void under slab (under 2 post lift) - can it be filled/mudjacked?

[j keith w]

I'm renting a space from a friend in a pretty old building (40s or earlier) with concrete slab floors. I've been in there for a year now, and am just getting the electrical sorted and installing my 4-post storage lifts and a 2-post service lift. Today, when setting a post on the 2-post lift, I over drilled the last (5th) hole and went through the slab - and discovered a void of about 8" below the slab. I pushed a wooden dowel down into it and was able to sink it another 20" or so into the soft soil, and the last 6" of the dowel absorbed moisture.
Previously, I'd learned from a neighbor that this property had historical flooding issues at the loading dock area (it's a dug-in style dock), but the city apparently resolved all those drainage issues a few years back. I suspect this is what has undermined the slab over time.

The slab seems pretty stout; there's no weird cracking or settling anywhere in the building, and I wouldn't have known had I not over drilled the one hole.
Is this repairable/reinforceable with mudjacking techniques? I have the space slammed full, and emptying it isn't a real option (I have two years left on my lease with a 1-2 year optional renewal; this is a temporary space while building a permanent future location elsewhere), so I'd love to shore it up if possible and get on with work/business.

Thoughts? Can it be as simple as hiring a firm to come and inject slurry into the void?

 

[manwithtools]

You could inject slurry or polyurethane foam. Only problem is, how much will it take to fill the void? That will impact costs for sure, I'd contact a couple of mudjacking companies and get their opinions...

 

[wssix99]

Whatever you put into the void is going to wash away and leave you back where you are right now if you don't find and fix the source of the erosion problem.

I recently was through an old building like this and voids were created by cracked sewage pipes. Every time a toilet flushed, more material went out from underneath the slab and into the sewer...

 

[jdewitt]

Whatever you put into the void is going to wash away and leave you back where you are right now if you don't find and fix the source of the erosion problem.

I recently was through an old building like this and voids were created by cracked sewage pipes. Every time a toilet flushed, more material went out from underneath the slab and into the sewer...

Per OP, it sounds like the original erosion problem may have already been fixed (flooding).

 

[j keith w]

Per OP, it sounds like the original erosion problem may have already been fixed (flooding).

Whatever you put into the void is going to wash away and leave you back where you are right now if you don't find and fix the source of the erosion problem.

I recently was through an old building like this and voids were created by cracked sewage pipes. Every time a toilet flushed, more material went out from underneath the slab and into the sewer...

yep, seems the water issue was solved years ago (or longer, hard to tell with small town gab.) So now we're left with at least a void and some weird cave moisture down there, from what I can tell. (I know my dog really likes huffing that 3/4" hole in the slab, that's for sure)

 

[wssix99]

The soft soil and moisture had me thinking that the flooding isn't fully resolved. At least, it wasn't remediated.

A 2 post lift transfers its loads through the slab to the dirt below. (Vertical and tipping forces.) Even after filling the void, the presence of mush below would give me pause under a 2 post lift in this situation.

If there isn't a concern for future erosion, I would look at cutting the concrete and pouring a repair slab due to the nature of the soil underneath. The repair slab would provide the opportunity to dig out all the really bad/soft soil and add a good stone base. The repair slabs are typically specified much deeper than the original slab, which allows them to sit on softer soil

 

[j keith w]

The soft soil and moisture had me thinking that the flooding isn't fully resolved. At least, it wasn't remediated.

A 2 post lift transfers its loads through the slab to the dirt below. (Vertical and tipping forces.) Even after filling the void, the presence of mush below would give me pause under a 2 post lift in this situation.

If there isn't a concern for future erosion, I would look at cutting the concrete and pouring a repair slab due to the nature of the soil underneath. The repair slab would provide the opportunity to dig out all the really bad/soft soil and add a good stone base. The repair slabs are typically specified much deeper than the original slab, which allows them to sit on softer soil

I appreciate all that - and I'd be on that line of thought if I was the building owner. Since I'm short-time, I may just not use the lift for the next few years and consider it vertical unutilized lift storage if there isn't a non-invasive way to shore it up in the interim. (If it were my building, I'd cut out the entire slab and repour - the building is that cool and worth it)

 

[Certified Drunk]

Cut the floor and put a couple footings in.

 

[ZRX61]

I appreciate all that - and I'd be on that line of thought if I was the building owner. Since I'm short-time, I may just not use the lift for the next few years and consider it vertical unutilized lift storage if there isn't a non-invasive way to shore it up in the interim. (If it were my building, I'd cut out the entire slab and repour - the building is that cool and worth it)

Pics!

 

[wssix99]

4 post lifts are a different deal. I would not give a second thought to filling the void and putting a 4 post lift on top. (The loads of a 4-poster are not much more than parking a truck on the spot.)

 

[j keith w]

Pics!

We think it was a city carriage house. Originally, it was three sided:

 

[j keith w]

Good news is that there is solid ground under the passenger side lift post, so this void is somewhat isolated?

Here’s the driver’s side post, though:



I’ve been all over the place with my scissor lift and forklifts, so I’m genuinely surprised to find the void. No cracking/heaving anywhere.

 

[phred]

How thick is the slab and does it have any reinforcing? Also most lift manufacturers require at least 4000 psi strength for the slab. With the small footprint of the lift I would be very concerned with it “punching” through over time unless there is rebar or wire mesh in the slab. I just relocated a lift and in the new location the slab was 3.5” and of unknow strength. I value my life so I cut out a section 4 x12. Excavated dirt out 18” below the slab. Added 6”’of gravel “4 rebar 12” ea way, one mat at 6” above the bottoms and one at 6” below top of slab. Doweled everything back into the original slab and poured It with 5000 psi mix. Yeah it was a lot of work and about 3k in materials but I know without a doubt the lift will not move.

 

[andyvh1959]

At minimum pumping in a fill type slurry that sets up over time is a choice, especially if the contractor can pump the slurry down deeper.

But I agree the best idea is to cut out a section of the slab over the suspect area and actually getting in there to inspect the issues. That type of work can almost be done by yourself, with time and some grunt. Then, drill pin holes into the slab around the cut section so when the new section is filled it is solidly connected to the original slab.

 

[MikeC55]

If the friend is giving you a really good deal on the rent, maybe just tell him about the problem but that you will pay to have it fixed. I agree with the others that some exploratory surgery is in order, at least for the 4 square feet or so around each post.

 

[Beamer]

I was involved in an old firehouse that had quite a bit of void space under parts of the slab. Amazingly a wheel never went through.
A number of cores were bored through and into the soil by a soil boring company to determine the extent of the undermining and then a slab jacking outfit developed a plan that addressed the entire floor and executed it. Parts of the area had masonry bearing walls sitting on thickened slab haunches, also partly undermined. It's amazing what those specialized slab jacking companies can do.