Frost lifting a barn skirt slab and binding slider doors

[s219]

So I noticed last week that my barn slider doors were dragging on the concrete skirt in front of the barn after we got a real cold snap. So I adjusted them up a little. Then it got colder and the doors were pretty much resting on the skirt and wouldn't budge, so I had to adjust them up again. Total adjustment probably 3/16", but I don't have a whole lot more vertical adjustment left if this ever happens in the future (I'd need to pull the doors and rip some wood off the bottom wear strip).

When I lived up north as a kid, we generally took extra precautions for frost (extra walls, barriers, etc). But down here in VA, while footings are down at least 12" to be below the frost line, slabs just sit on top with no extra precautions. Clearly, frost must have lifted my skirt slab a little, given the historical/abnormal cold. For those of you who have dealt with this before, will the slab settle back down when the weather warms up? Are there any steps I can take to prevent this in the future? I'd be inclined to dig down around the edges of the skirt and put in a frost wall or barrier (could be as simple as foam I suppose), but that isn't going to do anything for cold coming down from above the skirt, or for that matter, from the adjacent barn slab (barn is unheated). Might prevent moisture from getting right under the skirt slab though.

As of yesterday (when I made the final adjustment) it got up into the mid 40s and the problem persisted. But I noticed that some parts of the ground under the snow pack are still frozen solid, so I imagine it will take more than a brief warmup to thaw frost under a slab. Odd to see frozen ground here and mud there -- it's like spring skiing in Vermont in the old days....

By the way, I can't say for sure that frost lifted the skirt since I don't have any "before" measurements written down. I suppose it's entirely possible some other part of the barn framing settled and the doors dropped a little, but with the appearance of the problem coming with extreme cold temps, and affecting both doors, I am thinking it's no coincidence. Unless all the poles and wood in the barn somehow suddenly contracted by 3/16" and got shorter because of the cold.....

 

[DFB]

It should settle down ok once all the frost is gone. Is there any drainage issues you can address come Spring? Things like poor drainage only exaggerate problems and add to the heaving. Keeping the snow off of areas like these just allows the frost to run deeper too but there's not much one can do about that if you need a clear entryway access.

There has been a lot of similar problems around here this year especially with Commercial buildings where the doors swing out instead of in and code requires the outside to be level with inside with no step down for disability requirements. My bud works for the town and he told me they are thinking of temporarily closing one public building he maintains because the doors wont completely open at the moment, probably be at least 6-8 weeks before the frost leaves the ground around here.

With the new building at work the contractor built porch slab heaved enough to buckle the vinyl siding corner post and same deal with floating slab outside an exit door last year. The small slab in front of the exit door heaved enough that the door would only swing out about a foot. This summer I used an excavator and lifted the small slab up and out of the way and added crushed stone for drainage then replaced it. Again there is virtually zero step down clearance to meet code. Even the corner of another slab (one I set and I poured) 18 x 30 and 5 inches thick moved enough to create entry problems to our donut shop this year. First time in 3 years. Again an out swinging door. My solution...I want to put in a slider this summer. Job security

There was an 3 season room built on slab at my Maine camp that every single winter I had to raise the strike plate on the door to get it latch right and every spring put it back.

 

[Egon]

The soil heaves when water is wicked up by capillary action and the freezes. Replacing the present underlying material with a coarse aggregate that doesn't wick should fix the problem.

Come spring all will return to normal as the frozen ice lenses melt.

 

[dave1949]

My theory about things that frost heave and don't go back down, is that it depends on the spring weather.

If frost has lifted a rock or slab, it is perched on, or held up by icy dirt. If it rains or a lot or snow melt happens while it is perched by the frost, some dirt and sediment will be washed in underneath the rock or slab. It can't return to the level it was at because now there is some fill below it that wasn't there when it heaved up.

I believe this is how rocks gradually emerge from the ground over several to many seasons.

I like Egon's coarse aggregate solution. If that is decently drained there is less moisture to freeze and expand causing heaving.

 

[DFB]

Coarse stone also allows space for frozen water expansion

As Dave explained...in a field that's exactly how rocks do work their way to the surface.

IMO the weight of a concrete slab would negate most soil resistance from settling back down. If the rocks were that big the site certainly wasn't well prepared

 

[s219]

Thanks for the comments guys. The mention of gravel underneath got me thinking -- I only put a thick layer of gravel under the front half of the skirt; the back half where it butts up to the barn floor (and under the doors) was pretty much on dirt as it was already up to level and severely compacted from construction activity. I didn't even think to have gravel under there for frost reasons. On top of that, it has been wet as heck here all fall and into mid winter (before it became cold enough to snow instead). So I bet it was damp under there for sure.

I'm going to put a pencil mark to show the skirt level now and see if it drops back down as we thaw out. I might add some gravel around the edge of the skirt to promote drainage in the future (could only help). If this is a repeating/worsening problem in the future, I may need to pull up the skirt slab and add a thicker layer of gravel underneath. Makes total sense, but wasn't even on my radar when pouring the skirt.

 

[Billrog]

If your going to put anything under the slab put Styrofoam 1" will do it don't create a reservoir for water to sit with gravel. Keep any water away from around the edge of the building so it can dry out .I'm not so sure your slab will settle all the way back down.

 

[oosik]

I have seen slabs, pony walls, extended entry slabs frost heave and usually they will return close to where they were. In one case it did not and in another it dropped about an inch. Without exception, all situations were a water drainage issue. Water going where it shouldn't be, freezing and we all know the rest.

The solution is doing whatever it takes to keep water away from the cement structure.

 

[dave1949]

I think the purpose of the coarse stone is to have a material that drains readily--but there has to be somewhere, or a slotted drain pipe in the stone, for it to drain to/through to accomplish anything.

Dirt or sand will not drain readily, always retaining some moisture which can freeze and expand.

 

[DFB]

Drainage is all important in building. Grade patterns, proper fill, drainpipes. Even roof water run off should be controlled to keep it away from the structure.