4

I found a sinkhole under my concrete walkway, which extends beneath the steps leading to a canal. It's around 3x5'wide and 3' deep at the lowest point. I believe it was caused by a broken sprinkler line, as I found a broken extension pipe under the sidewalk. However, I can’t confirm if that line was ever capped off since it's embedded in concrete.

It also looks like someone previously patched a hole on the other side of the walkway, as there’s a block of concrete there, preventing me from accessing the void from that side. So my guess is that line was leaking for a very long time.

What’s the best way to fill this hole? Should I use expanding foam or concrete slurry? Would it be better to throw in a layer of soil or rocks first, then top it off? I assume completely topping it off with concrete might be difficult.

Also, will foam or concrete interfere with drainage, since they aren’t permeable? I want to make sure it won’t create more issues in the future.

Just to add there's 3 pvc pipes running under that walkway - water supply (extension made for a faucet in the backyard), sprinkler line, and another sprinkler line that was broken. Would you replace the section running under that walkway with some other material that would do fine in foam or concrete when I'd fill that hole?

Thanks!

Photo1

Photo2

Photo3

Magnum
  • 123
  • 6

2 Answers2

5

There do exist foams intended for below-grade use. A problem with foam, though, is that many of them have an exothermic cure: they self-heat while curing. This usually leads to a requirement that foam be applied in layers limited to a thickness of a few inches.

In the utilities and public works arenas "flowable fill" is a preferred material for filling voids where compaction is difficult. Flowable fill is essentially low-strength concrete consisting mainly of sand and water. A bit of liquid dish soap can be added to help the mixture flow. Portland cement powder also improves the flow, and gives it some hardness after cure. The amount of cement added is less than would be used for concrete - maybe 20% of what would be used in a concrete mix. Mechanical vibration, or even just shaking a stick in the mix, will help it flow into the voids in the space.

All your existing pipes should be fine in concrete, foam, soil, or sand. It would be a great idea to place a sleeve (a large pipe, like 4" diameter or so) so that future pipes, including repairs to those existing pipes, can be pushed through without any need to excavate or undermine the walkway again.

Greg Hill
  • 35,866
  • 42
  • 91
3

I've built a few driveways in areas with a high water table, and lots of precipitation.

They way you build a driveway/path/sidewalk is that you start with large rocks, then add 3/4 crushed rock, then sand or gravel. If you have a lot of clay in the soil, you might want to add some landscape fabric to avoid the gravel and sand mixing the clay. But in that sandy soil (= no compression) it might not be necessary.

Then make sure that you divert any runoff water so that it does not run under the concrete and undermine the work.

Cheery
  • 12,884
  • 1
  • 20
  • 47