Tank Pad above soil - Is frost depth a concern?

[adurbin]

I am a structural engineer and we are proposing to build a 13 foot diameter by 2 foot thick concrete tank pad. We are placing it 6 inches below grade on top of 6 inches of clean gravel. My question is does it need to go to the frost depth to resist upheaval? In other words won't the weight of the concrete (13' dia x 2' thick - total weight is 40,000 pounds) tank pad with the tank (8,000 pounds) on top resist any type of upheaval in the soil? Is thier a way I can calculate how much weight is needed to resist upheaval?

 

[fattdad]

I would imagine that the thermal mass of the tank would limit any chance of frost penetraiton below the tank. You could do a concrete turn-down perhaps to minimize frost penetration at the perimeter.

I used to work in the North Slope of Alaska. The concern of water tanks was that they would melt permafrost. To mitigate this concern, "cryoanchors" would be installed beneath the tank to keep the soil frozen and minimize the thermal affect (i.e., thawing) below the tank.

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

 

[MiketheEngineer]

Mother Nature is a bitch and she ALWAYS wins...

Unless this pad is heated and insulated - I would at least propose a footing going down to below frost line

By the time you form all that up - the few extra cubic yards of concrete to get to the frost line might be cheaper. Of course, then you might have to worry about exceeding the soil capacity

Good luck

 

[civilperson]

If the tank piping connections are sufficiently flexible, (2 degrees or so), then any movement due to frost heave will be easily tolerated. The large gravel under the slab will stop moisture from migrating upward and the enormous thermal mass of the tank/liquid will maintain equilibrium for a long time in sub zero weather. Very few tank failure due to frost in my experience.

 

[bridgebuster]

I agree with Mike. I just designed a footing for atall gas flare that weighs 54K. A two foot thick footing would work but I made it thicker in order to place it below the frost line.

I asked our chief geotech if it was overkill and if I could put it higher. He said heaving was still possible even with the heavy load.

 

[oldestguy]

You didn't say what soil type you are on. That magic statement of "frost depth" means nothing unless you know soil type.

Take a clean sand for instance. It has low moisture content (low heat of fusion per c.f.). Thus frost can go very deep. But the stuff is not frost susceptible, so who cares about frost depth, unless if is for necessary depth of buried water pipes.

For a clay however, it probably is frost susceptible, but has lots of water so frost penetration is not deep. The amount of heave within that "depth" depends on permeability. High plasticity means low frost heave.

The worst stuff is silt or silty clay.

As a side to this, how deep is frost depth at a lake? Not 4 feet, but why? Lots of heat of fusion to offset that cold penetration.

To get real crude about it,going to "frost depth" as locally defined will do it, but it may not be needed.

 

[cbosy]

I would unquestionably take it down to frost depth unless the soil is a clean dry sand or gravel.

 

[miecz]

We usually excavate to 6" below frost depth and fill with compacted gravel. The footing extends only 8" below grade. After reading this thread, I'm wondering if the 6" (below frost) is enough.