Storage Tank Foundation

[RFreund]

I am looking at a project where they are installing a cluster of 6 - 12' diameter x 30' tall storage tanks (for plastic pellets). From the information sent over if looks like they were planning to use a thick slab on grade to support all 6 tanks. This doesn't seem right. I mean sure we could design a slab but it would be subject to frost heave, etc. I'm going to inquire further but wanted to get a better idea of what is typically done. Is it possible that the tanks can accept the movement? In my area frost is 3'6" below grade. If they wanted to do some sort of thick slab I suppose you could put in granular soil or insulation but I would have thought maybe a large common mat which bears below the forst with ring foundation walls would be more of the norm?

EIT

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[SlideRuleEra]

There could be good reasons for a shared foundation mat:

1. You did not mention how close the tanks are to each other. If they are close together, having one foundation may not be any more expensive than six individual foundations.

2. The tanks are tall compared to their diameter (2.5 : 1 ratio). If wind or seismic are a concern, a shared foundation would easily solve that issue.

3. Perhaps differential settlement of six independent foundations would cause problems.

4. The tanks may need to maintain alignment with each other for conveyor, or similar equipment to operate properly. A shared foundation solves that. The fact that a thick slab is being suggested can indicate that this is a concern... a thick slab is also a rigid slab - minimal deflection under various loads.

I agree with you about questioning a "thick slab on grade". As an alternate, I would bring up for consideration a slab of appropriate thickness with the bottom below the frost depth and the top below grade (if that will be thick enough). Six pedestals, either ring walls with fill or solid concrete, could rest on the mat.

On industrial projects the mechanical and process requirements often dictate foundation and structural solutions. Economics is secondary - this is as it should be... and can make our jobs a lot more interesting.

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[oldestguy]

Hey,not all soil types heave. Not all soil types have the same depth of freezing. Insulation can be used to significantly affect these things also. If one really wants to know what will go on, you don't take a ccok-book code of "frost depth" as meaning much of anything.

 

[RFreund]

SRE -> What you describe is what I was suggesting, meaning a slab that meets frost depth with pedestal/foundation walls.

OG -> Very good points.

It sounds like it really depends on the equipment, but using a common mat down to frost with ring walls Or using a granular/non-frost susceptible material (or insulation) and keeping the common mat at grade could be another possibility.

EIT

http://www.HowToEngineer.com

 

[BigH]

I had a job in northern Ontario where the depth of frost was about 9 to 10 ft. Yet, the footings were put down at about 2 ft. Why? As OG said, not all soils heave due to frost. The soils I had were clean sands and the groundwater level was more than 12 below ground surface. Therefore I had non-frost susceptible soil, had no access to continuous water being added to the freezing zone. And, BTW, the other much older footings at the plant were also down about 2 to 3 ft.