What causes frost heave?

[EET]

I live in Northern Utah. Two years ago I had a new drive way poured and some sidewalk squares replaced. Last winter one section of the driveway began to rise. I called the contractor, he came out and said it was frost heave and would go down when it warmed up. It did so I didn't think anymore about it. This winter, the same section is doing the same thing again and so are the sidewalk squares. What is causing this and can I stop it? It is a real pain to snowblow and shovel over this raised edges.

Thanks in advance.

 

[JAE]

When water freezes, it actually expands (unlike a lot of other materials). During the winter months, soil that has moisture present will expand with the freezing and heave - usually/only upward.

To avoid this effect damaging structures, footings are usually placed below a pre-determined frost line depth so that through the winter the underlying/supporting soil will not move the structure.

In the case of sidewalks and driveways, you cannot extend these types of elements below frost depth (at least economically) so they are designed to move with the soil. To account for differential movement (among other reasons) concrete sidewalks and drives are jointed with control joints and expansion joints to minimize the damaging effects of this movement.

What you can do (minimally) is try to keep any excess moisture from penetrating below the concrete. If you can't do this, the new concrete can be doweled into adjacent concrete to avoid trip hazards.

I replaced my driveway near my garage some years ago and took the extra step to thicken all four edges of the concrete down to a depth of about 18". Now this didn't get below frost, but it did act as a sort of cut-off wall below the ground to limit the amount of moisture getting under the concrete.

Also...your contractor isn't totally accurate: I don't believe that the slabs will necessarily come down to the original level after thaw.

 

[Ron]

One other point is that to reduce the potential for frost heave, place a material below the concrete that has more pore space (such as a gravel bed), so that when moisture is present, it is not as likely to fill all the pore spaces. When it freezes and expands, it then only gets bigger inside the pores, with less likelihood of building enough pressure to push stuff around.

 

[CarlB]

Frost heave requires 3 things:
1. Freezing temperatures, causing frost level to penetrate the ground
2. Frost-susceptible soil (generally means silty or clayey)
3. A source of subgrade moisture

The heaving is due to more than just freezing in the pore space, there is water migration and ice lenses are formed.

Typical approach used in traffic areas (including sidewalks)when there is frost susceptible soil in the subgrade is to replace the top several feet with non frost-susceptible (NFS) soil - usually a clean sandy gravel. Probably 24" minimum should be used. In very cold climate sometimes 6 feet of NFS material is used, or a combination of NFS material and subgrade insulation. Makes for expensive roads.

Your contractor should be aware of these standard techniques. While true a heaved area usually returns to original level (or almost) upon thawing, there is the inconvenience and possibly damage that occurs. If the excess water is due to Fall rain that penetrated the subgrade, the first winter could be worst, then it's not as bad in subsequent winters. It seems not to be the case for you.

A 'fix' may entail removing the sections causing the most trouble, such as at doorways, garage entry, etc., excavate 24", place 2" of subgrade insulation, backfill with NFS material, then repave or replace concrete.

By the way, this a very common occurrence, unfortunately, here in Anchorage, Alaska.

Regards,

Carl

 

[JAE]

Carl...do you deal with perma-frost there in Anchorage, or is it only found further north?

 

[EET]

Thanks for the information.

 

[CarlB]

JAE,

A few "remnant" areas of permafrost have been encountered in Anchorage, but it's rare. You have to go a few hundred miles north before it's common (discontinuous permafrost), and about to the Arctic Circle before you have 'continuous permafrost', with some exceptions.

But in Anchorage we do have up to 10 feet of seasonal frost penetration, in areas kept clear of snow. Newcomers are usually surprised to learn we bury waterlines 10 feet deep!

Regards,

Carl

 

[RMPE]

Other obvious recommendations would be to direct any surface water (ie from snow melt) away from the slab areas, by grading or adding drains if possible.

 

[JAE]

Thanks Carl.