We’re stripping our basement floor in our 70 year old house. After doing that, we noticed a large crack going diagonally across the floor. The crack isn’t wide (less than an 8th of an inch) nor are the two sides uneven (it’s level) so it’s not a big deal. However, I started tapping the floor with a 2×4 and noticed a very drastic difference between the ‘solid’ slab in the rest of the basement and the slab within 2 ft. of each side of the crack. Near the crack, there was a very distinct ‘hollow thud’ compared to the ‘clink’ of the solid areas.
I assume this would indicate a bit of a void underneath that area, and, most likely, caused the crack. Out of curiosity, what could have caused a void like this under the basement slab? There’s obviously nothing I can do about it and it doesn’t appear to be anything serious (the house is still standing 70 years later) but I’m just curious what would cause voids like that under large slabs.
Replies
Me. It would drive me crazy, I would have to core the slab and look under it, and maybe think about puming some grout under there. but then I worry about everything.
nuthing but happy thought.
If you are in coal mining country, even if mining stopped many moons ago, that void could be a deep one. Core drill a hole through, and get down close to the new hole, listen carefully, and drop a marble through. Start counting . . . one thousand, two thousand, . . . a second per.
Feed us back the number of seconds until the marble hits bottom or splashes down, and we will calculate for you the depth of your void.
In some of those mines, you could drink a whole cup of coffee on the ride down the shaft in the elevator. Think of it, you might need a bigger core drill and maybe something like a lead sash weight, to be able to hear the little thud when your dropped item hits.
Get back soon! We want to get to the bottom of this!
You could have a drainage pipe running along the same path as your crack that was never properly compacted prior to the floor being poured.
Could be that not all the organic material was removed prior to the floor being poured and that it has settled leaving the gap.
Could be that some soil was put back because of overexcavating the foundation and never compacted.
Could be that your foundation sits on uneven soil conditions or partly on rock.
In other words it could be a lot of things.
But I would attend to it and fix it by pressure grouting underneath and filling the void.
Gabe
If the house is 70 years old, the slab has not shifted, and the crack is as narrow as 1/8" I'd forget about it. There are a lot better uses for you time, mind, and money.
What makes you think that the slab is as old as the house?
What makes you think that the void is as old as the house?
What makes you think that the slab section won't collapse tomorrow?
Oh never mind, you would have to think............
Gabe
Last week there was a news story that showed a 50's era house with single car garage.
The pulled the pickumup into the garage and the front end of the slab colapsed about 2 ft.
> What makes you think that the slab is as old as the house?
It's a guess.
> What makes you think that the void is as old as the house?
I don't know. Hence me asking.
> What makes you think that the slab section won't collapse tomorrow?
Nothing. Again, hence me asking. I doubt it's a mine shaft...but we're near indian burial mounds...(uh oh...now it's getting spooky!)
Actually, a drainage pipe makes a lot of sense. And old gutter drain runs down the inside corner where the crack starts.
It sounds like I should be a bit concerned about this...how does one investigate it? Drill a small hole? Tear up the slab?
Is pressure grouting akin to mud jacking?
> ...but we're near indian burial mounds...(uh oh...now it's getting spooky!)
Indian burials probably would have been well compacted before the house was built, but if it turns out to be Judge Crater or Jimmy Hoffa, that's another story.... ;-)
-- J.S.
Years ago I ran into a similar problem. I was helping the client ready the house for sale when I noticed the bedroom slab was a couple of inches below the base board. The slab pinged off with a serious echo. They stopped all the work and called in an architect friend. Last I heard there was a mini sink hole due to building over a dry creek bed. Seems the watertable had rose enough to cause their problem. I never heard what they did and never saw a for sale sign go up.
The coolest thing, when I was about 15 helping my dad remodel an old farmhouse. I was in the bedroom/converted back porch when I dropped my nail set. It rolled into a very wide crack in the wood floor. There was a moment of silence then plop! Like water. I called my dad and told him. We started dropping nails and listening to the splash.
The HO agreed to let us take up the flooring and look. We found an old cistern that know one in her family could remember being there. Seems they've owned the house since Oklahoma became a state.
I spent three days shoveling sand into that sucker. My dad ordered two 11 yard truck loads.
My dad said he didn't want to pump out the water before filling with sand. He was afraid of what might be in the bottom, maybe old uncle orville or worse.
It could be washout, if you've got ground water running under your floor. It could be a waste line that sprung a leak and has washed away things. Or, as Micro pointed out, it could be mine subsidance. You do need to investigate, if for no other reason than if you sell the house, you'll have to make a disclosure or be liable for a hidden vice lawsuit years later.
