Sawcutting a two-way slab

[csellc1]

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.

 

[JedClampett]

I'm confused. Is it slab on grade (supported by subgrade) or a two way slab (supported by walls and columns)?
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.

 

[csellc1]

the soil below the slab is/was unsuitable fill. Instead of excavating the unsuitable material out, the original engineer designed the slab-on-grade as a two way slab spanning between piers on deep footings.

 

[rowingengineer]

If it is a two way suspended slab on ground, the normal procedure would be to cut smallish openings in the middle of the slab between supports and under bore between these points. This way the structural integrity of the slab is maintained.

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[csd72]

Agree with rowing engineer, use the following article as a guide.

http://www.concrete.org/members/ci2807mota.pdf

 

[csellc1]

i had thought about small openings. Unfortunately, the local Building Dept needs to inspect the under slab plumbing that will be installed. based on past experience want to see 100% of the pipe not just small sections here and there.

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".

 

[HeavyCivil]

Cast iron or ductile iron?

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.

 

[csellc1]

This slab is in the original part of the building, built circa 1995.

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.

 

[ron9876]

This is where the structural engineer laughs and tells the mechanical engineer to find an alternate solution. If there is no other solution then you need to know if the soil has settled to the point that there will no longer be any support for the slab. If not then it will be expensive. If it is a regular layout you could cut the slab at about 1/5 points so that the cantilever that is left isn't overstressed and the adjacent spans aren't changed a lot.

But that sounds unreasonable. Must be a better solution from the mechanical engineer.

 

[csd72]

I would say that you need to explain to the building department that the structural stability trumps drainage as far as their priorities.

Why couldnt it be camera surveyed afterwards?

 

[a2mfk]

I'm no ME, and this may be way too into the project, but this is all for gravity sewer? What about digesters and pumps and going up wall chases and into the roof plenum instead? May not be any more expensive than repairing large sections of structural 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.

 

[wannabeSE]

If the soil can support the slab during renovations and it is not post tensioned, why can't you splice the bars transverse to the trench and replace (or splice) the bars parallel to the trench. To expose the reinforcing for splices, the concrete would be chipped away rather than saw cut. Splices can be lap, mechanical or welded.

 

[rowingengineer]

wannabeit,
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.”

 

[wannabeSE]

rowingengineer,

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.

 

[JedClampett]

Well it seems like a basic question, but it isn't.
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.

 

[csellc1]

Jed,

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.

 

[BAretired]

If the soil has settled away from the slab, it may not be suitable to support the plumbing without excessive settlement. The plumbing should ideally be suspended from the structural slab.

Trenching under the slab is one possibility. Slab replacement is another.

BA

 

[a2mfk]

You are not in an enviable solution. However, nothing has been done but the design, correct? Why not approach the architect to send the MEP back to the drawing board, as this may be much more economical for the owner than having you come up with solutions to put this under the slab? It may end up being an ad serv for the MEP engineer, but compared with 5000sf of structural slab, I think I'd at least want to know my alternatives as an owner.

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!"

 

[Inlander]

I'm assuming this is a waste line you need to install. If this is the case, what about cutting the small openings per rowingengineer and have the pipe bored under the slab in one piece (polypipe) or have the pipe inspected prior to installation. The entire pipe could be pressure tested after installation. Maybe the plumbing run could be checked along the way through slab cores between the reinf. Maybe the hole could over sized enough to adjust the slope of the pipe. After shimming the pipe to the proper elevations, it could be grouted with a sand slurry.

Alternatively, a2mfk has a good solution with the digesters and pumps.

Cutting the slab seems like a possible nightmare.

 

[csellc1]

My concern with directional drilling (and I also thought about doing this) is, as BAretired mentioned, will/can the existing fills support the plumbing?

I won't be able to install hangers from the underside to support the plumbing.

What a F'n nightmare this has become.