Deep frost depth 1

[BENDOG]

We are designing an industrial facility in Canada with a 7 foot frost depth. The facility has a lot of interior equipment foundations. If the facility is heated do these foundations also have to go to the frost depth?

 

[TDAA]

Minimum depth for frost protection is typically for exterior footings.

Sometimes it is advantageous to put them all at the same depth, depending on bearing conditions, but this is not likely required with your set up.

 

[Jalthi]

I don't think the heating will affect 7 feet below finished grade. But there are ways to insulate the footprint of the strcutre. In this case, your foundation can be shallower.

 

[CarlB]

In our locale (Anchorage Alaska), design frost depth is 10 feet. For continuously heated buildings, perimeter footings are placed at 42" minimum cover (to bottom), interior footings as necessary for stability, typically 12" minimum. Perimeter footing walls are insulated to direct heat to below footings (anf prevent freezing). Only unheated footings, such as for canopies, would be extended to 10' depth. Even Fairbanks (much colder) uses similar footing depths. This isulation/footing is like a simple version of a "frost protected shallow foundation". This approach allows for even lesser footing depths, with more protective insulating methods.

Brief googling indicate Canada codes call for 48" footing depth ay many locations, you could check with local authorities.

 

[BENDOG]

Thanks for everyone's input. It's been a long time since I worked on a job with a frost depth this deep. We typically are in the 3 foot range, so it's not a big deal to make all foundations the same depth.

 

[BAretired]

CarlB,

I was surprised to hear that you use footings as shallow as 42". I am near Edmonton, Alberta. Most of the geotechs in this area specify 5' minimum for continuously heated buildings and 8' deep for open, unheated foundations.

Our code specifies a minimum depth for clay soil of 1.2m (4'-0") but not less than the depth of frost penetration (which nobody knows with any degree of certainty). The predominant soil in this area is clay.

For buildings without basements, most contractors prefer drilled concrete piles, either straight shaft or belled, but steel screw piles are becoming more popular now.

BA

 

[VAD]

Bendog:

Other factors come into play as well such as frost susceptibility of soil, and ground water level. Of course we have the cold temperatures. The codes do not normally discuss these factors. Anyhow, the codes provide certain depths which we tend to use irrespective.

BA retired. The geotechs in your area likely follow the building code requirements. Also there are different shades of whet is normally classified as clay. Drilled concrete piles need to be designed for frost jacking forces and need to be acertain length depending on soil type and groundwater conditions. Much more to this topic than observation only. Observation of performance of what was built along with understanding of ground conditions often lead to proper solutions. Indeed screw piles are very versatile and have a multiplicity of uses and not to prop up camps in the oilfield.

 

[BAretired]

VAD.
The local geotechs are slightly more conservative than the building code, recommending five feet in lieu of the code's four feet. Our clay is often described by geotechs as lacustrine clay overlying glacial till. The depth of till varies considerably.

We design straight shaft concrete piles for frost jacking in a somewhat peremptory fashion. Conventional wisdom says that a pile should be three times as deep as the depth of frost penetration. On the basis of the code, the minimum depth of a straight shaft concrete pile would be 12'-0", even for nominal loads. However, some of the local geotechs are suggesting minimum depth of 16'-0".

I have seen twelve foot long piles jack out of the ground during winter, then settle back down when the frost came out. In one case, the piles did not settle back down but remained high throughout the summer (frost jacking).

I have heard of cases where piles kept frost jacking during successive winter seasons such that the elevation of the building increased an inch or two each year. After five or six years, the building was several inches higher than it was supposed to be. The grade slab, on the other hand settled back each summer.

BA