Frost line depth

[mar2805]

Hi guys!
How many of you take in account the frost line depth while defining the foundation depth?
Im asking this couse for my country wich is relativly warm, has mild winters rearly under 0"C for no longer then 7 days, minimum foundation depth is 80cm below finished ground level.
Converstaing with my friends from Germany and UK, many of them use rafts as foundations (residential hoses 2-3 stories) wich are usualy placed not deeper then 40cm below FGL.
In Germany and UK temperatures in winter can be under -5C for lonnger then 7 days.
If I want a raft foundation in my county I would have to use a ratf with an perimetar beam with an height of +80cm!
Any other thoughts on how to tackle this problem when choosing a raft for a foundation system?
Thanx!

 

[oldestguy]

Interesting question. Since concrete conducts heat probably about as well or better than soil, I see no benefit for a "frost wall". I suspect the heat from inside is generally sufficient to prevent edge heaving of a slab. In the colder climate of northern USA states, with deeper "frost peroration", many non-heated buildings are on a slab on grade, yet heaving of the slabs seems to be a minimal problem, at least in my experience.

Here a number of codes do define "frost depth" as required foundation depth for outside walls. However, that "frost depth" can vary considerably, depending on moisture content of soil. The lower moisture content of granular soils will allow deeper "frost penetration", but fortunately those soils are not usually types that heave.

 

[msquared48]

In Alaska, and many locations in the northern US, the frost depth is 48" or greater. Here in the Northwest, I commonly use 18" as a minimum, but greater where required by the local jurisdiction.

So, yes, I always consider frost depth when designing my foundation depths.

Mike McCann
MMC Engineering

 

[mar2805]

"in the colder climate of northern USA states, with deeper "frost peroration", many non-heated buildings are on a slab on grade, yet heaving of the slabs seems to be a minimal problem, at least in my experience"
Very interesting and totaly contradicts the regulation requirements.
Maybe regulations on these minimum foundation depth are taken as very conservative.

Does anyone here maybe has any ground temperature data at define depths that were monitored during winter days/months for your local town and can you draw an comparison for regulations requirements?
I found few of these for my local town and one town up in the north thats colder in the winter.
Generaly at 50cm in the coldest months, temperatures were never below 4˙C and at 100cm there where never below 6˙C!

Another problem that I can think of if going "by the book" is, even if I use edge perimetar beam for the raft, wich will ensure my foundation depth below the frost line, this still doesnt means that tempertures under the slab wont be below zero!
As oldestguys pointed, concrete is a good thermal conducter, wich will also cold the groound beeneth the slab very fast unless there is an thermal material attached to the perimeter beam as seen from the sketch attached.
So this would mean that you woudl also have to inslulte your foundation perimetar since your foundation solution is of an "raft action" and not just a strip perimetar beam.

Thoughts...

@msquared48
"I commonly use 18" as a minimum, but greater where required by the local jurisdiction"
So, you dont follow your local regulation codes!

 

[GeoPaveTraffic]

I always recommend foundations (isolated, strip or mats) be located below the published frost depth.

The real questions is why does your country have a minimum depth of 80cm specified?

Mike Lambert

 

[EngineeringEric]

80cm does seem deep if you are not in a freezing climate, 40cm is more like 15" so that seems like a decent minimum. we typically go for 24" below grade (60cm)

 

[EngineeringEric]

"We" meant my firm. But my firm and I always look at the minimum by code; As M^2 stated above

 

[BigH]

The point missing, in my view, above is that the "frost line depth" is just a line that frost can penetrate - but says nothing about the effect of the frost. Case in point, in northern Ontario we had a site where the frost depth was about 3 m (10 ft). The soil, however, was clean medium to coarse sand to depth. The groundwater table was down at 5 m (15 ft). So am I going to dig my footings down to 3 m when the material that would be affected by "frost" is non-frost susceptible? I think not and we didn't. When one is applying a frost line criteria shouldn't one think, too, as to whether the material is frost susceptible and where the groundwater is located?

 

[oldestguy]

Per BigH comments, it is unfortunate that many code writers know nothing about frost heave, and probably a lot of users of the codes don't either. My experience with inspectors that are covering many facets of a job also know nothing about frost heave. However, now and then there is an inspector that will say "If you say so, on some recommendation (and you are a PE), that usually takes care of it. I usually try to tie it to moisture content by saying "where is the most moisture content and what is the dept of freezing?" Answer: a lake. and shallowest freezing depth.

 

[msquared48]

@ mar2805:


"Thoughts...

