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Discussion Starter · #1 ·
I am trying to run an underground electric conduit through an area with very little topsoil. I have had numerous conversations with the local electrical inspector and various professional electricians about this work. The electrical aspect is quite straightforward but the excavation is potentially a huge problem. I will not know for sure until excavation starts but is seems almost certain that I will not be able to dig deep enough along the full run length without hitting rock.

One of the possible options to avoid blasting or cutting rock is concrete encased conduit. I am still not sure what the inspector will require if I go this route. From a cursory reading of the NEC, it looks like I will need about 4 inches of concrete above the conduit and about 6 inches on either side.

Any thoughts on the difficulty/cost of encasing a schedule 40 plastic conduit in concrete ?

My hope would be that I can avoid formwork and just pour concrete directly in the trench on top of the conduit. Is this realistic ?

If I hire a ready mix truck will they be able (willing) to drop concrete along the length of the trench ? The trench is quite long (~500 feet) but there is vehicle access along the entire length. I estimate that I will be buying about 15 yards of concrete.
 

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We've done this before for electricians, but it is pretty rare. Here, the electrician usually opts for rigid (heavy) conduit to avoid the concrete when shallower than 18".

The ready-mix truck should have no problem with spouting the trench, as long as it's safe to do so (think about overhead wires, buried pipes, septic field, etc..) and the access is good. As a matter of fact, most suppliers would much rather spout the concrete off than spend more time waiting for you to place it yourself. You'd be surprised how efficiently these drivers can be when you let them do their thing.

As for "how much?", my math say about 7.5 yards assuming 500' long by 12" wide by 4" ave. depth. I can only guess what concrete runs in your area, but for a 3000 psi simple mix, I'd expect a retail cost around $100/yard. And there is no need to form or finish the concrete off either.
 

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Discussion Starter · #3 ·
Thanks. This is exactly the info I was looking for. Very helpful to hear from someone that has done this before.

The heavy conduit (instead of concrete) is an interesting option but the electrical inspector here mentioned the concrete specifically so I am pursuing this option.

In terms of volume of concrete, the trench will be at least 6 inches deep (4 inch cover + conduit diameter). I also expect that I will run deeper & wider than this in spots because the trench will not be very uniform. That is how I end up with 15 yards.

Is there a delivery charge on top of the per yard charge for ready mix ? Should I be concerned about concrete against dirt in the trench (problems with curing etc) ?
 

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I work for a concrete company here running the batch plant. Concrete plants each have their own fee structure so shop around your area for pricing and ask questions including all surcharges. Ask what volume their trucks can haul. A 15yd order will most likely be split into more then one load. What I usually recomend is to order one or two 5 meter loads (we are metric here) and measure what is left for the last load.
Since the concrete will be used underground be sure to specify you want a type 50 sulphate resistant cement, the same used for all piles, NOT type 10 general use concrete used for sidewalks, driveways etc. Speaking also as an ex-mixer driver, filling in a trench with concrece from a truck is not difficult at all so you should have no problems as long as the truck has good access.
 
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