Sawcutting a two-way slab
Sawcutting a two-way slab
(OP)
I have a project in which the slab-on-grade is an 8" thick two-way flat plate slab.
The building is being renovated and the Mechanical Engineer has shown the existing slab being saw cut and trenched to install new under slab plumbing. There are at least two trenches that run the length of the building and also a few that run the width of the building. The width of the trench is about 2 feet.
I am having a hard time figuring out a way to have the slab repaired after the plumbing is installed to behave as a two-way structural slab again.
Any ideas or guidance would be appreciated.
The building is being renovated and the Mechanical Engineer has shown the existing slab being saw cut and trenched to install new under slab plumbing. There are at least two trenches that run the length of the building and also a few that run the width of the building. The width of the trench is about 2 feet.
I am having a hard time figuring out a way to have the slab repaired after the plumbing is installed to behave as a two-way structural slab again.
Any ideas or guidance would be appreciated.






RE: Sawcutting a two-way slab
If it's a two way slab, your problems are solved. The slab will collapse when they cut trenches through it and your only issue is what to do with the rubble.
RE: Sawcutting a two-way slab
RE: Sawcutting a two-way slab
ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION."
RE: Sawcutting a two-way slab
http://www.concrete.org/members/ci2807mota.pdf
RE: Sawcutting a two-way slab
The other issue with small openings is the piping install. Since we are under a slab, the mechanical engineer is specifying cast-iron. The pipe diameter varies from 4" to 6".
RE: Sawcutting a two-way slab
Not sure what kind of pressure you're dealing with, but I'd make sure they're using retainer glands or flanged fittings instead of push joints. That probably goes without saying since you're inside the bldg footprint. Dont want to have to rip up that nice slab if a pipe fails.
Also unlclear as to why pipes couldn't have been installed prior to slab pour. Could have used cable tray or something, or if they are DI just cast into the concrete.
RE: Sawcutting a two-way slab
I'm not sure what kind of fittings the ME specified.
We are renovating this area and constructing an addition. Our addition also has a structural slab-on-grade.
RE: Sawcutting a two-way slab
But that sounds unreasonable. Must be a better solution from the mechanical engineer.
RE: Sawcutting a two-way slab
Why couldnt it be camera surveyed afterwards?
RE: Sawcutting a two-way slab
But I also think the building department could be talked into an independent company doing camera inspection as suggested by CSD. Why not, they jack and bore constantly down here in Florida under roads, etc. for utility construction.
RE: Sawcutting a two-way slab
RE: Sawcutting a two-way slab
If the two way slab was required to ensure the slab didn't load the fill during the buildings life, why would/could you assume that it is ok to load this fill during renovations? What just because of the short term nature this makes it ok? What if the fill has already settled away from the slab, how are you going to get the slab back to level? This is not a way a structural engineer should be thinking.
Cutting the slab along its full length to install the underground plumbing is not a viable in my opinion. This opinion should not be investigated further.
The suspended slab action and strength needs to be maintained; only solutions that meet this requirement should be entertained.
All this BS about building officials not accepting ect, will not past muster, these guys are here to maintain safety once you explain it to them they should be happy with a camera or similar. They should not be able to demand anything that causes unsafe practices to be considered, my feeling is that they haven't even been asked. And a lot of assumptions are being made.
ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION."
RE: Sawcutting a two-way slab
Bearing capacity is often times governed by long term settlement. Since it is poor fill, differential settlement may be the key problem. Without any significant load on the soil, I would not presume that it has settled. A geotech should be consulted. That is why I made the statement "If the soil can support the slab . . ." I did not say that it could; I don't know that. I have not even seen the soils report. If the floor area is cleared for renovations, we are only talking about 100 psf plus construction loads.
The initial question is about repairing the slab. While it is good to point out pitfalls and dangers, the question was not about temporary shoring for the slab. The question was about repairing the slab (which seems like a pretty basic question). There have been some good suggestions (like boring). But no one answered the initial question.
RE: Sawcutting a two-way slab
If you cut a trench in a two way slab, you've removed at least one of the span support mechanisms. Since you're on grade, there's no way to support the slab from underneath. The slab is likely to fail, or at least redistribute its supporting mechanism in a permanent (and unfortunate) way.
And even if the slab could be temporarily supported, how do you repair a slot cut in it? You could dowel in reinforcing, but only from one side at a time. So then you have a line of laps in the slot. Pour the concrete back and you're done. I wouldn't seal it, but someone might.
I don't know how big of an area we're talking about as its not clear in the question. But if the ME is dying to put plumbing pipes under the slab, I would tell him you have to rip out the whole slab, column to column, enough to feel confident about the two way design, and repour it. Actually it's not that expensive.
RE: Sawcutting a two-way slab
That is exactly what I said....after I stopped laughing at what the MEP had proposed/done........and yelling at the architect for not informing me sooner and not informing the MEP about the slab.
Unfortunately, I was not given a copy of his drawings until after the drawings were sent out for bid and contract awarded.....
So now I trying to figure out a way to make it work. And the more I look and think about it and discuss it with others, the more the answer is remove the slab and replace. The area is about 5,000 sf.
I called the PCA to discuss my dilemma. One of their engineers also recommended removing and replacing. He also suggested trenching underneath the existing building (which I had also thought of) since we are building along side.
RE: Sawcutting a two-way slab
Trenching under the slab is one possibility. Slab replacement is another.
BA
RE: Sawcutting a two-way slab
None of this is your fault, sounds like the architect dropped the ball unless he did not know this was not a traditional SOG. So I would not be so worried about being the perceived "bad guy" in this situation. Tell the architect "I can do lots of things with this slab to make it work, but your client is not going to like any of them!"
RE: Sawcutting a two-way slab
Alternatively, a2mfk has a good solution with the digesters and pumps.
Cutting the slab seems like a possible nightmare.
RE: Sawcutting a two-way slab
I won't be able to install hangers from the underside to support the plumbing.
What a F'n nightmare this has become.
RE: Sawcutting a two-way slab
RE: Sawcutting a two-way slab
The pile spacing will be dependent on how far your slab can span.
RE: Sawcutting a two-way slab
The disadvantages to hydrodemolition are usually related to the amount of slurry they produce (don't let it go down existing drains), noise and cost. However, I would bet it would be cheaper than removing the slab.
Anywhere near my area, where building foundations have to be supported by piles, so do the services, otherwise you get a lot of cracked water/sewer lines. Has the mechanical considered this?
RE: Sawcutting a two-way slab
If it still worries you then you can use the holes mentioned above to fix hangers around the pipe and hang it from the slab.