Home>Discussions>EXTERIORS>Lifted concrete driveway slabs (not tree roots) - how to lower?
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marco1
Lifted concrete driveway slabs (not tree roots) - how to lower?
marco1

Hello All:

I have an interesting problem that I'm not entirely sure how to solve. The driveway to my garage has been lifted up to the point where large portions of it are now hollow underneath. Let me describe the driveway first to help you understand the issue. The driveway is in two planes - the first is flat (well, it should be and probably once was) and it extends out from the garage about 10 feet. From there, the driveway slopes down to match the grade of the slope in front of the garage - the length of the down slope portion is about 17 feet. The width of the entire driveway is about 30 feet.

Now the problem: the point at which these two slabs meet is being vaulted/pushed upwards, which has created large voids of hollow space under the driveway, running from one side to the other. You can see the space from the side and there is probably a good 3 - 4 inches of void between the slabs and the gravel base. The lifting is so great that the (previously) flat slab is no longer flat. I placed a six foot long level on this "flat" slab, with one end at the point where the two slabs meet with the other end running back towards the garage. At six feet back, it is off level by 1.5 - 2.0 inches.

I'm not entirely sure what has caused this lifting. There are no mature trees nearby, so it is not tree roots. The house is about 10 years old and this problem has existed since I bought the house about four years ago, but I think it has gotten a little worse over that time. Perhaps there are some issues from an improper pour from when the house was built - not sure about that. Perhaps the summer heat here in mid-Missouri caused it, but the problem does not go away in the winter, it's permanent. I think the most compelling suggestion I have heard has to do with the location of the house/driveway. The house is located at the bottom of a long steep hill and my driveway connects to the concrete road that runs down that hill. Someone suggested to me that the weight of the entire road on that hill is slowly pushing down on my driveway (like a concrete glacier) and has caused it to buckle up.

Open to any thoughts on the problem - but I'm really wondering what to do about fixing the problem. Would it be advisable to use a concrete saw to cut a small strip (perhaps 3" wide) out of one of the slabs where they meet in the hope that the two would then drop back down? Any chance that might work? Also, I'm not sure how dangerous that might be given the weight of those slabs?

I've tried to add a photo, but can't seem to get it to attach.

Appreciate any feedback.

A. Spruce
Re: Lifted concrete driveway slabs (not tree roots) - how to lower?
A. Spruce

How much of the slab is unsupported? If it's only a few inches, I'd be inclined to look into mud jacking to pump concrete into the void to support the driveway. If several feet are unsupported, then you've got an issue with the ground either lifting or settling.

One of the things that few people take into account when they pour a concrete slab is the ground it is poured upon. Soil is active, it moves with temperature and moisture fluctuations. Depending on where you are at, you may be subject to deep frost lines and heaving soil (due to freezing ). Could be that a wet spring has saturated the ground, causing it to swell, or the opposite, a very dry summer, causing the ground to shrink. Where I live we have no frost line to speak of, but we do experience prolonged periods of wet weather and dry weather, causing the ground to move way more than anticipated by builders, splitting foundations and slab style homes alike.

I doubt the street has anything to do with your driveway issue because you'd be seeing side shift and likely stress cracking due to the shear motion the road is exerting on the driveway. Likewise, I would expect to see sidewalk and street damage around the end of your driveway as well.

marco1
Re: Lifted concrete driveway slabs (not tree roots) - how to lower?
marco1

A. Spruce, thanks for the feedback.

There is definitely several feet of unsupported slab - under both the flat and sloped slabs. It is vaulted up where they meet and the void underneath extends several feet under both slabs, across the entire width of the driveway.

I get what you're saying about the ground underneath and you may be right. I can't really tell how thick the gravel bed underneath is, but perhaps it wasn't thick enough. Maybe the ground settling post-construction, maybe the shrinking and swelling over the past ten years?

I'm just not sure what to do with it at this point. Is it better to just mud jack and get it supported? Is there any way to really lower the slabs at this point? I think if they could be supported it would be alright. The only issue is that the "flat" slab now slopes toward the garage and if we get a heavy rain, some water does run that direction and a little gets under the garage door - not a lot, just enough to annoy me.

Thanks again!

A. Spruce wrote:

How much of the slab is unsupported? If it's only a few inches, I'd be inclined to look into mud jacking to pump concrete into the void to support the driveway. If several feet are unsupported, then you've got an issue with the ground either lifting or settling.

