HomeMy WebLinkAboutVault DFRCobalt Geosciences, LLC
P.O. Box 82243
Kenmore, Washington 98028
Date: September 26, 2024 Project: Plat Job No.:
Contractor: Location: 19805 108th Ave SE Weather: Clouds
At your request and authorization, Cobalt Geosciences, LLC has prepared this report to discuss the
detention pond design and construction. We understand that a thickened base was not installed as
part of the vault to resist buoyancy. Normal concrete vault construction in areas where seasonal
interflow may be present typically do not include a thickened concrete base beyond a typical slab
and foundation.
We have visited the site and observed that perimeter drainage will be placed partway up the vault
to remove any groundwater that becomes perched on the denser till and in fill around the vault.
Provided the drain is completed per plan and with our verification, the risk associated with
buoyancy will be mitigated. Buoyancy is typically an issue with lightweight aluminum or other
material cylindrical shaped detention systems.
We also understand that there will be over 5 feet of soil cover compacted over the vault upon
completion along with some dead storage. These factors indicate that uplift forces will be mitigated.
Again, vaults are more apt to be affected by buoyancy if they consist of lightweight materials, not
concrete. The structural engineer has also commented on buoyancy and uplift.
Concrete vaults with some dead storage and soil cover typically do not experience issues unless in
areas where there is regional persistent groundwater or under fully saturated conditions, neither of
which are present at this site or area.
Sincerely,
Cobalt Geosciences, LLC
Phil Haberman, PE, LG, LEG
2063311097; phil@cobaltgeo.com