I have seen the following equation used in multiple engineering design manuals including AWWA M45, M55, and pipeline design books going back to the 90s.

Rb = Buoyancy Reduction Factor = 1 - 0.33 * Hw / H

Hw = groundwater height above buried pipe

H = depth of soil cover above buried pie

Usually this factor is used to determine the vertical dead load pressure on a buried pipe beneath a water table as follows:

Vertical Dead Load on pipe = Rb * γ * H

γ = soil unit weight

I'm struggling to understand how the buoyancy reduction factor is derived even though it is a simple equation. Google searches don't come up with anything as the results are all on buoyancy of objects immersed in water (not where the buoyancy reduction factor equation is used).

Any help is greatly appreciated!