How Thick Should the Concrete Foundation Be to Counteract Buoyancy Underwater?

[ahmed2810]

Hello, I have a question about buoyancy of concrete in water. If a submersible pump station is placed under water, and it has under it a concrete foundation to hold it in equilibrium.
How can someone calculate the minimum weight of the concrete foundation?

The pump station has a diameter: 1.6 meter
density of the water is: 1000 kg/m^3
Density of the concrete is: 2300 kg/m^3
the height of the submersed item is 2 meter under water

How thick must the concrete foundation be to counterbalance the buoyancy forces?
PS: please show me equations, and not just an explanation

 

[SpecGuest]

Hi,
I guess the pump has a negative buoyancy in water, so it will stay at the bottom even if there is no concrete.
The buoyancy of concrete in water is simply 2300(conctete)-1000(water)=1300 kg m^-3

You may also need to take pressure forces due to currents (if any) acting on the pump.

 

[ahmed2810]

Thank you for the answer, it is not the pump itself that is going to be in water, but the pumpstation (the sump), and inside it there will be the pump. On the bottom of the pumpstation there will be a concrete foundation. This has to hold the structure down because the buoyancy will act upwards. So my question actually was, how thick and heavy must the foundation be to counterbalance the forces.

Therefore 1300 kg/m^3 is not the answer, as this is just the difference of the density of both of the concrete and the water

 

[BvU]

Hello Ahmed,

You may need to collect some more information: you provide a diameter of 1.6 m for the pump station, but: is it cylindrical ? If so what height (so you can calculate the volume). What is the weight ?
And about the prospective location: still water or lots of current ? etc.

AS SG says, the concrete provides 1300 kg/m3 pull-down force, but that is a static situation.

 

[SpecGuest]

You need to estimate what is the positive buoyancy of your pump station.
Without this number, how do you want to calculate the amount of concrete needed ?
1300 km/m^3 is the exact answer for concrete apparent density in seawater. This is Archimede :)

Once again, you also need to know if there are other forces applied to your system (I mean other
than the only gravity force) like a transverse pressure force due to currents that could move your
pump station. All these numbers are required to scale the amount of concrete needed to counteract
all the forces.