One of the things that few people take into account when they pour a concrete slab is the ground it is poured upon. Soil is active, it moves with temperature and moisture fluctuations. Depending on where you are at, you may be subject to deep frost lines and heaving soil (due to freezing ). Could be that a wet spring has saturated the ground, causing it to swell, or the opposite, a very dry summer, causing the ground to shrink. Where I live we have no frost line to speak of, but we do experience prolonged periods of wet weather and dry weather, causing the ground to move way more than anticipated by builders, splitting foundations and slab style homes alike.

I doubt the street has anything to do with your driveway issue because you'd be seeing side shift and likely stress cracking due to the shear motion the road is exerting on the driveway. Likewise, I would expect to see sidewalk and street damage around the end of your driveway as well.

A. Spruce
Re: Lifted concrete driveway slabs (not tree roots) - how to lower?
A. Spruce

The problem with an unsupported slab is that it can and will crack, even without use, drive a car over it and you're asking for trouble. Having the flat portion sloping back towards the garage is not a good thing, however, if you're not getting much pooling or leakage under/around the door, then it isn't something to worry about for the time being.

Without a site inspection it will be hard for anyone to say with any certainty what the problem is, I'm suspecting a combination of heave and constriction of the ground, more than anything else. IMHO, you have three choices, mud jacking, replacement of the affected area, total replacement. I would call in several local concrete guys in your area for a site consult and have them give you options and pricing for possible repairs. Ultimately, I think you're in for at least partial replacement, if not total replacement due to the drainage issues towards the garage/house, it may not be for a few years yet, but this is likely the only real fix that doesn't leave residual issues or cause other problems.

marco1
Re: Lifted concrete driveway slabs (not tree roots) - how to lower?
marco1

Sound advice. Thanks!

dj1
Re: Lifted concrete driveway slabs (not tree roots) - how to lower?
dj1

Is there an expansion board between the "flat pad" and the "sloped pad"?

Depending on where you live, changes in temps can cause one pad to push against the other. I'm not saying that that's the case here, but a close inspection will definitely be needed.

I'm just wondering why they chose to pour the slabs this way, as apposed to a one long slope. At about 27 ft long, one continuous slab would make more sense.

ed21
Re: Lifted concrete driveway slabs (not tree roots) - how to lower?
ed21

I wonder if you have a problem with water runoff somehow undermining the slab or in the worse case a sinkhole developing. It could be the earth wasn't compacted under the slab and it is just settling unevenly.

Mastercarpentry
Re: Lifted concrete driveway slabs (not tree roots) - how to lower?
Mastercarpentry

I think ed21 is onto the problem's cause. I'll bet there's some uncompacted (or poorly compacted) fill dirt under there. The proper solution for that isn't for the meek- a full demo and re-do with a better base and better compacted. But there may be another solution based on some "if's". If thew concrete is still solid and uncracked everywhere, and if the hollow areas extend all the way across about equally, and if those hollows are at the end of the sections and not the middle, then you could cut the slabs straight across to let the raised portions drop right where they sit. That won't fix the lack of soil prep under them but it will at least hopefully allow things to hold together for awhile.

If you try this, the cuts need to be pretty close to the peak- say within a foot. And you'll need to make a series of shallow cuts- if you make one deep cut then the slab could crack into pieces before you get a straight line cut all the way across it. By making shallow cuts it should break along your cut line. A skilsaw with a diamond blade should cut deep enough for up to a 6" slab thickness. The main downside of this is going to be that now you will have 4 separate slabs with each laying in a different plane (ie no longer flat) and they might settle even worse over time. Or they might not.

dj1 is also onto something here- if the driveway abuts both the garage slab and a concrete roadway at the other end there needs to be some expansion joints in there (not control joints but gaps filled with the correct material) so that summer sun can expand the driveway without causing it to bind and buckle. If you're in that spot, after making those other cuts you need to deepen the 3 cuts to a full slab thickness so that there is at least 1/8" gap open somewhere on the hottest day. You need to keep rock out of those gaps when winter cold opens them up.

Almost no builders and few residential concrete contractors have a clue about what needs to happen under slabs, foundations, footings, and driveways. Only in industrial and some commercial work do you ever see a soil Engineer involved. All the builders and concrete guys know is the minimum code requirement and whatever rumor they learned coming up in the business (if that ever happened). You can't just cut and lay a given thickness of assorted gravel fill down then place the slab and expect it to work every time. Most of them think it's OK to put a driveway over soil fill when it isn't. And most of them do not understand the need for expansion joints or how they differ from control joints. Concrete is an extremely good building material but even it is only as good as what's supporting it and there's a whole lot more to it than form, pour, and finish.

Phil

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