Exterior Large Equipment Pad with deep frost depths 7

[BSPE90]

The projects I typically work on are in warm climates where frost depth is not too much of an issue (my company generally thickens the concrete mat down to frost depth). We have a project now in New England and frost depth is apparently 4-5 ft up there.

What is typically done to design an economical outdoor equipment pad?

One solution I have heard is that you can remove the soil below the pad down to frost depth and replace with large granular fill. But one engineer has a concern that it can fill up with water and freeze which would still cause heaving issues.

 

[jayrod12]

I assume the equipment can't accommodate some vertical seasonal movement?

Is the soil frost susceptible?

If both of those are correct, then we've done a couple of different approaches:
- Structural slab at grade supported by piers on footings below frost
- Piled slabs
- Frost protected shallow foundation using rigid insulation.

 

[dauwerda]

One solution I have heard is that you can remove the soil below the pad down to frost depth and replace with large granular fill. But one engineer has a concern that it can fill up with water and freeze which would still cause heaving issues.

Frost heave isn't really caused by just the moisture in the soil freezing (and the subsequent small volume increase). It becomes an issue when ice lensing happens. This is when a horizontal layer of ice forms and continues to grow in thickness as water is drawn into the freezing layer through capillary action in the soil. If you have non-frost susceptible soil (large granular fill), capillary action can't take place, therefore ice lensing can't take place. So, even if your large granular soil were full of water, that water may freeze but it won't cause a detrimental heaving action that can happen with frost susceptible soils.

 

[dhengr]

BSPE90:
I agree with Jayrod12 & Dauwerda with one exception. I’ve never quite understood the “Frost protected shallow foundation using rigid insulation” concept in this type of application. It might allow the warmth of the lower soils to inhibit freezing a little bit, but otherwise this slab is sitting out in the middle of nowhere, with no internal bldg. warmth to improve the foundation freezing conditions. So, what does it accomplish, it just takes a little longer to freeze? The real questions are; are the local soils suspectable to frost heaving, and what is the local water table level? Don’t make this foundation a hole which will take on water. You should certainly remove all organic soils, and other deleterious materials, and use a granular material for any compacted fill which is not suspectable to frost heaving. Set the slab slightly higher than the surrounding grade and grade the immediate area to drain away from the slab. Then, the slab should be made thick enough, strong enough, to move slightly as an integral unit. Thickened edges seem reasonable to strengthen the edges which take the major beating from construction equip. and the like, maybe even driving up onto the slab. And, finally mechanical connections should made to tolerate some movement in all directions, wherever possible.

 

[dik]

With our highly plastic clays, you can have supercooled water at -40C... the soil is so tight that the moisture, to form ice lenses, does not flow. We do have silts, however, that are bad and sometimes fines >200 are in the order of 10-12%.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

With insulated slabs... they generally work OK and the geothermal heat from the soil below reduces the chance for freezing... If there's any activity within or heat from equipment they are well insulated. The insulation skirt, in these areas goes 6' or so from the perimeter. Frost penetration in these areas is about 6'. When I did the Polo Park addition about 30 years back, in the parking lot area... they found frost at 13.5'... not recently, but we often had a week or two at -40C (C and F are the same at that temperature). I remember digging fence post holes, as a kid, and hitting ice at 3' down in late June.

Our clays don't really freeze... the problem is with the silt material (really frost heave susceptible) and you can have 6" or 8" ice lenses.

My understanding is that when the initial ice crystal forms, and attracts water molecules, this creates a -ve pore pressure that draws water from surrounding areas to increase the size of the ice lens. If your soil is 'too tight' then the capillary flow cannot occur (with our clays). That's why you need a fine grain soil (like silt) that water can readily flow through. The fine grain soil provides a much greater distance for capillary flow so lenses can be larger.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

Compared with Dik's case, your is more manageable. Other than deep pile, I might contemplate a table top type foundation, with base mat placed below the frost line, and standing pillars extend to the ground to support the top slab. The space in between shall be backfilled with non-frost susceptible soils. Depends on the anchorage layout, you might be able to fasten directly to the pillars without requiring the top slab.

