Can foundation weight allow avoidance of frost depth? 2

[StructureMan44]

A contractor is suggesting the use of 1ft deep, very wide concrete slab to support heavy rotating equipment. The local jurisdiction has a required frost depth 42in. Can a very heavy foundation (while still below the allowable bearing capacity of the soil) eliminate the need to go below the 42" frost depth?

 

[structSU10]

No. You either need to use frost free material below the foundation down to the frost depth, insulate the foundation, or bring it to frost depth.

 

[KootK]

Nope. The forces generated by frost action can disintegrate mountains. Restraint isn't much of an option. It sounds like it might be a good candidate for a shallow frost protection system though. You know, a blanked of strategic insulation below and beyond the pad.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

Can your equipment tolerate movement? If so, then frost heave may not be a really big concern. Often use stiffened slab foundations for electrical and mechanical equipment that 'can move'.

Dik

 

[BridgeSmith]

In similar situations, we typically provide a layer of free-draining granular material under the slab. For foundation soils that are mostly granular or silt, we would typically replace 1ft of the native material with the non-frost susceptible material. Native foundation material that is heavily clay might need more than the 1ft to mitigate frost heave.

I don't believe you can count on the weight of the foundation to prevent frost heave; the expansion pressure of ice is huge.

 

[dhengr]

StructureMan44:
As important as anything, is a well drained site for the slab and the piece of equipment, reaching some distance beyond their foundation foot print, and draining to daylight. If you have native soil which drains well, then removing any organic materials and soft soils is a good place to start. Then, as mentioned above, a foot or two of compacted base material which is not susceptible to frost, is a good base for the foundation slab. The whole site should be high enough so it drains away from this found. and you don’t want to dig a bathtub which holds water as you excavate the organic and soft soils. You might want that slab to be a little thicker and heavier under a piece of rotating equip. to take into account any vibration issues.

 

[amec2004]

For heavy rotating equipment foundation, the answer is NO. You have to go below frost depth. The cost of repair and damage is very high as the rotating equipment normally has very high tolerance requirement.

For non-critical structure foundation such as walkway or light pole etc, the answer is Yes. You can provide

1) 12 inch thick compacted frost stable granular fill

2) 12 inch thick *RIGID* void form such as Dow Styrofoam Highload

3) Apply 2% slope around the foundation to drain out the water

Please note the word *RIGID* above, *RIGID* means you can maintain the soil bearing capacity even after providing the void form between foundation and soil

Dow Styrofoam Highload Void Form

AISC Steel Connection Design Software

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[oldestguy]

OK, plenty of advice. However,the so called "frost depth" commonly is something people have found is the depth you go to to avoid heaving. However, frost having all depends on soil type and water, plus many other factors. Silts and low plasticity clays heave the worst. High plasticity clays are so tight not much water moves to cause frost lenses. Most low silt content sands and gravels don't heave. In low silt content granular soils, usually of low moisture content, the depth of below freezing temperature can be two or three times the so called frost depth, but no heaving takes place. Not knowing soil type many go by the cook-book method (frost depth), not necessarily the most cost effective for the site. It ain't that simple.

 

[amec2004]

Some recommendation is to replace the cohesive soil completely with frost stable granular fill to avoid frost upheave, but that replacement cost is normally very high.

The key point here is the *rotating equipment* foundation which need the dynamic analysis to avoid resonance and keep foundation natural frequency to be +-20% away from machine operating frequency.

Foundation natural frequency highly depends on the soil characteristics for damping stiffness. If the soil around and below foundation frozen/thawed during season change, it will change foundation natural frequency
significantly and may hit the machine operating frequency.


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[dik]

OG: When we did the addition to one of the large shopping centres here about 30 years ago, in the parking lot, we found frost at 13'.

Water as a liquid, in a supercooled state, has been found in our highly plastic clays at -40. It's really the silt that causes frost heave conditions.

The reason I switched from cardboard voidform about 30 or so years back was because I did a report where the cardboard void had collapsed and the space filled with water, froze, and jacked the grade beam. Initially used aerofoam 'skins' taped together (they had a lower 'crushing' strength) and then switched to a plastic product made as a void form.

Dik

 

[dik]

amec2004: have to find out from the equipment manufacturer to see if movement is possible. Don't know what heavy rotating equipment is... I've used stiffened slabs for large diesel units without concern... just checked with the manufacturer. I've also done heavy presses, on 'real' foundations, where the movement allowance is measured in 1000's of an inch.

Dik

 

[amec2004]

We live in Canada, northern neighbor of US.

We have more than six months winter in a year and the lowest temperature in the north site could reach -40~-50 degree or -58 to -76 Fahrenheit

Frost upheave and adfreeze is a common issue we should tackle with in most foundation engineering works.

AISC Steel Connection Design Software

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[racookpe1978]

amec2004

Your diagram answered my question about gravel-insulation-concrete sequences under the foundation.
Thank you.

 

[dik]

amec2004: Yup and Winnipeg, for its size, is the coldest city in the World... nothing as big with a higher freezing index. C & F converge at -40.

Dik

 

[oldestguy]

Here is another factor. How stable is the replacement granular fill below the slab? Reasonably uniform stuff will migrate under the influence of vibrations. I've seen loss of support for compressor foundations.

 

[mcdermott2]

FROST HEAVE:
If you do consider a slab on non frost susceptible fill down to frost depth, be careful what sort of fill you specify. If your native soil has low permeability, and you go in with an open graded, free draining fill, be careful that you don't create a "bathtub" for water to collect under your foundation. Choosing CLSM as your fill down to frost depth would be one option to mitigate this possibility.

If you do go with an open graded fill, consider wrapping in a nonwoven geotextile fabric lest the voids in that open graded material fill with undesirable material

Below are several resources that might be helpful for the problem at hand:
Report on frost susceptible soils (Figure 3 provides a good summary):

http://1204075.sites.myregisteredsite.com/downloads/TS/EB204P/TS204.3P.pdf

Report on the viability of CLSM (flowable fill) as frost resistant fill:

https://www.nh.gov/dot/org/projectd...tofFreeze-ThawandFrostHeaveonFlowableFill.pdf

Others have provided info on insulated foundations as a means to prevent frost heave. Below are some additional resources that speak more to that concept that you might find useful:

https://www2.cs.arizona.edu/people/jcropper/desguide.pdf

http://www.structuremag.org/wp-content/uploads/2014/10/codesstandards1.pdf

ASCE 32-01 - Design and Construction of Frost-Protected Shallow Foundations

ROTATING MACHINERY FOUNDATION DESIGN:
If this is indeed a heavy piece of rotating machinery, be careful how you are designing your foundation to resist vibration. A full dynamic analysis should be performed to ensure that resonant frequencies of the machine + foundation are outside the acting frequency range. From the natural gas turbine compressor design world, the following rules of thumb are good starting points (but are not a substitute for vibration analysis):
-Foundation / machine weight ratio > 3
-Length / Width ratio of foundation < 5
-Thickness of support block > 2'
-Width of foundation / Height to centerline of rotating shaft > 1
-Eccentricity of foundation center of gravity to machine center of gravity < 5%

Resources:
Look for articles / books by Arya and Pincus. There may be some available online.

https://www.amazon.es/Design-Structures-Foundations-Vibrating-Machines/dp/0872012948