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Repair below slab on pile foundation?

Repair below slab on pile foundation?

Repair below slab on pile foundation?

(OP)
We need to replace piping beneath a slab on pile foundation. The piping is 4 ft beneath the slab.

I'm pretty new to engineering work (only EIT) and for the life of me can't figure out where to start. I'm sure people have figured out how to replace piping many times before, but I don't know what the standard way to do this is. I'm hoping someone here has experience with this.

I obviously cant just chop out a huge section of continuous 2 way slab, but how else can one get underneath? Is there some way to support the slab during construction? My head is spinning.

- Aaron

RE: Repair below slab on pile foundation?

Aaron - There are several other factors, things like:

The size of the pipe. What you do for a 1" pipe may not be the same as say an 18" pipe.

The use of the pipe. Pressurized? Gravity Flow? Fluid carried by the pipe?

The pipe's destination. Comes up through the slab? Passes under the slab and out the other side, at a more or less constant elevation 4' beneath the slab?

Is there some reason the new pipe cannot be rerouted or does it have to follow the existing path?

Unless there is a really good reason, investigate abandoning the existing pipe in place and rerouting its replacement. You asked about the "standard way to do this". That's how you start.

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RE: Repair below slab on pile foundation?

(OP)
It's a 4" grease waste pipe at a kitchen, runs under the slab to a settlement pit. There is an approximately 20 ft section being abandoned/replaced including a couple floor cleanouts. I don't have a full set of drawings but it looks like it avoids the pile cap, but does run under a column strip.

I can't see how rerouting helps; whichever way it runs, it has to run under the slab. Unless it's possible to tunnel under 30 ft from the edge of the building, but then they would need another settlement pit, and still have to cut out 5 or 10 feet of slab to connect a couple spots.

What about supporting the slab from the top with WF beams during construction, and mechanical rebar couplers to put it back together? Sounds crazy expensive for a pipe replacement. And would likely need extra shear reinforcing for the joint?

RE: Repair below slab on pile foundation?

So this is a structural slab supported on piles, but placed on soil or fill? If the slab had been formed over a crawlspace, would there be a problem? Perhaps the money saved during the initial construction can be used now to create a service tunnel.

RE: Repair below slab on pile foundation?

I had a project where we did just that. Expansive soils crushed the pipes under a structural slab on void. We had the contractor basically create crawl spaces below the slab with access pits and ventilation. Expensive, but the facility was able to stay in full operation.

RE: Repair below slab on pile foundation?

If it is a straight run of pipe, you could place a new 3" pipe inside the existing? We have done that before with some success.

RE: Repair below slab on pile foundation?

We've also seen local specialty contractors use directional auger drilling to place underground piping underneath existing slabs, etc. You might try asking around your area of local contractors to see if they can put you in touch with any like that. We would drill in underground subbase heating pipes under freezer floors from the outside truck dock area. Somehow they control the direction and just auger where they need to.

Your situation may require a straight-run but with the knowledge of where your support piers are it could be done from the outside to the point directly below the pipe/slab penetration point.

If you are in the center (middle strip) areas you can cut small openings without compromising the slab too much. Perhaps even a small opening through a center strip if it is away from the supports.

The boring would attempt to intersect a small excavation below the floor. Then a floor patch to finish it off.

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RE: Repair below slab on pile foundation?

Aaron - Several experienced engineers have provided very good approaches for a solution. The only thing that I can add is that this project is as much civil and piping as it is structural. You mentioned that your "Head is spinning". Take a balanced, multidiscipline look, as suggested by others, and the project will not appear to be quite as daunting.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Repair below slab on pile foundation?

You could also use a helical pile or pin pile system to come back in and support the slab if you end up having to cut critical reinforcing. Here's a sample system from ABChance:

http://www.abchance.com/products/helical-brackets-...

RE: Repair below slab on pile foundation?

While there have been several very good approaches provided already, I would like to approach a slightly different approach for consideration.

Often large areas of suspended, structural slabs in parking garages are jackhammered and replaced due to deterioration of the concrete as a result of concrete delaminations. Through slab repairs are commonly done. Could you consider opening perhaps three 4'x4' areas of the slab, one at each end of the 20' run of pipe and one in the middle. You may also be able to justify removing a 4' wide strip the full 20' length. Concrete removal is via jackhammer, following good rehabilitation practices and the reinforcing steel remains intact. I do fully admit that normally an opening in a suspended slab would be shored, however if the construction loads are low and the surrounding soil can be accounted for to provide some support around the sides, it may be possible. A vacuum truck could be used to remove the soil. A few rebar could be removed to allow for piping to be easily installed (and then welded back in place). Fill the hole with u-fill up to the level of the existing structural slab, then replace the area of structural slab using structural concrete.

Costing will obviously be an issue. If possible, re-routing is usually the best option, however, if you are stuck, there are several options to consider.

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