@msquared48
"I commonly use 18" as a minimum, but greater where required by the local jurisdiction"
So, you dont follow your local regulation codes!"

Your sketch has absolutely nothing to do with the frost depth required - the depth of the foundation below grade with the intent of minimizing the chance for heaving due to freezing BELOW the bearing level of the foundation footing.

What your sketch implies is the use of rigid insulation to limit the lateral transfer of energy from the inside to the outside and vice-versa. It has absolutely nothing to do with frost heave and/or frost depth.

Contrary to your statement, I do follow the local code, and as a minimum!






Mike McCann
MMC Engineering

 

[mar2805]

Hi guys!
I talked to a friend who works at the Civil Engineering University and the minimum foundation depth due to frost danger has change for my region from 80cm to 70cm.
The depth is defined due to minimum temperatures that could accur in 50 years period for a given region.
In my region the minimum temperature forcast are -15`C wich we found unbelivable!
In 40 years there has never been an temperature drop under -5`C in my region.
We agreed that this forcast is somewhat exaggerated but you never know what might happen and if it does youll be to blame to not designing by regulations.
So maybe better to bring those foundation to 0.70m depth and also benifit from aditional ground surcharge but at the cost of more concrete and reinforcement beeing instaled.

We talked aboth "what if" you dont go by the book but still want to design foundations at shallower depths. The "solution" was to dig out the "frost suspicious" ground and fill it up with well graded sand and gravel (fractions from 0.6mm to 60mm) wich have to be compacted and after compaction we usualy run plate test fi30cm wich determines the constrained copressibility modulus ( I dont know how you call this in USA but it ranges from 20 MN/m2 to 60MN/m2 for well compacted granular fills).
What do you think?
Attached image

@msquared48
"What your sketch implies is the use of rigid insulation to limit the lateral transfer of energy from the inside to the outside and vice-versa"
Yes correct but it will also prevent the tempeartures going below 0`C under the raft wich will ensure me not to worry aboth slab heaving forces IF founded in "frost suspicious" ground.

 

[pdzeng]

In the UK the maximum frost depth is generally considered to be 0.45m below the surface. This was based on data gathered from some very cold winters towards the mid-to-late 19th century and the frost line hardly ever reaches that depth in practise. Typically UK building codes specify that foundations be located a minimum of 0.75m below surface level.

 

[BigH]

The point I tried to make was that frost lines/depths of freezing do not necessarily imply that a soil (or footing on soil) will frost heave - would one put a footing to frost depth if one encountered massive granite at a depth less than the frost line depth? Perhaps it is now such a sad state of affairs that codes have permeated geotechnical engineering to the point that one follows the code rather than using sound engineering principals. Good luck digging out 450 mm of granite rock just to get below the code's frost line.

 

[pdzeng]

I don't think that we are in a situation where building codes superseded engineering principles just yet. As with anything it is important to justify your decisions, and if this is done then in my experience good sense tends to prevail. However, I have not yet worked on a site where ground conditions prohibited the use of foundations beyond prescribed minimum depths, so maybe I'm in for a shock!

 

[dhengr]

It seems to me that BigH is right on the money on several accounts. The are plenty of instances where the footings or slabs don’t have to be below the frost line, and you still won’t have frost heaving problems. But, this takes some engineering experience and judgement to determine with any confidence. And, it seems that these qualities are becoming less and less common in our profession or in the agencies which watch over us and our work. There are also plenty of conditions which will alter this magic frost depth number, and a good foundation designer had better pay attention to them too. “It is now such a sad state of affairs that codes” and building dept. thinking have eliminated any need for common sense and engineering judgement and you can spend hours arguing with some dummy, with an important title on his business card, who doesn’t know the difference btwn. well drained sand and gravel or fat clay. And, if you step across the state line, the frost line might change by 18". How do it know to do that? But, following the code allows any dummy to be a builder, just read that convoluted crap and follow it to the letter and anyone can build anything, and the BO will get payed and be happy too. However, it does seem that the sketch the OP’er. shows has more to do with designing an insulated foundation, to minimize cold inflow into the bldg., than the frost heaving we are talking about. This too, seems like a bit of a black art the way it is codified and bureaucratically controlled. We can follow this crap until we are blue in the face, and won’t save much energy, if we can’t convince people to save water or energy in a hundred other simple ways.

 

[mar2805]

I wouldnt call it black art just common sense
Found an interesting article:

http://www.huduser.org/publications/pdf/fpsfguide.pdf