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There is a sidewalk running over an old tunnel. It's near a grade school, and is used often by faculty and students. The tunnel is completely open, but blocked at both ends so there is no access under the sidewalk. A 6-inch hole has developed in the sidewalk and is now a trip hazard. Any idea how I could patch this hole, so it will hold until next summer when I can collapse the tunnel? The hole is next to a welded-shut manhole and is open into the tunnel, with a piece of steel (almost rusted through) spanning the hole.

Any ideas???

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    Rather than patch, would it make sense to just cover the entire area with a single sheetgood of some variety? Even a piece of tread plate (painted yellow to avoid more tripping) might do the trick. Cover, rather than patch since you intend to demolish it anyways? – BrownRedHawk Sep 25 '15 at 14:34
  • @BrownRedHawk Sounds like an answer (and safer than any patch). – bib Sep 25 '15 at 14:44
  • I see cracks extending from the whole are you sure this thing is structurally sound enough for people to be walking on right now I can't see the whole entire area so I can't say for sure but it might even be a good idea to keep traffic off of it – user41750 Sep 25 '15 at 23:14
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Immediately, and until you are done patching it, put an orange traffic cone over the hole.

Slip a flat stick tied to a wire in the middle through the hole. Pull the wire up to make the stick cover at least part of the bottom of the hole - you can perhaps twist the wire on the rusty rebar to hold it in place. Push the remaining ends of the wire down below sidewalk level.

Wet the sides of the hole.

Mix a VERY stiff (relatively dry) mortar or cement patch mix and place in the hole. For best results, don't try to fill it level at first - some will drip out the remaining hole at the bottom unless your stick is a very close fit. The aim of the first batch is to really plug the hole. When that has set up a bit, but before very long (a couple of hours) come back and use a slightly wetter mix to fill to sidewalk level.

Alternatively (depending on the size of this tunnel and the expense of "collapsing it next year") call up a company that "pumps grout" and have them show up with truck, poke a nozzle in the hole, and fill the tunnel with "grout" (more or less low-grade concrete, though there's probably other technical differences.) Depending who does what in your area, "cellular concrete" might be another option (effectively "foam cement.")

If it's your tunnel (I presume if you plan to collapse it next year), I'd be just as concerned about the vehicles parked on top of it breaking through (given the obviously deteriorated condition of the sidewalk, and something parked right on top of it in your picture) as of people tripping, liability-wise.

  • Thanks for this idea.. I couldn't think of a way to create a bottom for the patch, but I think this could work. The vehicle in the photo is a little golf cart parked on the grass. We have considered the possibility of collapse, so the sidewalk will be eliminated altogether. Also, thanks for the tip on pumping grout into the tunnel. I've got a large amount of material to use as fill, so it's just a matter of bringing in a machine to do the digging, so it might be more cost effective than the grout option. But thanks! Something to consider. – MadTownSarah Sep 25 '15 at 16:27
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You should contact the city and/or owner of the property/tunnel, and alert them to the problem. It looks like a city manhole cover, so that should probably be your first call. If they don't own the tunnel, contact local utilities (gas, water/sewer, electric, etc.) until you figure out who owns it.

You should not personally try to fix the problem, unless you are the owner of the property/tunnel, or have the owners permission.

Patching the hole could result in a larger hazard, than a large visible hole. If your patch fails, it could end up injuring somebody. Which could potentially lead to legal action against you.

While you wait for them to fix the problem, you can place a traffic cone over the hole, or rope off the area.

  • I work for a small college, and this is an internal manhole (our property). The manhole and tunnel have been out of use for many years, and the sidewalk really needs to be ripped out now. Thanks for the consideration though- always need to be aware of potential hazards here! – MadTownSarah Sep 25 '15 at 16:29
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as Ecnerwal described using wire attached to a narrow strip of plywood, but if you were to use a fast set concrete mix (some types will harden in minutes) it could be mixed loose. Once poured into opening with the board plugging the bottom it would flow easily filling every part of the void. Level to grade and snip the wire flush to the patch.

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