 

[dik]

and if frost penetration is below 6'? With that mass, I'd be looking at long term settlement issues...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[DayRooster]

dauwerda - That is an interesting concept. I have thought of this issue before but never heard anyone explain how it would not heave. Not even geotechnical engineers I have asked on past projects. Truly interested to learn more. Is there any concern if the surrounding soil is clay and the non frost susceptible fill allows water to perch underneath the footing then freeze? Or will the explaining ice simply find its way around the foundation (thru the same path it came in) and not cause any problems? Assuming the traditional frost heave is not a factor based on your explanation.

BSPE90 - Another option I haven’t seen throw out yet is to use CLSM as the non frost susceptible fill underneath the foundation (to frost). Just want to make sure it’s low enough strength that it’s excavatable in case it’s poured a bit too high. That being said, worth seeing how the above conversation plays out too.

 

[dik]

I generally use 6" of compact Gran 'A' (which is nearly impermeable) and often provide a 4" dia perf perimeter drain to free drainage.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[HTURKAK]

I agree with Mr dauwerda , jayrod12 and , dik . Their posts giving valuable info. regarding the soil behavior at cold climate...
I wish to give triple pink stars but not possible .. I just want to add some more info. based on having worked at cold climate.


Let me explain with experience based on lakes at cold region; during winter season at beginning, if it does not snow for several weeks, you may observe the ice thickness may reach to 10-15 ft. But if it snows, the ice depth will be very limited and risky to drive on. The snow blanket insulating the water below together with geothermal balance, the ice thickness developing may limit to just one foot.
The rigid insulation is balancing and behaving as retarder due to day and night temperature difference. So, the frost depth say 6 ft specified at local code may occur at outside of insulated SOG, but the frost depth below SOG will be around just one ft.

I want to add that, if the soil is collapsible ( silty clay with some organics) the ground may settle every year at the end of frost season.

My suggestion to BSPE90 will be;

Check the probability of ponding of surface water, check the soil for frost susceptibility and collapsible or not. If there is no risk of ponding, frost susceptibility and collapsible soil, just remove the top soil and provide 1 ft granular fill below the pad having thickened edges..

 

[bones206]

My preference is to replace soil with gravel and improve drainage around the equipment pad. I think that gives you the best bang for the buck. I only spend money thickening the slab edges if scour is a concern. Piles are appropriate in some special cases, but frost-jacking is a possibility that should be considered.

I did have one project a couple years ago in Rhode Island where the contractor and AHJ teamed up against me to say my generator pad design didn't meet code requirements for frost-protection. They rejected the concept of soil replacement and improved drainage and said that I must add 4.5' foot deep frost legs around the perimeter of the slab. They couldn't explain why it was required other than "the code requires foundations to extend to frost depth". As long as they replaced the silty soil with gravel and MEP used flexible connections, I didn't really care since the owner was willing to pay for the "frost legs".

 

[dik]

bones... frost depth is not an issue if movement can be tolerated. In these environs frost heave is approx 1", if it happens... it can be considerably more (6").

Other than 6" of compacted granular A (for levelling) I would be adverse to filling the area to frost depth with granular (big sump)... with the 6" of compacted fill, this is drained to free drainage. If your local code requires that you go below frost penetration... you might talk to them about this not being an issue, else happy digging. Should have added... the use of several inches of expanded polystyrene insulation to modify the depth of frost penetration. DOW has a good technical data sheet on this.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

and if frost penetration is below 6'? With that mass, I'd be looking at long term settlement issues...bigsmile

Do you mean the frost can reach deeper than predicted for the local? I couldn't connect long term settlement and footing depth though

 

[dik]

Don't be confused... I was trying to be funny... with that mass of concrete, you might have to look at long term settlement.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

Yeah, the foundation I proposed can be much heavier than the equipment, but believe or not, the soil wouldn't feel much difference but the weight of the equipment, if heavy weight concrete is not used.