Compaction of frozen soil

[Estew]

I have tried to find information on the compactability of frozen soil. I can not find anything that would help the technicians explain to the contractors that the soil will either not compact or will required additional effort to achieve the specified results. I have tried to experiment with running proctors on frozen soils, using sand and clayey sand. The results were inconclusive. I did not have the most controlled environment to perform this study. I would like to know a certain temperature of the soil that would make for difficult compacton procedure. I know this will be different for each type of soil.

 

[CarlB]

I'm not aware of any specifications, or acceptable practice, that allows compaction of frozen soils, or allows placement in freezing conditions. Here in Anchorage, for City projects, fill placement is not permitted when air temperature is below freezing. Sure, if the material is well above freezing and is compacted quickly it can be done, but as a pracical matter it should not be allowed. Once soil is frozen it forms hard clods that can not be further compacted using normal methods. There are many instances of where it was tried, only to have problems once the material thawed.

Maybe if you are using dry pea gravel you may have success using material that is frozen, but if the soil has any fines and is frozen it is unlikely the material can be compacted.

If someone finds a way to compact frozen soil they could start a whole new industry!

Regards,

Carl

 

[dirtsqueezer]

I've been a soil tech for about the last year. I don't have any documentation to offer you, but I've got practical experience.
Here's where you run into problems filling in cold weather:
I've been on more than one project where the fill reads 95% or better, but there is a lot of movememt (pumping and yeilding; deflection), and I have to fail them anyway because the soil isn't 'firm and stable'. Obviously, your base won't move that way when it's frozen, so you won't be able to spot when this happens. This would be especially bad in, say, a parking lot, where you are using crushed rock, which is a very dense layer and does compact on a softer base. As soon as that ice thaws, wham! Instant recipe for potholes. And I've seen some pretty big potholes.
What it will really come down to in the end, I think, is the quality of the companies you work with. Winter construction work is always gruelling, and the contractor, if they have any useful experience, should know this already.

 

[DRC1]

Frozen soil will compact, but since much of the material is bound by ice, and a significant portin of the voids is filled with ice, it will lossen when it thaws. If you do get good results with your procter test it would be interesting to know the results. When we backfill in the winter, assuming the subgrde is ok, we,ussually use crushed stone. Lean concrete or flowable fills are also options. Wating till spring has been done also.

 

[jbp]

Working with frozen soil will almost always yield unacceptable results. One time I allowed a relatively small amount of frozen silty clay to be buried deep in a road fill. The material was approximately 15 feet below grade. I justified it in my own mind that if the frozen soil is thoroughly broken up during compaction, the moisture reading should indicate the water content even if it is frozen. We placed it, obtained 95% compaction, moisture was a few percent above optimum, and about 10 years later, no problems yet.

 

[GThomas1]

I'm certain that you've already put more effort into this problem than is necessary. We all know that it is impossible to achieve true compaction with frozen material. Remember basic chemistry - what is the relationship between frozen and thawed elements? Therein lies your answer. Yes, it is true that frozen soils may be compacted with extreme effort (e.g. dynamic compaction methods), however this remains true only if that material remains frozen. When it thaws, as stated by DRC1, the voids will collapse and the material will be less dense. Bottom line is: If the frozen material is placed in a friable condition, and densified using dynamic compaction methods, and will remain in a frozen state indefinately (such as above the arctic circle) then the density will remain as well. If not, you're wasting your time. Simply use leancrete / flowable fill, or wait for better weather. A good reference to read is "Soil Engineering: Testing, Design, and Remediation" by Chen, F. H. (Fu Hua), CRC Press, 2000 [ISBN: 0849322944].

 

I am building a concrete patio approximately 14 inches from grade. I have filled the area with pea gravel except the outside permimeter where I have left a 1-foot width for solid concrete. My question is what is the best method to compact the pea gravel prior to filling with concrete?

 

[dirtsqueezer]

I would water it with a hose, if anything.
In the construction industry, pea gravel is said to be compact as it is dumped into the ditch. It is common to place it around a pipe without any compaction whatsoever.
I just recently heard, however- in fact, I think today, that there is a locality that uses 'stingers' to compact their pea gravel. A stinger, as was explained to me, is a hollow metal rod with small holes in one end and a hose attachment at the other. The idea is to stick the spray-end in the pea gravel (or other fill), and it is compacted by lubricating the fill and allowing it to settle on its own.
As a footnote, it occurs to me that concrete would probably soak into that pea gravel pretty well. If you pour your concrete so that it flows out of the chute (as opposed to 'breaking off'), it seems like in the end you would have some pretty sturdy pea gravel. Watch out, though. The more water you add to your concrete, the weaker its compressive strength. Good luck!

 

[vlinker]

I know that Steve Adamczak of Shannon & Wilson has performed a laboratory study that might help you out. I'm sure the results are published someplace in the geotechnical or cold regions engineering literature. Contact Steve at SA@shanwil.com. He can direct you to the publication.