Opinions - Exterior Mat Foundation above Frost Lines

[CardsFan1]

I am just looking for some opinions on how other engineers deal with exterior mat foundations.

I have a cooling tower that is about 12 x 10 foot in plan. The frost line in our area is 30". I told the contractor to dig a 12x10 foot by 30" deep and pour a 3 foot thick mat.
I know that sounds excessive. But I believe that since it is exterior, a turn down does not gard against heave since the soil in the middle can heave. So rather than digging a 30" hole and filling it with compacted well drained rock, sloping the sides, and pouring a 12" mat with 30" inch turn downs, I say just fill it with concrete.

Concrete is cheap compared to fixing a tilting cooling tower. If it was a transformer or a generator pad, I would use a 12" mat with a turn down and let it go.

Thoughts?

 

[Once20036]

I`ve always heard that you need three things to create heave:
1) Cold
2) Water
3) Frost susceptible soil.

If you can eliminate any 1, you shouldn't have heave.
To eliminate #1, you could add heat (not economic)
To eliminate #2, stop the water, via your turndown. The middle will not heave because there's no water.
To eliminate #3, excavate, replace with gravel, then pour a shallow pad on top (whatever you need for the loads).

Personally, I like #3, but I haven't met very many geotechs who will sign off on it.
I`m not a huge fan of the 3' mat option due to the impact that the thickness has on temp and shrinkage steel. Much more than 3' thick and youre getting into ACI "Mass Concrete" where the temperature differential across the thickness of the ooncrete could create cracking issues.
You could excavate, backfill with lean concrete, then a 12" structural mat, but that isn't much different than the turn down option that you're not a fan of.

Assuming the geotech wouldn't sign off on the gravel option, I`d have used turndowns.

 

[CardsFan1]

Thank you for your input!

Have you successfully used Option#2 in a similar Midwestern silty clay type soil?

 

[oldestguy]

I'd not worry one bit with what you have planned. It is assumed any piping is somewhat flexible and is insulated if necessary. It also would be good if surface drainage is provided for any excess water from the system. I consider any "frost depth" for the area to be only approximate, possibly half or double of what will happen there.

 

[CardsFan1]

oldestguy, Can you please clarify as to which you would not worry about. Using the three foot thick mat or the 1 foot with the turn downs?

 

[jayrod12]

I think he's saying that he'd likely ignore frost altogether and just provide a pad thick enough to resist the loading applied to it.

I assume there's likely some global overturning depending on the height of the cooling tower, how much mass do you require at the bottom to counteract this overturning?

 

[oldestguy]

OG here. Some may say you must support from below "frost depth". Why, when this unit can be built to take some frost movement without damage and the service connections can be made to allow for movement. The unit will still function. Besides that so called "frost depth" may have come about due to local experience, GENERALLY and likely is different and local conditions can vary substantially from place to place. In some areas, with soils that are known to heave substantially, maybe do something special, but then totally eliminating heave for project like this is really not needed. By substituting for possible heaving soil via the deeper slab, significant reduction from the worst case can be expected. Without the local area being known for lots of frost heave problems, jobs like this usually do not need a lot of extra work to eliminate heaving. For a fuss-budget builder, with a generous budget, maybe go whole hog, but that's very unusual for most construction I know of. Most of my many years in construction has been in northern states with frost action very common. You don't always have to eliminate it.
Thus, the 3 foot mat is likely to be moved less than the one foot thick mat. Then, assuming the one foot mat can handle slight differences in support (bending) a slight more heaving may result. Also, slightly more tilt may occur with thinner mat, how the unit works may take slight re-leveling now and then, if at all. Thus, either probably will do the job. In my mid-west area, in general, I'd expect winter heaves no more than an inch in most places, but that depends on many things.

 

[dik]

Can you use a couple of inches of extruded polystyrene insulation to cause the 'earth's heat' to rise under the pad... have done this lots of times where movement cannot be tolerated. We typically have 6' of design frost penetration. As a kid, I've dug fenceposts in July and encountered frost at 3'.

Dik

 

[oldestguy]

Sorry I didn't answer the choice question first, but did try with an edit. Now Dik has something you may want if any question. I'd look at 3 inches closed cell to be safe, but it has to be extended out maybe 3 feet in all directions or the cold will come in under the edges. At slab grade that outside stuff needs earth cover to keep it out of sun.

 

[dik]

OG: that's what we do... but often extend out 6'...

Dik

 

[WinelandV]

Why, when this unit can be built to take some frost movement without damage and the service connections can be made to allow for movement.

Just a word of caution here - we recently did an industrial facility upgrade (central Iowa), and between the tank manufacturers and the owner's resistance to the added cost of flexible fittings, there was not much give in how everything fit together. Best make sure that you get the entire design team (geotechnical, mechanical, electrical, owner, and yourself) on record as to what settlements can be tolerated. Hopefully everyone will say that it's no big deal.

 

[dik]

winelandv: You need some flexibility built in... even for pile foundations.

Dik

 

[haynewp]

I usually get a recommendation from the geotech for a depth of non-frost susceptible fill to be put under exterior pads like this.

 

[WinelandV]

dik,

We were able to talk them down off of a requirement of "zero allowed settlement/heave". They got a nice ring wall foundation to below the frost line. Even still, we had them load the tanks and sit for a while prior to making the piping connections.