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#28885 - 09/03/03 09:24 AM underground vault question  
rmiell  Offline
Joined: Nov 2000
Posts: 242
La Junta, Co. USA
I have a question concerning placing pvc conduit through the walls of a concrete underground vault, which will be poured after forming. I will have 30 2" pvc conduits (3 rows of 10 runs, horizonally) entering one side of this vault, and 6 6" entering another side of the vault. The specs show the layout, but does not show the procedure for installing them into the sidewall forms.

I have used voids or sleeves inside the plywood sides of the concrete forms before, but never for this many conduits. Is using voids, or sleeves, or drilling the plywood form and installing the conduit, the best way? If drilling, how close to actual conduit size do I have to be, to keep excess concrete from flowing out of form? Or is there some trick to seal these before pouring?

The specs also call for the 6" PVC to have bell ends flush with the floor when it enters the building next to this vault. Will duck tape work to seal the ends while the floor is poured, or is there another way? What is the purpose of limiting it to the floor level? I believe these runs will be entering into the bottom of high voltage switchgear, so I will probably extend stub-ups a few inches after the floor cures. Sound ok?

Any hints on doing any of this would be appreciated.

Rick Miell

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#28886 - 09/03/03 05:22 PM Re: underground vault question  
elektrikguy  Offline
Joined: May 2002
Posts: 132
I had a similar situation to yours and came up with a solution that might work for you. I had 15 2 inch conduits through a concrete wall below grade and the contractor was adimant that he would not let me drill holes in the forms as they reused them throughout the entire job. It was a 10 inch wall. I cut pvc and put couplings on both sides of the pvc pipe which were cut in such a way that the end of the couplings would measure 10 inch exactly. I duct taped the ends of the pipes and fastened them to the steel(of course using alot more in that area for tieing the pipes off and keeping distances equal.The second form would sandwich the pipes. After the pour all that was required was a knife to cut the tape off the end and I was able to allow for backfill up to conduits and could run the pipe directly into the "wall" and continue inside as needed. I have also done this same thing using an FA adapter on the the inside form and was able to changeover to emt immediately for a very clean look. Of course, if you are able to drill holes in the forms I would go that route but be sure to leave enough for a coupling outside the pour but no more as the concrete guys will have a heck of a time getting the form off sometimes and ruin your work. BTW you will be ok with an 1/8 to 1/4 inch around the pipes. Good luck.

#28887 - 09/03/03 11:01 PM Re: underground vault question  
George Corron  Offline
Joined: May 2001
Posts: 717
Lorton, Va USA
typically, you secure the conduits well using pegs and the plastic stand offs, and butt the bell end up against the plywood. Once the plywood has been stripped, you simply "tap" the concrete slurry with a hammer to remove it.

If you want, you can also form a window on the manhole (BTW, if you pour it in place, it's called by the ACI "Cast in Place" - sorry, American Concrete Institute). The problem with that method is the conduits must be grouted in afterward and that can be a pain, though it makes a very neat job.

Use the plastic forms rather than some other method, they make your life LOTS easier, AND they are designed to allow the 1 1/2" spacing to allow for the aggregate of the concrete to be vibrated in between the conduits, anything else causes a weak pour. Back to ACI on that one, that's why electrical specs typically call for the 1 1/2" spacing.

I prefer to cast them in place that way. Precast manholes typically come with a window you can knock out, but we're back to the grouting.

If the bottom of your manhole is below the conduit bank, there is little choice but to either 1) pour the manhole in 2 pours, [that allows you to backfill up the the level of the ductbank] or 2) cast a window into the structure, or 3) core bore the holes after the pour.

Geeez, did ANY of that help?

#28888 - 09/04/03 03:13 PM Re: underground vault question  
rmiell  Offline
Joined: Nov 2000
Posts: 242
La Junta, Co. USA
all good information. Thanks, guys.


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