HomeMy WebLinkAboutVault AddendumCobalt Geosciences, LLC
P.O. Box 1792
North Bend, WA 98045
www.cobaltgeo.com (206) 331-1097
November 13, 2024
Jassu Dhaliwal
Elitehomes2015@outlook.com
RE: Geotechnical Addendum
Detention Vault
19805 108th Avenue SE
Renton, Washington
In accordance with your authorization, Cobalt Geosciences, LLC has prepared an addendum to
discuss detention vault buoyancy. The site is underlain by glacial till which becomes denser with
depth. There is a zone of seasonally perched groundwater (interflow) that develops at shallow
depths on the denser till.
Perched interflow and seepage from precipitation will accumulate at the base of the vault unless a
perimeter drain is present at its base. We note that the plans indicate a drain around the vault at
an elevation where drainage is possible (about half way up the vault). Below this level, all backfill
will become saturated and remain saturated at most times during a typical year. Vertical
infiltration in the dense till below vault will be minimal to nil.
Since the soils are very dense and regional groundwater is not present, buoyant uplift forces are
anticipated to be minimal to nil. Any base rock below foundation and slab areas of vault are
anticipated to be saturated due to accumulation of precipitation and interflow that migrates
through vault backfill. This zone is likely 4 to 6 inches thick. The underlying dense to very dense
till below the rock will be moist to very moist but not saturated below the base rock and vault.
Sincerely,
Cobalt Geosciences, LLC
11/13/2024
Phil Haberman, PE, LG, LEG
Principal