Frost Penetration and Movement 3

[someengineer]

Hello Engineers,

Does anyone know at what angle frost will be transmitted through soil as a result of vehicular impact loading? I imagine it would be a function of the soil type. Does anyone have any information regarding this?

I need to bury a pipe beside a road, below the frost line and I am concerned with additional frost penetration from the road (from impact loading) being transmitted horizontally - since the frost depth under the road is deeper than the frost depth adjacent to it.

Thanks!!!

 

[oldestguy]

Somehow that old idea of impact still goes around.

If you believe you can change the movement rate of heat transfer one way or the other, try pounding on your frying pan when cooking your breakfast.

Heat loss direction and rate, which is frost penetration, can go in all directions controlled by moisture, insulating factors, temperature gradient, soil type etc.

By the way how do you know the true depth of frost penetration? Take the city of Rhinelander Wisconsin with clean granular soil of low moisture content. Water mains at 8 feet depth freeze, while generally building codes seem to list the frost depth as 4 feet. It all depends on many factors, one of the more significant being the moisture content of the soil. The higher the moisture content means lesser frost depth. The extreme case is a lake. There ice thickness is usually less than nearby ground "frost depth". Will driving cars on the ice make it thicker? It probably will reduce the insulation effect of packed snow on top and be interpreted as an impact effect.

Your best advice will be coming from engineers that study arctic conditions and soils involved. There are some university types that have formulas for this, but unlikely will impact enter into the equations.

 

[darthsoilsguy2]

snow is the definition of insulation. keeps the cold wind from freezing the soil deep

 

[CarlB]

A- "snow is the definition of insulation. "-fairly true, acts as insulation
B- "keeps the cold wind from freezing the soil deep" -not quite, wind doesn't affect frost penetration much. Snow has insulating value (see part A)

 

[kelowna]

Oldestguy, thanks, you made me laugh and I did need it. I do not understand someengineer's post either.

I know very little about this subject (although more these days in my last project) but I never heard of frost penetration having anything to do with impact loading.

 

[ishvaaag]

Well, humour is good, yet consideration like this are those that bring new questions to solve.

It is a feasible scenario some truck impacting in congealed roadside soil and transmitting by rigid body a force; something of interest if one wants some service warranted even to such extent.

I myself yesterday in one of these forums called attention to flexural, lever or prying action, whatever, at fillet and butt welded tubes against a plate, subject to tensile force; to some this is a non-issue, for others a subject of interest.

If you read the Mechanics as told by their original authors, Jouguet, you will see that many of the originators of the theories and calculations we use today had not them (or not all that they devised) right, and in the end was all polished by the work of many other people, a task at which we all are at.

"Impact factor" or "safety factor", well, both have had, still have practical use, and in this case we even can visualize the impact of concern. And curiously as the intervention of oldestguy points, this uses to appear when some lack of proper knowledge stands (if not always) so well ... we have a question at least supported by this pair of concepts, and a bit of good humour.

 

[someengineer]

Thanks for your posts.

We had a geotech determine the frost penetration and he determined 2 depths. One for regions around roads and another for underneath the roads (deeper frost penetration). His justification for the deeper frost depth was that as vehicles drive over the road, the loading drives the frost down deeper (my understanding was that it was a result of impacting, but i could be mistaken).

I am no expert on this and this is why we had a geotechnical engineer determine frost penetration for us (he has a doctorate so i would like to assume he is correct!)

What i am trying to determine is if that frost penetration under the road will transmit horizonatally to some degree.

I suppose the best option is to consult our geotech again, since there is some debate on this subject.

 

[bpattengale]

I've always been under the impression that the deeper frost penetration below roads is due to the lack of snow cover. The roads are generally kept free from any insulating snow cover which allows the frost to penetrate deeper then the adjacent ground with the insulating snow cover.

In my area of the country (Minnesota/North Dakota)the years where we don't get a lot of snow but the temps are just as cold are usually the years we get the most watermain breaks and frost out is usually later. The lack of snow cover allows the frost to go deeper those years.

