Concrete Incased conduit.

[dcsva@aol.com]

I would like to know if anyone can detail the procedure for putting PVC conduit in the ground incased in concrete. (ie. how much concrete on top, lay the conduit on a bed of rocks, etc.)

This is referencing a 480vac/3ph/60Hz-50amp circuit, 950FT from a school to a pump station. I just want to protect it.

Don't beat me up with this question, I have never actually seen this done.

 

[sjaniga]

Re: Concrete Incased conduit.

dcsva,
there are probably many ways to do this. One way we did it for an airport job was that the excavator came and dug a trench 6'x6'x2500', then some fabricators came and made rebar supports, like wine racks, they were spaced every 6', then we layed our conduit(2" rigid)through the rebar, then the masons came and filled it full of concrete. I'll never forget the day that semi pulled in with 16,000 feet of 2" rigid.

 

[websparky]

Re: Concrete Incased conduit.

Typically I have always used "chairs".
They are available from most supply houses.
What they are is plastic rectangular shaped supports with a curve molded in the top side of the rectangle that supports the conduit. They come in different sizes matching the conduit size. They are placed in the bottom of the trench and spaced according to the support intervals of the NEC per conduit type and diameter. They hold the conduit up from the bottom of the trench about 3 to 5 inches. Be sure to use some sort of "hold down" so the conduit will not move or float during the pour.

Hope this helps.
Dave

[ July 09, 2003, 06:31 PM: Message edited by: websparky ]

 

[dcsva@aol.com]

Re: Concrete Incased conduit.

thanks for the input. It has been very helpful.

 

[flightline]

Re: Concrete Incased conduit.

The "chairs" that websparky spoke of are made by a number of different manufacturers. One manufacturer is local to us in Chicago, and is Underground Devices and they allow for side to side uniform spacing as well as stacking if tose things are needed. Carlon has them as well, as I'm sure others do as well; it really depends on what your supply house can buy.

At the airport at which I work, our standard spec, and that's all it is, is 3" all sides, and between. At one time our ductbank included rebar in both longitudianl bar and rectangular cage, tied up in a sort of spine arrangement. This requirement has since been waived and no reinforcing is required under airport spec. Depending on the level of seismic activity and intensity in your area, it might be prudent to include it, but you may find the cost to do so is one of the larger line items on the encasement bid.

Our spec also calls out that the ductbank be bedded on a layer of crushed stone or sand. And remember to add some handholes or smaller manholes laong the way if you can.

I will try to locate our spec and e-mail it tou you if I can.

Hope it helps.

[ July 09, 2003, 09:10 PM: Message edited by: flightline ]

 

[bennie]

Re: Concrete Incased conduit.

Why the concrete? UD cable, direct buried, is a lot more cost effective and durable than concrete encased PVC.

 

[websparky]

Re: Concrete Incased conduit.

Hi bennie,

I'm guessing, from past experiences, that they don't want to dig deep and/or they are concerned about physical protection of the conduit in the event of future excavating for another project.

Or, maybe they feel it will provide extra protection in general and extend the life of the run.

Just my two cents.
Dave

 

[bennie]

Re: Concrete Incased conduit.

Concrete encasement of duct banks is mainly to prevent the driving of a rod, or drilling into them.

This appears to be one conduit run. My friend, the Evil Knivel of backhoes, would go through this like butter. He can feel an earthworms heartbeat through the controls, but he will crush it anyway.

Use a rotary ditching machine. Dig an 8 inch wide trench, 3 feet deep. Lay the cable behind the ditching operation, and begin backfill right away.

 

[dcsva@aol.com]

Re: Concrete Incased conduit.

bennie and websparky,
there are actually 3-2" conduits.
1 - 480vac/3PH
2 - 120vac/1ph
3 - phone line
Any comments...

 

[don_resqcapt19]

Re: Concrete Incased conduit.

dcsva,
Is this a straight run? Have you done any calculations on the pulling force that will be required to install the conductors and the sidewall pressure that will be the result of the pulling force?
Don

 

[bennie]

Re: Concrete Incased conduit.

The purpose of conduit is to provide a hole from point A to B. When A to B is too great of a distance, the hole is impractical.

120 volts, at this distance, is also not practical. 277 volts is borderline for clearing a fault.

Concentrate on running a feeder to the pump station and making your own 120 volts at the site.

The pump controller will have its own protection devices.

There is direct burial telephone cable.

 

[dcsva@aol.com]

Re: Concrete Incased conduit.

Don,
We have anticipated 3 pull boxes to give us 3 straight runs of about 300FT each.
No calculations have been done.
Is there a book that has these formulas in it or do you know of a good reference for this information.

 

[dcsva@aol.com]

Re: Concrete Incased conduit.

Bennie,
Where can I get supporting information to tell these guys not to run the 120VAC circuit because it is impractical and may not be able to clear a fault?
I have a customer that is head-strong about running that 120vac circuit.
I told him we should do, as you said, and make our own 120 at the site.

 

[don_resqcapt19]

Re: Concrete Incased conduit.

dcsva,
The only place I ever remember seeing the pulling force and sidewall pressure calculations is in the Southwire Power Cable Handbook. Unfortunately that section of the handbook is not online. With straight runs, I would guess that you could use a single pull point in the middle of the run. You should consider the use of rigid 90s if there are 90s in the run. The pulling rope will often cut through the PVC 90s and sometimes this will cause damage to the cables. I've been told that there is a product called "mule tape" that is used for pulling and eliminates this problem, but I have never used it.

I agree with Bennie that a fault on the 120 will likely have too much impedance to open the OCPD and you should use a transformer to make your own 120.

Don

 

[iwire]

Re: Concrete Incased conduit.

Originally posted by websparky:
Typically I have always used "chairs".

That's what I call them too, here is the best photos I can find and a link for info. You said you have 3 pipes you can get mix and match sizes.

You need base "chairs" and if you stack you will need intermediate chairs.

Carlon

Originally posted by websparky:
Be sure to use some sort of "hold down" so the conduit will not move or float during the pour.

Take this to heart, PVCs want to float, if the concrete is to thin or if they use a vibrator (concrete) the PVCs will try to make it to the surface.

Stakes and tie wires at each set of chairs will do it.

Bob

[ July 10, 2003, 04:45 PM: Message edited by: iwire ]