Running electrical conduit under foundation trench

nowImonboards

Hi all,

I need to run some flexible electrical conduit under the side of a concrete slab by a wall. The existing length of conduit up to the point it meets the slab area runs under the garden 600 mm below the ground level. I was going to continue this depth under the slab. However, as it is by the wall this means it will be under the trench foundation section of the yet to be poured slab. I have surrounded the conduit in a layer of plastering sand and plan to fill in the trench with crushed rock, electrical warning tape of course and compacted 804 to bring it up to the level that the foundation will be poured. I'm kind of worried that the weight of the foundation, slab and anything above that may crush the conduit, defeating the purpose. However, I'm hoping that the surrounding compacted material will be enough to take and disperse the weight.

Anyone have any thoughts on whether my concerns are valid or whether it should be fine?

Thanks in advance.

10-10-20

If you use proper electrical conduit for direct burying then the round shape of the conduit in sand and rock will stand up to the weight of the overlying layers. It would normally take a vehicle to crush the proper stuff.

nowImonboards

It's not the red ESB type stuff or the real heavy duty galvanised steel conduit but it's not the cheap contractor pack stuff either. I would say it is somewhere in between. It's the PVC type that wraps metallic spiral. I think it is this stuff but can't find a data sheet for it:

https://techaido.com/en/products/inset-b10456-gsi-28-d-28-schwarz-schutzschlauch

I did ask in the electrical wholesalers when I bought it but they just looked at me funny and said it was suitable for burying. However, the instructions were "just f@$k a bit of topsoil on it and it'll be grand" so didn't leave me bowled over with confidence.

I'd say I could squeeze a 5cm length it in one hand to the point where I could get the internal sides to touch but probably wouldn't be able to completely collapse a length of it.

10-10-20

https://www.boards.ie/discussion/comment/122545672#Comment_122545672

Oh, that's flex tubing, I'm not sure I'd be trusting it under foundations anyway as the wiring might not feed through it if there are contortions, etc.

Even if you just got some PVC piping like bathroom waste pipe, then 'chucked it under' sand, that would hold up as it's essentially the same as a soil/sewer pipe. Then you could feed that flex pipe afterwards.

Other options:

https://www.goodwins.ie/ducting-50mm-60mm/c-2747.html

nowImonboards

Hmmm…..

Looks like I'm digging up my conduits again so :-/

I do have a few lengths of waste-pipe here so maybe I'll just try to run the conduit through that as and extra outer shell protection so I can still keep one continuous sealed run form one end of the conduit to the other.

In that case, do I even need the sand? If I try to remove the crushed stone that is already on top of the sand protecting the conduit now I'm sure it'll all get mixed together so I'd have to create the sand channel again if it's needed.

Thanks for the input. Much appreciated.

nowImonboards

[removing duplicate post]

Lumen

My limited experience with buried conduits is, assuming you want to get anything substantial through the conduit in future (like 3x6 SWA), to go big and keep bends to a minimum (radius and number).

With the 50mm red ESB conduit I found two 90 degree bends OK but three was pushing it.

That said, I am no expert in feeding things through conduits.

10-10-20

The sand is in there to prevent the bigger stuff from puncturing the pipe. If you're not driving a truck/JCB over the foundations then I don't see a massive need for such layering.

nowImonboards

Thanks both,

I was imagining that there could eventually be a fair bit of weight on top of the foundation at some point (otherwise what's the point of the foundation) that would transfer to the layers (and conduits) below. I really have no real reference or experience to quantify in my own head what the likes of a JCB vs a conservatory wall would amount to in the lower layers.

Either way I guess I have nothing to lose by running a section of the conduit through the spare length of wastepipe I have.

I do have a length of 100mm sewer pipe I could use to really future proof additional runs as Lumen has suggested but wondering whether there is more a chance of something that size collapsing at some point and affecting the foundation above? I know it is only 100m of potential shift but I have no idea what that would do to a slab.

I have nothing really to go on here so hoping to stand on the shoulders of you giants :)

10-10-20

If the foundation shifts in relation to the sub-soil surrounding it, then you probably have bigger issues than a cracked conduit. I'd just go with a wastepipe and get it done. 👍️ 😁

nowImonboards

Done 🙂. Thanks again to all for the input 👍️