Actual frost depths

[pbonarrigo]

I am trying to find some type of map which show the actual depth of frost for different areas, maybe a break down by county (USA). I am from Mass, the state building code states 4'-0" foundation depth accross the entire state. I am looking for something that would give some actual depth of frost for the coastal area. Inaddition, I am finding that older versions of the building code allowed a foundation depth of 3'-6", now as people add on to their house, we find that their foundation depth does not meet the current code. As a structural, I feel the need of a Geotechnical review. For a home owner who wants to add say a 20'x20' addition this is complex and costly problem. I have searched every web site I could think of. Thank You in advance.

 

[BigH]

I'd suggest the following:

1. Contact various local geotechs - in MA, you have some very very good ones. See what their experience says in various locales.
2. Contact the MADOT (I hope I have the moniker correct) - they likely have good maps in that this is important, too, for their projects/roads/bridges.
3. Contact the local cities to see if they have any records where such have been determined.

Now, if these are additions to existing houses and the foundations are at 3.5ft, and there is no sign of distress, I would opine that you should be able to seek a variance in the building code to permit a 3.5ft foundation in additions. Why did the building code change? Of course, there are variations across the state and such 'all inclusive" building codes reflect the "worst case" rather than "specifics". In such cases, I would hope that you have the option of making adjustments based on sound engineering principals and experienced judgment rather than rely on some plutocrat siting in the capital . . .

 

[jheidt2543]

Other thoughts:

1. Try contacting the state agricultural department, they usually have a pretty good handle on frost depths. In our state, there is a County Extension System, kind of an outreach network of the state government. They have a "County Extension Agent" that responds to questions like yours.

2. Try contacting some local cemetaries. The grave diggers keep track of frost depths too. You know, people are just dying to get in, but can't if the ground is frozen. (Sorry for the morbid humor, but I couldn't pass it up!).

3. IMHO, if the code changed, I would "grandfather" the older portions of the house. Just provide a notice to the Owner and meet the code with the new addition. Trying to extend the existing foundation the additional 6" below frost could cost more than the addition. Even if you added fill around the house, you could end up with drainage problems. If there haven't been problems with frost affecting the foundation in the past, leave well enough alone.

 

[PEinc]

Check out the following thread.

thread507-68846

 

[SirAl]

Frost penetration varies with soil type/condition, exposure condition(i.e. cleared surface vs. snow covered) as well as temperatures. I believe that the USCAE have come up with a graphical illustration showing frost penetration depth as a function of freezing degree-days for various types of soil. Where frost cover is not sufficient, a rigid insulation can be applied to protect foundations from potential frost heave.

 

[ishvaaag]

In any case what pbonarrigo says checks well with what Heinrich Schmitt says in his Hochbaukonstruktion, that before 3 to 4' where thought to be enough in temperate zones and now it is 4 to 5' what is thought to be adequate.

Yet maps on frost are for this. In p. 371 in chapter 7 of

Foundation analysis and design 5th ed.
Joseph E. Bowles
McGraw Hill

you have one map indicating the depth of frost, based in a survey of Bowles himself. The entire state of Massachusetts has there 0.9 m or 3 ft frost depth. However, it is the -based in some specific data- opinion of a foundation expert, not the code.

 

[Focht3]

Be careful with Bowles' book - he had a habit of representing the average value - when a range of values was more appropriate. Using his values as gospel can get you into trouble -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[PEinc]

NAVFAC DM-7.1 Soil Mechanics Design Manual, Figure 7, Page 7.1-42 has a U.S. map with contours of Extreme Frost Penetration (in inches) Based Upon State Average. The 50 inch contour passes along the southern edge of Massachusetts. The 60 inch contour passes just above the northern edge of Massachusetts. Search on-line for the manual.

 

[PEinc]

For DM-7.1 Design Manual, try the following link:

http://www.vulcanhammer.net/download/general_soil_mechanics.htm

 

[Focht3]

You need the services of a geotech familiar with frozen ground issues to look over your shoulder...

One major issue: ice lenses. Imagine an ice lens forming under a portion of the foundation, but growing at a fast rate - say an inch a week, but not uniformly. With a "swell" pressure of 5+ tsf.

That's an extreme case - but it can happen. How do you design for that? Generally, you put your foundations below the frost zone, insulate the foundations to obtain as few thermal variations as possible. Try to avoid plumbing leaks - or at least mitigate their impact if possible. And other things that are beyond my limited experience with frozen ground. (My experience has been with large commercial freezers in the "sun belt" from Texas to California.)

I expect that there are other issues as well. I'm sure others will chime in -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[DRC1]

Have you talked to the local building official? He is the guy who will approve or dissaprove the permit. Generally as long as the existing house was prmited at the time of construction, it would not have to be revised unless there is a huge change in the construction. I don't think that if the houses have not experienced a problem, that 6" will be a problem for the old foundation. A new foundation for the addition may have to meet current code. Best bet again is to start by dicussing the problem with the local building official. Also the local branch of the Soil Conservation Services should have frost depth data.

 

[peek]

pbonarrigo,
can't you add 6" of dirt around the foundation base thus satisfying the code. Also,if this fill is made to slope away from the house, it should help drainage too!

 

[ishvaaag]

A depth of the frost line map by US Weather Bureau can be seen in

Practical Foundation Engineering Handbook
Robert Wade Brown ed.
McGraw Hill 1996

p. 3.147

 

[ecanuck]

Hi Pbonarrigo:

Finding a map of the "average" frost depths should give you a fundamental idea of frost penetration which can vary with:

1: Nature of the soil - is it fine grained or granular is is satuated or dry above the water table? Is there groundwater flow - sometimes the ground doesn't freeze at all with sufficient groundwater flux.

2: Will there be snow cover or will the area around the structre be a parking lot with snow removed? or otherwise can you simulate snow cover with insulating foam buried a foot or two - extending horizontally surrounding your structure, including a retrofit of the old structure? (see industry information about that).

3: Then thre is the active approach - to somehow allow heat loss from the building to reduce frost penetration.

You said: "As a structural, I feel the need of a Geotechnical review."

That is a good idea if you cannot satisfy yourself of the above considerations.

 

[EIT2]

For related dialogue, check out thread507-76186.