How is frost depth determined / calculated? 1

[mcdermott2]

Frost depth is an aspect of the majority of foundation design that I do, but thinking about it, I realize I'm not sure how the actual frost depth is determined. I see STP1358, "Remote Field Methods to Measure Frost Depth" (from the ASTM website) outlines several ways that frost depth can be calculated. Are the methods described here typically used? If frost depth is determined for a county, how many tests do they perform before the county is satisfied with their estimate of frost depth? Is climate change taken into account in frost depth calculations? It seems to me that there are a ton of variables that could affect frost depth, so am just curious how these numbers are typically determined.

 

[JAE]

Usually based on local knowledge and geotechnical references over time - but also maps such as THIS

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faq731-376

 

[oldestguy]

Well,not getting into detail, using the map will be a number, not necessarily meaning anything correct. First off what is the soil type? What is the percentage passing the number 200 sieve? What are you trying to protect? What is the water table depth? Will frost heaving be of concern? Right, many variables as to potential problems as well as effects. Each site is likely different.

 

[MTNClimber]

Not sure how they are actually developed. I assume by local experience and not by tests. I just use refer to the state, county, city, or town building codes. I highly doubt that they account for climate change.

I've been in the field during long extreme cold periods and I can attest that the local building code's frost depth was pretty conservative in that area. Building code frost depth was 4'-6" and I never saw frost deeper than 2.5'.

 

[mcdermott2]

Thanks, all,

I am familiar with maps like the one JAE provided, and am familiar with state-amended IBC provisions that speak to frost depth (as well as county, city, town building codes). The question is really how do they come up with these numbers?

It sounds like the consensus thus far is that these are based on local experience. Anyone have any insight beyond this?

oldestguy - I think you started getting towards the root of my question -- soil is variable, so how can a county provide a generic frost depth without considering things like soil type, % fines, etc?

Thanks!

 

[fattdad]

I just googled the search, "How is frost depth determined in North America." Interesting reference to, "Frost tubes," a term I'd never heard of before! Also, some reference to grave diggers. There's always somebody digging a grave and they (apparently) make remarks on frost depth. Now where's that clearinghouse!

f-d

ípapß gordo ainÆt no madre flaca!

 

[MTNClimber]

Looks like WisDOT has a network of frost tubes for determining when to set weight limits on roads. Pretty interesting.

https://www.youtube.com/watch?v=cGvOoYYKAS0

 

[oldestguy]

Locally there is a need for some way to set footing depths for houses, etc. to avoid frost heaving in almost any local area. But, for some soils it is not necessary to go to that depth, yet you can't get that detailed usually. However some countries do have lesser depth rules for non-frost heaving soils. The simple way, but not necessarily least expensive, sets a depth that usually works. That then is the local building code depth requirement. Not highly scientific, but you do something in the way of rules.

 

[heligeo]

Simple approach to frost depth calculations use either Stefan (one layer) or modified Berggren (multi layer) equations. Stefan equations ignores the volumteric heat of the frozen and unfrozen soil, so overestimates frost penetration.

Modified Berggren Equation

depth of frost = (correction factor, based on soil properties and latent heat & climate data)*(thermal conductivity, freezing index, time)

Frozen ground engineering by Andersland and Ladanyi is a great resource for frozen ground. Other equations relate climate averages to ground temperatures in a pretty straightforward way. Surface cover (snow, vegetation, or lack of) can have impacts on the thermal regime as well.

 

[GeoEnvGuy]

I thought they just asked the municipal water engineers which water pipes freeze and at what depths.

The first stage of site investigation is desktop and it informs the engineer of the anticipated subsurface conditions. By precluding the site investigation the design engineer cannot accept any responsibility for providing a safe and economical design.