Square concrete tank - too tall to work? 2

[Screwtape]

Currently working on the design of a rectangular shaped concrete tank consisting of two 60' square tanks that share a common interior wall. It's retaining 21 feet of water, the bottom of the tank about 10 below grade, and the top is open (see sketch).

I've done several concrete tanks but none as tall as this one. Ran through a preliminary design and I think I can get the vertical and horizontal moments to check using a tapered wall section. But overturning is an issue even with a thickened footing (30" thick) under the walls. Soil pressures are not even close to allowable, as the geotech has given me 2500 PSF to work with.

Any suggestions or tips for tall walls such as this? I'd rather avoid piles if possible. Will probably reach out to the client to see if circular tanks could be used but not sure there is enough room on the site.

 

[JoelTXCive]

In multi-celled tanks, I always assume that 1 tank could be empty for maintenance. I think you have to.

These things are 50+ year assets. The person managing the facility 15 years in the future might not have any idea that the partition walls were not designed to have water on 1 side only.

 

[BAretired]

I am still wondering what the water is used for. Is it for fire fighting? Without a roof, the water will not be potable, so fire fighting is about the only use that occurs to me. If the tanks could be connected with an opening near the bottom, water level could be balanced each side of the internal wall.

Edit: Just read the post by Joel. So cleaning the tanks is the only time the pressure would be unbalanced, is that right?

BA

 

[r13]

BA,

It is a waste water retention tank. The open lid allows for free access of oxygen, which is required for bio-process. If closed, methane gas will be the by-product.

 

[BAretired]

Okay, thanks retired13.

BA

 

[Screwtape]

BA:

Not necessarily. The tanks have high and low water levels but I'm not sure if levels are always equalized or if they can act independently. I don't have the plans in front of me, but in this case, the difference is around 10 feet or so. Not as bad as one side full and the other empty, but it is an unbalanced condition that is possible.

Unbalanced loading is one of the first questions I ask about projects like this. With adjacent channels or cells, I typically design for the worst case scenario, unless the project engineer tells me otherwise.

 

[r13]

BA,

You are welcome. Learnt a little on waste water treatment plant at one of my old employment.