Frost line depth
Frost line depth
(OP)
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!
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!





RE: Frost line depth
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.
RE: Frost line depth
So, yes, I always consider frost depth when designing my foundation depths.
Mike McCann
MMC Engineering
RE: Frost line depth
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!
RE: Frost line depth
The real questions is why does your country have a minimum depth of 80cm specified?
Mike Lambert
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"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
RE: Frost line depth
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.
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Found an interesting article:
http://www.huduser.org/publications/pdf/fpsfguide....