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Advice about a void under our concrete floor

My first shop was in an old one bedroom house with a 4 inch slab floor, I drilled 6 - 1 1/4" holes right next to the perimeter of the lathe and put an 18" long 1" pvc pipe in each one with some tape on it so it just poked thru the slab then filled the pipes with sand. One stayed almost full, one only went down a little, the other 4 got filled several times, the vibration from the machine shook it down under the slab. It took several months before it quit eating sand but eventually it quit, I beat it down in a few times with a slightly turned down 1 1/4 aluminum bar and big hammer then after a week or so of that I poured in some fixall. The problem was noticeably better after a few weeks and almost gone after it quit taking sand. I had planned on doing the same around the 2 vmc's but after the floor quit shaking I ended up not, I should have done by the vmc's first. Vibration can be a powerful tool
 
That is what she said. Seriously though, that is pretty clever. I only worry if the sand itself will migrate over time? I suppose if you leave those tubes there and keep them filled it could be sort of self maintaining if the substrate keeps settling?
We were there 5 years, the problem that came next was there was not enough floor, the condition of the floor was still good. though I had thought later I could have wet the sand and possibly got more in. In the end we went from 1200sf to 7000sf and a better floor but you have to start somewhere.
 
I have sawed floors and poured foundations for individual machines- it’s worth doing.
However when we had a new shop built- we gave them the foundation spec for the heaviest machines and told them to make the entire floor match the mfr spec.
This was also with it - machines stay in alignment, year after year. And we can reconfigure as needed and not have to worry about the floor.
 
I have sawed floors and poured foundations for individual machines- it’s worth doing.
However when we had a new shop built- we gave them the foundation spec for the heaviest machines and told them to make the entire floor match the mfr spec.
This was also with it - machines stay in alignment, year after year. And we can reconfigure as needed and not have to worry about the floor.
Sometimes what is there is what we get and we have to make the best of it.
I think it was Mark Twain said " any tool is the right tool if thats the tool you have"
 
I have sawed floors and poured foundations for individual machines- it’s worth doing.
However when we had a new shop built- we gave them the foundation spec for the heaviest machines and told them to make the entire floor match the mfr spec.
This was also with it - machines stay in alignment, year after year. And we can reconfigure as needed and not have to worry about the floor.

Do you remember what that floor spec wound up being? How thick was your slab? Obviously it is a great investment that will pay for itself but the up front cost of doing it this way isn't insignificant, especially in my area. Did you go with in floor heating?
 
Sorry, I didn't take the time to read all the posts. I have dealt with this a bit. Urethane foam injection should reduce vibration, but maybe not totally. 7k is very high unless you have a large area with big voids. I often checked voids by drilling small holes in slabs and probing the soil. You can get a good idea as to the depth of the voids and the compaction of the soil.

It helps to do some research or otherwise figure out why you have voids (sloped site, raised grade level, etc).

Mud jacking (concrete injection) can be less expensive than foam and probably better from a vibration standpoint. The downside to this is that if there is a relatively deep level of poorly compacted fill the weight of the new concrete can cause further settlement.

If you only need to deal with the location of one or two machines it may be easier to cut out the slab, remove loose fill and fill with compact stone and concrete.
 
Do you remember what that floor spec wound up being? How thick was your slab? Obviously it is a great investment that will pay for itself but the up front cost of doing it this way isn't insignificant, especially in my area. Did you go with in floor heating?
We went with 14 inches of concrete, double layer 3/4 rebar, 12 in center's.
The subgrade as I remember was 20 to 24 inches of crushed compacted rock.
We took the spec directly from our Mazak HC 4000 horizontals manual and gave that to the contractor. The spec covered the rebar schedule and the subgrade compaction requirements.
We did not put in floor heat in. We did upgrade the insulation and the building is fully Heated and cooled.
It was as you say not cheap, but concrete was definitely cheaper in 2016 than it is today. I chose to forget what it added to the cost of the building - but it was substantial.
And it was probably overkill to do the entire shop floor that way - the other dozen CNC machines in the shop are under 16,000 lbs, but that one is over 24,000 lbs.
it’s been a benefit even on the one manual lathe in the shop - leveled it when we set it up, and on the occasional use I personally get to use it - I am amazed it always turns straight. The same lathe in the previous shop with 6in floor needed re level spring and winter to keep it turning straight. And that shop did have 2 in of styrofoam 2 feet deep around the foundation. But the cold still moved the slab.
 
Yeah I know, you said that in your original post. But your having them now, soooo??
You implied that the only way to isolate the machines would be to put them on individual pads. While I'm not going to say that vibration couldn't be measured across the two machines in the old shop, it didn't cause any issues. I'm just looking to get back to that state.
 
We've got two machines and old Haas VF-OE and a newer DS30Y. We ran these two machines ~3 feet from each other in our last shop and everything was fine, apart from tripping over ourselves. So we moved to a new shop down the street and have way more room. Problem is when the lathe winds up it will shake the floor enough to cause chatter in the mill! While we could try to improve the vibrations of the lathe or tighten up the mill we know that there's an environment issue as well. We've had a couple of different people out to look at the floor and there's definitely a void under the slab. How much of a void is a great question that doesn't seem easy to answer though. I did reach out to a NDT company who said they could inspect it for ~$1500 and tell us where the void is but not how deep it was. The other option is to have a company put fancy expanding foam in through a series of holes and support the slab. They do this a lot to correct slabs that have failed and bring everything back into level-ish. I am the property owner so for better and worse this is on me to correct. The foam company gave me some references that all checked out but they haven't done this for someone running machines and looking to cure vibrations before. The cost to do the foam work is ~7k so it's not bad IF it solves the issue. The alternative is to cut it all up and repour the slab. which sounds awful, disruptive and expensive but once it's done it would be nice. I haven't had that quoted yet.

Anyone been through this before by chance? Any other words of advice apart from buy a different lathe that runs smoother?
Best option is to do new concrete with subgrade and base abatement.

Next best is high strength grout injection.

Both are expensive so make absolutely sure that what is creating the problem has been fixed. If there is no plumbing under the space to wash out underneath the slab it sounds like you could have water running under the slab from storm runoff around your building or site. It is worse than and likely not just differential settlement unless there was a well or similar pit filled in underneath that area.
 
My new shop was cut down to hard packed subsoil for the slab.......since the endless rain of the last 12 months ,the sub soil has become saturated and I have had to excavate a cutoff drain on the uphill side .........Downhill of the shed is literally turned to slop ,too soft to walk on ,and standing water on the surface .
 
My new shop was cut down to hard packed subsoil for the slab.......since the endless rain of the last 12 months ,the sub soil has become saturated and I have had to excavate a cutoff drain on the uphill side .........Downhill of the shed is literally turned to slop ,too soft to walk on ,and standing water on the surface .
Sounds like you need not only a surface trench drain on the uphill side, but an entire subsurface french drain and grid tile several feet down that the roof gutters are also tied into in order to dewater your building area. Potentially even a well sized basin for the dewatering.
 








 
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