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Undermined patios & walks


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I typically see some thing on the order of 3" - 6" of undercutting and in the picture that you see it was about 6" and ran for about 6' of the walk where it wrapped a slope.

I have seen what happens with driveways when this happens. Kachunk, piece breaks off when the car drives over it.

With a walk I am thinking of a concentrated load like wheeling in a refrigerator.

I have gotten complaints before when I don't mention it.

At what point would you mention it if a few inches is not a concern?

Chris, Oregon

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At what point would you mention it if a few inches is not a concern?

If I can stick my hand in with my thumb on the edge and there's still space beyond my fingertips. I honestly don't know how I came up with that, I certainly didn't base it on any published standards.

I have seen what happens with driveways when this happens. Kachunk, piece breaks off when the car drives over it.

With a walk I am thinking of a concentrated load like wheeling in a refrigerator.

Yeah, I could see that happening with a driveway. I think it would have to be a pretty big void for a wheeled refrigerator to break a sidewalk though. Of course, the one in your picture looks like it's not very uniform in depth.

You asked about packing it with gravel. You would have a hard time packing it tight enough to offer enough uniform support. The only thing I can think to do is have it mudjacked, that is having grout injected under it.

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Originally posted by Chris Bernhardt

For simple undermining at the edges of patios and walks, what are some suggested course of action?

I can't see why one couldn't just pack the edges with crushed rock or gravel and correct grading and drainage issues to prevent reoccurence.

Chris, Oregon

Riding mowers can snap these edges.

I've had success dry-packing the void with grout. Ram it in with a length of 2x4 and wait for soil moisture to harden it. This method is pretty easy and works remarkably well. It's way cheaper than mud jacking and any halfwit can do it.

- Jim Katen, Oregon

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