Structural Slab on Piles - Piping

[ash060]

I am working on a project that has a first floor slab supported on concrete piles. The soil is all muck underneath. How are conduits and piping typically supported in this case. I was thinking of hanging a duct bank under the slab, but I am not sure. Also, what is the method of construction for these slabs? Do they use stay-in-place forms for the slab soffit or something else.

Lastly, for vapor protection do you cover the soil with visqueen or do something else.

 

[SteelPE]

Not a great expert here but I will give it a go.

I usually hang the piping that is placed under the structural slab on grade by hanging them from the structural slab on grade. They system should be designed so that the "muck" under the slab can sink w/o adverse problems to the building. If the piping is resting on the "muck" it will sink when the "muck" sinks and you will have a problem. I usually leave the spacing and type of hangers up to the piping contractors but make sure something is shown on the drawings and the GC knows of the reason why they should be there.

Most of the structural slabs I have constructed have used grade as the form for the underside. It is usually leveled off then reinforcing is placed and concrete poured. Depending on your soil conditions results may vary.

I don't know what visqueen is, but I assume it is some sort of plastic sheathing. I usually show a vapor barrier placed under the soil that is used to for the structural SOG but don't get to worked up if they place the sheathing directly under the slab. If you have grade beams or footings you will need to figure out how to block the vapor from coming up through the beams and footings (not that hard to do).

 

[ash060]

So the piping is after the concrete is poured? How would they get under there.

 

[SteelPE]

Well, I didn't lay everything out step by step.

The piping is placed before construction begins on the slab. One thing to keep in mind is what cover you need to use for your bottom bars. If the slab is cast against soil you need to use a cover of 3". If you use some sort of form (like a void form) you can use a smaller cover.

 

[ron9876]

Visqueen will alow you to use "normal" cover. I think the code indicates that. I allow for a bit more construction tolerance since it is hard to get the soil completely level. If you want to hang a duct bank then detail some reinforcement to carry the load and design the slab for the added load.

I assume that you will be placing the slab on the soil right? If not then the question can get more complicated but you would have to remove the forms.

 

[JedClampett]

I agree with those who hang the pipe and conduit from the slab. To do this we usually encase the pipes and conduites and have the reinforcing develop into the slab. make sure the slab can carry to piping and encasement load to the piles.
As far as Visqueen, make sure it's heavy enough to not get damaged during construction, at least 10 mils. Even though I don't think much of vapor barriers, I think even less of them when they're poked full of holes.

 

[MiketheEngineer]

I generally refrain from putting any or as few as pipes under the slab - because some day you will digging them up or at least stopping them up. Then you get to re-install and re-route the pipes up the walls and over the ceiling - where is what I suggest you do now...

 

[ash060]

Mike

How would I do this for the sewer lines? They will be gravity fed.

 

[JedClampett]

Put the toilets on the roof.

 

[hokie66]

I don't disagree with the plumbing comments, but if the slab is on piles and is cast on "muck", you will need something better for supporting the reinforcement and placing the slab. A "mud slab", or working slab, is typically used where I am, but it is also possible to use granular fill material to form a working platform. The vapor barrier would go on the mud slab or granular fill.

 

[SteelPE]

I would imagine the soils are bad but they are at least going to have to be able to support the equipment during construction. I imagine if they can support the construction equipment w/o major problems I would think they could be used as a form.

I worked on a project about 2 years ago where the top soils could support construction equipment but once you broke through the top layer you were in trouble. I remember the GC trying to place a catch basin and having it sink out of sight under its own weight. It was actually kind of funny because we had extensive discussions about placing the utilities on piles which the contractor said wasn't necessary. I guess they were wrong.

 

[derim01]

@ash60
"How would I do this for the sewer lines? They will be gravity fed. "

For story below pipes, sump pit with pump can be used.

 

[ron9876]

SteelPE now that is bad soil.

 

[SteelPE]

I was actually fill. During construction it was believed that the building was being constructed above an old dock by evidence of massive granite blocks that formed a wall that went down as far as an excavator could reach. One side of the building was constructed on decent soil (but still needed piles) while the other side was constructed on this fill muck stuff. I don't know what they did about the catch basin. I remember the conversations about the utilities because I was happy my responsibility ended at the building with a few small exceptions.

 

[MiketheEngineer]

$hit lines are generally not pressurized - but if the soil is that bad - they may need support??!!