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
I disagree about investigation being necessary to avoid future liability. When a home is offered for sale, all you have to do is truthfully disclose what you know about the condition. You are under no obligation to investigate for someone else what you don't know yourself. If you investigate, find a significant problem, then hide it, you may be liable. If you doesn't bother you and you don't know any more about it, you would not be liable.
" If you doesn't bother you and you don't know any more about it, you would not be liable."
Well...it didn't bother me until I posted my question in here...now everyone's got me wondering... ;o)
Yeah, but now he knows there's something wrong; if he doesn't find out what, he'll look pretty stupid trying to convince a judge there was no bad faith involved.
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
Not long ago I had occasion to watch someone look inside a wall using a 'borescope', a flexible fiber optic inspection tool with a small light. It's a little bit like what they use for endoscopy. Get your hands on one of those, drill a small hole (1/2"?) and see what's down there. A good rental shop should have one.
Can't help but think of what the home inspector said to me when inspecting my old place before I bought it. We were walking around in the old out-building, basically two walls and a roof abutting the house and the garage. He said, "Hmmm- this part has a concrete floor, and this part has a raised wooden floor. Wonder what's under that wooden floor? Then again, we probably don't want to know...."
A poor guy in Hamilton, Ontario, decided to break up the concrete patio that covered most of his backyard so his kids would have some lawn to play on. The garage had a gravel floor which was low in spots- he had lots of time on his hands, being unemployed, and figured he's save a few bucks on disposal by using the concrete rubble to level out his garage floor. He started digging in the garage and unearthed a drum...then another, then another...Turns out the former owner was doing a little moonlighting as a hazardous waste disposal agent for his employer...
Instead of covering it up and quietly putting the house up for sale, the guy called the Ministry of the Environment for help. They helped, all right- they gave him a clean-up order on the property...Now he's after the former owner's estate for the money to cover the clean-up costs- good luck...
If you are concerned, you could call a plumber and have him/her send the scope into the crack. You may have to open it a little, but most of the newer scopes/probes are very small. In reality I would open a 3-4" diameter hole and check it out with a flashlight. I don't think I would use pressure grout as you do not want to raise the floor, just seal the cavity. For a small void use "soupy" readymix and pour it in. For a large hole I would call a concrete supplier and order a slurry mix and dump it in.
Regards
Mark
Mark:
Thanks for the info. If this is doable as a DIY project, I might tackle it.
Is the pressure grout the same as mudjacking? If so, that's not really that expensive, is it? The pressure has some appeal in that it may be able to force itself everywhere it needs to go a bit better than just pouring stuff in, but I certainly don't want any undo pressure from underneath. The slab is perfectly level...just a thin crack running through it.
Ugh. Old houses. Charming, but a lot of work.
Kind of like kids, I guess.
;o)
My house is 80+ years-old. I’m am glad they don't build them like they used to. Mine is junk! Looks nice, well now it does, but has poor bones. The basement floor appears to have been floated by someone on their knees and using their hands. It looks like remains on handprints and it slopes every which way. As I was walking through the basement not to long ago, my foot fell through. I needed two bags of redi-mix to fill it. To make it match I used my hands to float it. It appeared that there were some organic materials in there and over time it decayed and formed a void. Again, I probably would not use pressure grout as the floor is not structural. If you inject it with pressure, it will have a tendency to push the floor up. Without a column above it, the concrete floor will rise and cause more cracking. The reason this reply is wishy-washy is because I do not want to lose my license, as i do not know the specifics of the task.
Regards
Mark
I can't resist telling this story.
A few years ago, in the house we lived in at the time, we developed a sinkhole in the driveway--I turned in one day and my tire pushed a chunk of the concrete down about 6" or so. It got worse, and it was right next to the sidewalk (think: neighbor walking dog at night, busted ankle, lawsuit) so I got a couple of buckets of gravel, filled the hole, and poured a bit of mud to finish it. About three days later I'm outside and I notice water running down the driveway. Looking closer I see it's coming out a crack in the driveway about half way up. Somehow I put 2 and 2 together--wife has just drained the bathtub, the sewer lateral is under the driveway, and the patch I made clogged the sewer so now it's bleeding out the cracks. Turns out I was right. Most of the neighborhood houses have 'street traps' made out of clay pipe. They eventually collapse and the driveway or sidewalk caves in. You can see sinkholes developing all over the neighborhood. Bottom line: $3K for a trenchless sewer replacement. I was lucky because my patch only partly blocked it, and it worked for a few more days while I got someone on it.
Be careful what you shove down thru a hole in a slab.