20-101386-Infiltration Considerations Memo-04-22-2021-V1GWENGINEER�
Memorandum
17425 NE Union Hill Road, Suite 250, Redmond, Washington 98052, Telephone: 425.861.6000
To:
From:
Date:
File:
Subject:
Laura Bartenhagen, PE, ESM Consulting Engineers
Debra Overbay, PE, GeoEngineers
April 22, 2021
22247-001-05
Woodbridge Building A, Federal Way, Washington
Infiltration Considerations
www.geoengineers.com
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This memorandum presents our opinion about the use of infiltration at the Woodbridge Building A site located
in Federal Way, Washington. GeoEngineers previously completed a design report for the project titled
"Proposed Greenline Building A Project, Federal Way, Washington" dated March 9, 2017.
Subsurface soils across the site include dense to very dense glacial till (hardpan as described in Section C.2.6
and C.2.7 of the King County Surface Water Design Manual). Glacial till is considered a hydraulic restrictive
layer due to high fines content and as the result of being overridden by glaciers. For this reason we previously
recommended stormwater detention at the site. In addition, it is our opinion that the infeasibility criteria
outlined in Section C.2.6 and C.2.7 are present at the site as follows:
• There is potential of downgradient flooding impacting the building area based on the existing
topography, surficial glacial till soils and close proximity to the building footprint.
• Infiltrating water below a permeable pavement area has high potential of compromising the adjacent
impervious pavements and building slab.
We trust this memo meets your immediate needs. Please contact us if you need any additional information.