 

[BigH]

Frost penetration and frost depth effects are really two different animals. As OldestGuy indicated, even in very cold climates, they recognize that footings do not have to go full depth of frost penetration if they are in non-frost susceptible soil. If the soil is non-frost susceptible (meaning that there is no significant change in volume (i.e., water freezing), you can put footings down fairly shallow (I did this in northern Ontario - frost depth was 9 ft (3 m) but the soil supporting structure was clean sand (non-frost susceptible) the water table was below 12 ft (4m)meaning that even if the soil had some potential, the effect of freeze heave would not occur. While the building foundation might be acceptable at 4 ft, water filled pipes would not necessarily be - they are full of expanding water. Concur with thoughts that roads, due to non-snow cover, see deeper penetration and that impact loading seems to be a bit far fetched.

 

[darthsoilsguy2]

air voids percentage of the soil matrix may be less for the road area assuming same material and less compaction... theoretically, less air voids should translate to lower R-value. perhaps the impacts of traffic create this compaction.

hmm...possible derivation of the origins of impact theory?.... oh, the mystery remains

To me, winter will always be both freezing cold and windy.... why i went south like a duck.

 

[oldestguy]

Snow is a great insulator, as do dense grasses, layers of leaves. Also, lack of sunshine (wooded areas), newly roofed over earth can have an adverse effect.

Asphaltic concrete versus Portland Cement will result in differing frost penetration, since that dark stuff is warmed by sunshine and the white stuff reflects heat.

South facing areas get beneficial sun heating to off-set the process to some extent also.

So the idea that road traffic has a frost driving effect probably is the result of some of the things mentioned above.

On this subject of frost under roads, can someone come up with a good rule-of-thumb for how to scientifically tell highway "commissioners" in this area when to take off the spring-time road bans for trucks over 10 tons. Southern Wisconsin. These days it is just a guess.

I'd like to suggest something like "when the frost heaves are gone at approaches to culverts", but that still means the soil can be supersaturated and yet quite weak. Fancy tests like the Dynamic Cone are beyond their capability.

These road bans are a significant problem on lower class highways affecting construction progress.

 

[CarlB]

Looks like Minnesota has some rational basis, may be applicable to Wisconsin-

http://www.lrrb.org/pdf/200018.pdf

http://www.dot.state.mn.us/ofrw/PDF/springloadReport.pdf

 

[BigH]

Thanks CarlB - will look at it later. I always thought that using a dynaflect during spring thaw to determine the effective modulus / deformation would be a good way to tell when such bans could be lifted.

 

[concretemasonry]

bpattengale -

I also have seen the differences in frost penetration in Northern Minnesota. - It is all matter of exposure since the warm winter sun heats the pavements (unfortunately the days are too short and the clear nights are too long), but frost penetration can be minimal.

The exposure is a very big factor. Doing surveying at a plant in Northern Minnesota in February after a week of of -30F temps and an earlier long, cold winter with a good snow cover we found the frost has only gone down 6"-12" in the snow covered areas.

At the same area we had a recently built (less than a year earlier on a 40 to 60' fill access road that serves as the general plant construction access for 2500 employees per day, associated deliveries by semis and an errant 65-75 ton dump truck. When we attempted to dig a trench perpendicular to the road for utilities, we encountered frost about 15' deep (code for frost was 48" at that time), while the surrounding areas had only a foot or two of frost.

I don't think this was due to "vehicular impact", but due to the lack of snow cover repeated loadings that compacted the fill additionally increasing the thermal transmission/heat loss in the areas subjected to the loadings in comparison to the protection provided by the small snow cover (12"-24" on the level). This is an extreme difference because of the conditions and the fact that the road/fill was very new and exposed while the surrounding areas were protected by the insulating snow.

P.S. - A section of the road settled when a 150' fill settled 15' on a weekend, but that was not problem when the mining company was looking for placed to dump large amounts of coarse fill (1' -4').

The exposure (temperatures and snow cover) can have a large influence. Because of this, work around a road can be highly variable when it comes to frost penetration. If a road heaves, it is usually not noticed since it usually is a general movement and the is no firm point of reference to measure from. For a building, it is a different story, so everything depends on the exposure and criteria. Air temperature is not as big a factor as exposure and cover is.

Dick