Frost protection of foundations 1

[SKJ25POL]

Is there any other methods or reason that we can arguee that it is okay not have the foundation to frost depth?

Does an equipment pad located outdoor needs to go to the frost depth?

In what cases we can go above the frost depth?


Thank you

 

[CBEngi]

Locally our frost depth is 4'. Sometimes for equipment pads (Slab-on-grade) we put rigid insulation under the slab and extend it out 4' on all sides. This has the effect of forcing the frost to come down around the insulation at 45 degree and not pass under the slab.

I wouldn't use this method if the equipment on top was sensitive to movement. This method reduces frost heave but doesn't eliminate it.

 

[jayrod12]

If the equipment can accommodate significant movement vertically then technically you don't need to provide any frost protection

 

[KootK]

The usual alternate method is referred to as a shallow frost protected foundation design (CBEngi's method). Here are some references:

Link
Link

If it's a mission critical application, I'll usually solicit recommendations from a local geotechnical engineer.

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.

 

[TLHS]

For minor structures that are non-critical you can discuss with your client whether they want to take a risk.

I've worked in a lot of northern canadian industrial sites where it's impractical to always build to frost and the amount of insulation and distance you have to carry it past the edge of footing sometimes makes things impossible.

If you've got an electrical cabinet where you can build some slack into the cable, it's silly to build a footing two or three meters into the ground. Same for small scale equipment that could take some degree of movement and isn't process critical.

Alternatively, you can look at the history of the site. Not all soils are frost susceptible and not all water tables are in a position to cause problems. You can quantify that, to some degree, but you can also examine the site. If you have pavement nearby that's been in place for a decade, you can take a look for instances of frost heave. Maybe you have well draining soil and are near the edge of a hill, or have a deep drainage channel nearby that would keep a high water table from forming. You can also look at detailing of existing structures on the site. If they have shallow equipment foundations and they work adequately, you may be fine.If you aren't comfortable doing that, find a geotech that is.

In all of these cases, you should explain the risk-benefit situation to the client.

"Standard practice is <something>, the cost of doing it this way is <lots>, you may be able to do <something cheaper> because of <reasons>. There is a <negligible/small/medium/significant> risk of <something bad happening. I recommend <blah>."

As long as it's not a life safety issue and you have a somewhat sophisticated client with the ability to understand the consequences to their equipment, then there's no issue with approaching from this direction. Just make sure that you document the basis of your design.

 

[msquared48]

Between the spread footings of the mainframes of a PEMB, I generally do not extend to frost as the area is generally not bearing. The end walls, however, I do.

Mike McCann, PE, SE (WA)

 

[oldestguy]

Going by "cook-book" rules for depth of "frost" implies you are ignorant as to what really happens in cold weather in varying soil and ground water conditions. Fortunately most of the time it works. If you are in a critical situation, and know what rally happens down there, you can significantly vary from that "cook book".

 

[bigmig]

ASCE 32 is the guide on shallow footing design and specification for buildings. Frost heave is very much a function of water content of the bearing soils. In otherwords dry soil will heave much less. In addition, heavy traffic actually drives frost deeper (i.e. roadways). Investigate the expansion coefficient for ice.....compare that to your water content in your soils under the worst case condition. From there you can calculate your estimated expansion (i.e. heave). Key word....ESTIMATED.

I have seen frost heave lift buildings several inches over decades. The ice lifts, fines fill the voids, the building repeats the process next winter.

 

[TLHS]

bigmig,

I've never seen a real basis for the argument that traffic impact drives frost deeper. It doesn't seem to make any sense, and the only actual writing I've ever seen on it is people disagreeing with it. There are all sorts of reasons why frost depth would vary on roadways (cleared of insulating organics, generally kept clear of snow, different fill materials, etc), but I don't think impact driving frost deeper is one of them.

 

[SKJ25POL]

Gentlemen,
Thank you very much for your responses.
Great information.

Just was wondering when using rigid insulation how much it reduces the frost depth?

Let's say for regular 4ft frost depth area, does it depend on the foundation size and thickness?

Thank you

 

[jayrod12]

The geotech's around here generally say that 1" of insulation equals 12" of footing depth. The insulation is typically specified to extend horizontally the same distance as the expected from depth. So for your example, if you wanted to put the footing basically right at grade (you need at least 12" of soil cover over your insulation, or some other method of UV and damage protection) you would need 4" extending out 4 feet. For an 8 foot frost depth you'd want 8" extending 8 feet.

However, that is strictly a very conservative rule of thumb and as many others above have noted, it does not necessarily apply to all areas or applications.

 

[dicksewerrat]

TLHS,
Traffic does send the frost deeper. I dug up 1,000's of feet of trench in MPLS. Frost depth in busy roads, normal winter 5-7 feet, residential roads 46 feet in the yards adjacent to both roads, 2-3 feet. And yes when I was there we installed and repaired sewer pipes all winter.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[TLHS]

I don't dispute that frost depth is often greater under roadways, but it's likely due to lack of insulation from organics and snow, different fill materials and higher densities (lower void ratio results would presumably result in higher thermal transmission).

I can't see a mechanism by which smacking something would move heat. If I stick a steel rod in the fire and then smack it against the ground a bunch, the heat's not going to transmit any faster than if I just left it alone. You also aren't physically driving particles deeper with traffic, or you'd also be having huge settlement problems.

Minnesota MOT does a huge amount of frost related research and none of the formulas I've seen in their papers take into account the amount of traffic on the roads. They have lots of experimental data and if they saw correlations between frost depth and the amount of traffic they'd take advantage of it.

You can also see that snow cover makes a big difference:

http://downloads.geo-slope.com/geos...Effect of Snow Depth on Frost Propagation.pdf

 

[oldestguy]

TLHS observations sound like a true observation, but as to why perhaps is a little more complicated than "traffic driving down". Of course lack of insulation is a factor, but what about sun energy reflection vs absorption, wind cooling (due to traffic moving the air), transmission of heat differences doe to pavement thickness, etc.
These all can be assigned some mechanism that makes sense, maybe. Impact doesn't, especially way down there.

 

[JStephen]

Water expands when it freezes, and applying pressure to ice near the melting point can melt it. But I would have my doubts as to whether that affected ice formation under a roadway, especially one without heavy use.

 

[Spartan5]

Regarding the "driving" of frost down into pavement and underlying soil. The lack of insulation is a big factor when the surfaces are clear. Lawn areas (where not compacted with traffic) have lots of trapped air. The lack of this protection makes for ready conduction of the heat in the ground to the cold air. Worse yet, when the snow is compacted on top of that surface, it both concentrates something very cold with a high latent heat capacity directly onto the surface which increases the flux dramatically over that of pavement simply exposed to the air (consider your face in the cold air vs. your face with snow falling on it vs. your face having compacted snow pressed directly onto it).

The lack of cover also drives energy loss due to convection. You can stay warm in the sun on a still cold day if you're not moving. However, once the wind blows, the transfer of energy increases significantly. Like a lawn, hair on your body can help to mitigate that. But like pavement, a bald head chills quite readily with the wind blowing over it.

Finally, there is the loss of heat through radiation on those crisp clear nights. Exposed to the absolute zero darkness of the sky, the pavement will give up it's energy quite readily. That's the very reason we get frost and/or black ice in situations when the air temperature is still above freezing.

All of these things serve to drive the ice deep into the unprotected ground. At least as I understand it anyway.