Hollow under driveway slab. How to fix?

[Andy Hill]

Got a strange one here. In the past year, one section of my ~13 year old concrete driveway (roughly 10' square) has developed a couple of cracks and settled 1/4" - 1/2" in the center relative to the edges. By itself, that wasn't enough to really bother me (other than a "that's weird"), but this weekend I noticed that section actually had a noticeable "drumming" sound compared to the other parts of the driveway when I dribbled a basketball on it. Not only did that make me lose concentration during a cutthroat game of "HORSE", it also made me start worrying a bit.

Anyhow, there must be a bit of a hollow under the driveway in that area. No water, sewer, or sprinkler lines in the vicinity, so I don't think it's water erosion. Not sure how to go about addressing the problem, 'tho. Just get a mud-jacker out and cross my fingers, or are there other things that need to be investigated first?

[MaryL]

I can't answer your question (no knowledge on my part). However, I may be able to make a suggestion about what caused the problem. When I bought my house several years ago, a large part of the concrete drive had collapsed. It was probably much worse than you described, and I negotiated for a reduction in price that covered removing a large section of the driveway and replacing it. When I had that done, I also paid to have the rest removed and replaced because there was no rebar, and I thought it was best to do the whole thing. Anyway, I asked about what caused the problem because (like you) there was no drainage problem or water lines in that vicinity. It turns out that a *gopher* had burrowed under one side of the driveway, and the previous owners failed to fill it. As a result, heavy rains continued to wash out the area, and the concrete eventually collapsed!

MaryL

[effi]

if by mud jacker you mean a company that will pump some kind of material under your slab to relevel it, supposedly those technologies are affordable and feasible

don't just cross your fingers, have someone who knows what they're doing look at your situation if you don't fully understand it

[Evon]

That could be a sink hole forming. I had one in my parking lot - opened it up and it was large enough to swallow a car. 110 tons of large stone and a new blacktop job looked good until they dynamited for a sewer line across the street. Now hole is back and the city has no liability - limestone county sink holes form w/o notice. David and Goliath fight coming.

[Gideon]

A few caveats:

1) Determine why the cavity has formed. If this is a result of significant re-grading of your property before construction and subsequent settling of the soil under the concrete, then this is generally a one time problem. If it is from rodents burrowing, a sink hole situation, etc. - then you must address the cause before addressing the void.

2) Mud-jacking slab-jacking techniques are generally temporary fixes. From my observations in our neighborhood, such repairs may degrade in 5 years or so and need to be repeated. I haven't had a chance to observe a driveway which has been jacked, but the load-bearing requirements for a driveway will severely tax the slab-jacking. Note that slab-jacking will create a small number of mud-cement pillars under the slab and most of the void will still exist after the procedure. The small area of soil under those few pillars will now support all of the weight of the driveway and whatever is upon it.

Gideon

[John B]

Lucky for you your house wasn't built on that sink hole site! I can just picture the house going down upon detonation of that dynamite.

[John B]

I'm with Gideon on this point. You might investigate the history of your lot. Was there another structure before your driveway was poured? Suppose there was a basement, etc. Then inadequate soil compaction would explain your problem...and portend of a greater threat. Do you live in Florida? (Sink-hole territory?) "Gideon" wrote

[Andy Hill]

No known prior structures. AFAIK, the area was just being used as horse pasture before being built on. It's old river bottom (lots of river rock, mixed with sandy areas and the usual river deposits). The area used to flood every few years, so it wasn't a great area for living structures. It hadn't flooded since dams were placed upstream in the mid-40s, so I would've thought that any organic muck left over from floods would've had 50 years to decompose. Maybe not.

Grew up in sinkhole territory back in the midwest (along with the occasional collapsing coal mine). Nothing like that around here.

Once the weather warms up a bit, I might just have to piss off my wife and dig myself a bit of an inspection hole. The problem area's close enough to the edge of the slab I should be able to get a look-see without displacing more than a few dozen yards of dirt :-) . Nothing like a little idiot stick work to make me appreciate a desk job.

[John B]

As long as you don't hit yourself with the pick, the exercise will be good for you!

"Andy Hill" wrote