant Parking Lot, Everett Washington for Providence Regional Medical Center," dated October 1 7, 2012. Generally the site consists of a blend of the following soils: • 1-4.5 feet of very loose to medium dense silty sands with some gravel and organic material; • Several feet of soft to medium stiff organic silt; or • Glacially consolidated soils consisting of medium-dense to very dense silty sand with varying gravel content and stiff-to-hard sandy silt and clay. Groundwater is expected at depths of roughly 13 feet below existing grade, which is deeper than the anticipated excavation for the proposed project. One key conclusion of the geotechnical investigation was that the anticipated subgrade infiltration rate is less than 0.25 inches/hour. As a result, stormwater conveyance has been designed without any groundwater infiltration allowance. OTHER PERMITS The below additional permits have been considered for the project: R.1 Industrial Stormwater Permits N/A R.2 Construction Stormwater Permits Appl i cabl e The pr of ect pl ans i ncl ude a Tenpor ar y Er osi on Cont r oI PI an for const r uct i on. Pr i or t o const r uct i on, an NOI v‘j I I be r equi red t o be filed. R.3 Endangered Species Act N/A R.4 Section 401 Water Quality Certifications N/A R.5 Hydraulic Project Approvals N/A R.6 Aquatic Land Use Authorizations N/A R.7 Basin Planning/TMDLs N/A (combined system) R.8 Underground Injection Control Authorizations N/A (see not e bel ow) The pr of ect consi der ed i nj ect i on weI I s as a means of r educi ng f I ow i nt o the corrbi ned system It yes det er rri ned t hat i nj ect i on wel I s woul d not be i ncl uded i n t he pr of ect f or pr i mar i I y t wo reasons: 1) t he carrpus soi I t ypes are t oo var i abl e to r el y on bel ow-grade conveyance I ayer s ( r equi red f or successf ul i nj ect i on) wi t hout subst ant i al addi t i onal geot echni cal i nvest i gat i on, and 2) use of i nj ect i on %el I s would r equi r e pre-t r eat rrent oper at i ons and rrai nt enance cost s t hat r ender ed t he concept cost- i nef f ect i ve. Storm Drainage Design 3 Utility Plant Expanded Parking Facility, Everett, Washington