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taadz 08-30-2012 08:33 AM

Settlement under Garage Floor
Looking for recommendations/options.

I had my driveway removed and new asphalt put down recently. During the removal process we noticed a hole under the garage floor. I dug a little more and was able to shine a flashlight and noticed a rather large cavity under the floor, most likely due to settlement over the 25 years the house has been there. I estimate the cavity to be around around 8' x 4' by 3' deep. Needless to say it is no longer supporting a good part of my garage floor. Using the contractors backhoe we were able to push enough dirt under the front of the garage where the driveway starts, to support the asphalt. But i know this will soon erode back. I marked with spray paint on my garage floor where the cavity is, I was thinking that i could cut a 3' square hole and just fill this void with gravel. I've noticed a slight bow where water collect, now I know why. How do people usually deal with these type of problems? Also, it's a 2 car garage with no signs of cracking or other evidence that this cavity exists. Short of the area where the water collects (run in from heavy rain) I would never of know this problem was growing under the floor. Thanks. Tom.

rjniles 08-30-2012 08:41 AM

I doubt settlement could cause a 8' x 4' by 3' deep hole. It sound more like a sink hole. If you try to cut a hole in the slab the whole floor may collapse in on you. I would hire a structural engineer to evaluate and recommend a solution.

jcarlilesiu 08-30-2012 09:52 AM

I agree, I would hire a structural engineer. The initial review and recommendations will probably be cheaper than a removal and replacement.

With exception of welded wire fabric installed during construction, the garage floor is lacking tensile strength and is not designed to be an elevated slab. As such, and as if settlement continues to occur, the probability of a failure and collapse may increase. Even if settlement doesn't continue, stress caused by loading the slab may cause a failure.

The good news is, there is quite a bit of methodology to resolve the situation without removing and pouring a whole new slab. It is possible the SE may recommend coring holes to pump new slurry under the slab to re-support the slab.

The issue with these recommendations is that no guarantee that settlement will continue and you could find yourself in the same situation down the road.

Good luck.

rjniles 08-30-2012 11:12 AM

Here is a write up on mud jacking:

taadz 08-30-2012 11:21 AM

Thanks for the info and links. I had thought about the floor giving a little if I were to cut a hole in it.. I'll look for an SE in my area..

GBrackins 08-30-2012 11:54 AM

rjniles and jcarlilesiu are spot on in their recommendation to see the evaluation of a professional engineer. the solutions they can come up will probably save you money in the long run instead of trying various options only to find out they didn't work.

Good luck!

rjniles 08-30-2012 12:19 PM

Also, I would not park a car in that garage until you resolve this. That could get real ugly.

concretemasonry 08-30-2012 01:03 PM

That is not a unique situation and if may not even be a structural problem at all.

I am a structural engineer and bought a 25 year old townhouse about 7 years ago and was familiar with the situation. So far, I have not done anything because it is not high enough on my list.

In many areas, it is a common situation (possibly a problem) depending on the type of structure. If you have a true floating foundation slab that takes the loads, it certainly can be a structural problem, requiring a structural/soils engineer.

In my case, the townhouses (and most homes) here are built with stem walls below frost (48" here). The common method of construction is to put in the strip footings and stem walls (usually block) to get up to the planned grade. After that (especially at 0F), the foundation is capped and the framing and finishing can be done under cover. During construction, debris and scraps(wood dry wall, etc.) are frequently thown into the garage area and may accumulate. One of the last steps is pouring the basement slab supported all sides by the strip footings and compacted soil after the mechanicals are completed. After that, the garage slab and the sidewalks and driveway are poured after exterior grades are set and garage floor drains or patterns are set.

In the garage area, soil is dumped and compacted to cover the debris and to be the base for floating garage slab. Ultimately, the scraps deteriorate and remove support from under the slab. In this case, the slab is just a covering for the soil and surface to park on.

The 3' you found is exceptionally deep, but once the support is gone it makes little difference how deep the void is.

In my case, I will have the slab and some soil removed. Then, some good soil well compacted and a new slab with mesh poured. - A one day job for a 2 car with reasonable access. My neighbor did the same and after looking at his situation, the settlement was only1" to 6" and generally in the center.


danpik 08-30-2012 04:29 PM

We had a similar problem under the factory floor at work. Several large voids spread thruout the 150,000 square foot facility. We called in a company that drilled holes ofer the voids and pumped grout in to fill the voids. We also had them jack several low/sinking sections. This was done over 10 years ago and there has been no problems since. In fact one section that was lifted now has a 200,000# mold press sitting on it. Price was real reasonablr too.

jomama45 08-30-2012 08:38 PM

As Dick mentioned, it's not a huge job to tear-out the floor, remediate the base below the floor, and repour the floor, and we typically complete them in a day. What is uncommon though is that you have a 3' deep cavity, which I can say I've never seen anything even close to. I've seen entire floors non-supported after the base settled a few inches, but never that much depth...........:eek:

As for mudjacking, I suppose it could be an option, but when the cost for pumping a few yards of "mud" is figured into the cost, I don't think it's going to be the best option..........

Tham 08-30-2012 08:58 PM

Very interesting thread. Let us know.


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