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.

 

[vtsnowedin]

An old contractor once told me you could build a road out of horse feathers if you could just keep them dry.
Up here in New England we put four feet of granular clean ,sand gravel, crushed gravel or crushed rock under a paved surface to keep it from heaving when frozen and below that we shape and drain the subgrade so that water is directed away from the roadbed as much as possible. Underdrain pipes six feet below edge of pavement are standard in any earth cut.
Your slab will probably settle back pretty much to where it was as your frost is probably not too deep and hasn't been there for many weeks. Next summer you can try adding side drainage that gets the water three feet below the bottom of the slab and drains off to daylight well away from the building. If that doesn't work you can either install doors that can function with the slab frost jacked up or redo the slab. If you redo the slab cut your subgrade down a uniform two feet and drain away the lower corners of the subgrade. then backfill with sand and crusher run that meets spec. (Less then 12% silt in the sand portion). Two eight inch layers of sand and one eight inch layer of crushed gravel or crushed rock and it will never move in Virginia again baring an earthquake.

 

[CobyRupert]

The internet says (thus it must be true!) that water expands ("heaves") its volume about 9-10% when it freezes (lifting the soil that it is saturating). = Drainage! (or putting a bigger covering over it)

 

[vtsnowedin]

The internet says (thus it must be true!) that water expands ("heaves") its volume about 9-10% when it freezes (lifting the soil that it is saturating). = Drainage! (or putting a bigger covering over it)

It can be much more then 10%. Ever notice when you put a straw in a glass of water how the water level is higher then the level in the glass? It is caused by the surface tension of the water. The smaller the diameter of the straw the higher the water will go. In a fine soil like loam or clay the small spaces between the soil particles act like straws, very small straws, that suck up water from below and keep adding it to the ice lens that is forming as the cold strips away heat from the water. If enough water is present and the soil fine enough you can get heaves that are two feet or more higher then they are in summer.
Granular clean material on the other hand has straws too big to pull water up so the frost heave can't begin to grow until the road is frozen all the way down through the sand and gravel to the top of the water in the subgrade.

 

[DFB]

When I lived in Maine the area I was in was mostly clay soil. I had never seen soil in my life that would heave like that. It would lift and swell just like a sponge during the wet seasons and then when it would drain and go back it would leave voids that if you stepped on it would collapse down. Coming from a 3 generation family of sand gravel mining and processing that my grandfather had built up I was amazed at the soil behavior. And of course with tillage if the timing wasn't right when it dried it was just clumped up in balls hard like concrete! With a lot of new house construction around there they would only dig down 3 or 4 feet at most to set footings and actually fill in and build up around the exposed foundation walls with drawn in sand fill...expensive operation!

Travel a lot of secondary roads here in New England you almost never fail to come across a large bump especially in late Winter or Spring most usually a large rock under the blacktop pushing its way up from the frost

 

[s219]

The more I think about it, drainage would have to be a factor here. The barn is on a gentle downhill slope. Grade slopes away from barn on the sides and back, but there's no way to get around the fact that the natural grade slopes toward the front of the barn overall. I did proper sloping of the skirt and driveway of 1/4" per foot going out 10' from the door opening, and that seems to divert surface water just fine (could see it flow away from barn even during a downpour). But surely dampness/moisture continues to work towards the skirt through the soil below.

What I will probably do is dig a trench around the front of the barn and skirt and go down 12-16" or so to get below our typical frost level, fill with #57 gravel, and then carry the trench around to the swale on the sides of the barn until it meets up with grade. That will make both the top of the skirt and the underlying fill act as somewhat of a high spot compared to the trench.

 

[mjcountrybumpkin]

Just curious , how much snow is on the roof. If a decent amount could it be flexing the structure downward ?

 

[s219]

Just curious , how much snow is on the roof. If a decent amount could it be flexing the structure downward ?

There was about 8" of snow compacted by 1" of sleet on the roof for a while (it's mostly all melted now due to sun and warmer temps). Certainly possible the added weight has settled the pole footings a little more (barn was only built last May/June). I will take a look at all the poles to see if I can tell if they settled any, relative to the floor slab. Each pole had some expansion strip placed around it so that it wouldn't stress or crack the floor slab if there was movement.

 

[DFB]

Ha know what some amateur volunteers did this weekend?

That public building I had previously mentioned that has a big issue with getting the doors to fully open outward because of slab lift, was to be used for public restrooms during a winter festival being held this weekend and the building inspector had made his ruling that only a limited number of people could be in the building at any one time given the bottleneck the narrow door openings would create.

So my buddy told me last night one of the hubby's of one of volunteers took an 41/2 angle grinder and ground the concrete walk down in front of the doors so they would fully open...now my buddy says a day later the door doesn't open all the way again!