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I have 2" low volt conduit that runs 50' underground from a communication mast on an out building to a basement. The conduit is crushed at the sweep 90 where it enters the basement. I currently have fiber internet running through the conduit - the fiber installers were able to get a run through. I wanted to switch and now I am trying to have coax pushed through but the installers couldn't get it through. I pushed a 50' endoscope and saw the crush damage - probably would have been visible from the basement without the scope. The fiber passes through the narrow crush.

There is no pull cord.

I installed the conduit before backfill. I probably should have sleeved it or used something with thicker walls maybe schedule 40 pvc but this 2" db2 (direct burial?) low volt electrical conduit is what my electrician told me to run. What do people normally run? Trying to carefully backfill is a bit of a joke so just wondering what the better practice is. Possibly I should have done 90s and kept the conduit closer to the surface rather than the gradual slope up to the surface.

I was thinking I could try to hookup my central vac to the conduit and suck a mason string through the conduit into the basement to use as a pull cord. I think this is still my best option. Since the crush damage is close to the basement I am wondering if I could somehow uncrush it?

I could dig an 10' pit but my side set backs are 4' and the grade is probably another 2' higher 4' away and I'd need to setup shoring. Also I have a 1" pvc irrigation pipe running 8" under the surface grade with irrigation wire and 12/2 landscape wire so I'd have to work around that. Even if I do dig it, would I be stuck cutting and replacing a segment or would there be some way to uncrush it?

Aren't there some plumbing bladder type things that you inflate? Maybe I could get something like that through the crush gap and inflate it?

Maybe I could put a ring of schedule 40 pipe into the 2" pipe and have it slightly smaller cut a relief and then push that ring through the pipe till it got to the crush damage and then force it into the crush area to force that to expand. I don't think I could get it into the crush area maybe I could do this after to reinforce that area though.

When the installers were out I mentioned possibly using the fiber as a pull but they didn't want to risk damaging it or damaging and not being able to get their pull through either and being stuck without internet. They mentioned bring out a rodder instead of fish tape - I am not sure what this would look like or if it could help get through the crush restriction. I am going to run the endoscope from the other end and see if there are any other areas that were crushed. It did seem like the installers were able to get by the crush damage they just weren't able to get the two fish tapes to hook up.

I've attached an image of the conduit. The black in the image is the existing fiber line - the only thing in the image is the grey conduit and the black fiber line. You can see that the pipe is pretty much pinching the fiber.

Thoughts? Other ideas? crushed conduit

Fresh Codemonger
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    Careful backfiling is quite simple (no joke) if you are not afraid of using a shovel until there's a good layer of material over the conduit. Then again, schedule 80 PVC is well worth the small increment in cost for robustness... – Ecnerwal Oct 16 '21 at 13:37
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    The information I find on "DB2 conduit" (never heard of it before) indicates that it's specifically intended for concrete encasement, not "direct burial" at all. The concrete would then provide the crush resistance it obviously lacks. -50 points to your electrician. – Ecnerwal Oct 16 '21 at 13:45
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    IF you can successfully plug the ends around the fiber you could try pressurizing it. If the basement will stand the mess (the cable should) water is generally easier than air for doing that. – Ecnerwal Oct 16 '21 at 14:15

1 Answers1

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I would try a nylon pull through or the string and vac as you suggest.

Trying to expand the crush may damage one of the existing cables.

I bought a long nylon pull through many years ago - some 20m and while annoying for short runs it works well for sequential and long runs.

Solar Mike
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    With the force required to expand it, the fibre is really likely to be crushed. String and vac can work well though getting started with a wide pipe and a constriction might be a bit tricky. Perhaps a little parachute on the head of the line might be needed, or start with a flat streamer rather than string – Chris H Oct 15 '21 at 10:16
  • Thanks, researching nylon pull through now. Any thoughts on how I should have run underground low voltage conduit that would have been more fool proof? Seems like even a straight 90 running up the side of the house would be better than this crushed / pinched scenario. – Fresh Codemonger Oct 15 '21 at 16:31
  • I am wondering if the vaccum will even work... since the pinch point is going to reduce the airflow the portion of the conduit beyond the pinch won't have very much suction... Guess I won't know till I try. – Fresh Codemonger Oct 15 '21 at 17:52
  • @FreshCodemonger put 2 vs on it - nick the wife' one :) but the air speed will be higher in the reduced section. Should be fine on a 2" pipe. – Solar Mike Oct 15 '21 at 18:00