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I have am unusual situation. I had to use 160mm pvc pipe to create a wide duct for services connections that must pass about 500-660mm deep through a new footing that's about to be poured. The pipe is already laid, and the trench cut, and neither can be moved. On the verge of pouring, an issue was raised about the risk of the pipe crushing.

The pipe is standard UK orange "polypipe" style drainage pipe. It does NOT have to remain waterproof - it's being used to create a 160mm duct through a footing. Once the pour hardens, the idea is that loose service conduits can be threaded through it. (The large diameter is because one of the pipes is a water pipe with a large plastic valve, and if it ever needs to be replaced, a 110mm duct might be too tight, whereas a 160mm duct will have ample loose space so it won't get stuck or catch on anything.) The pipe was new but die to adverse circumstances was laid 16 months ago and has been exposed somewhat to the outside environment for a year; I can't be sure if this has actually affected its strength; it seems in good condition and has mostly been shielded from the sun due to the trench itself.

The concrete being poured will be a deep trench about 300mm wide, and the pipe runs horizontally across the width. When poured, the pipe will be submerged by about 500 mm at the top and about 660 mm at the bottom. It's completely surrounded on all sides by the concrete, and doesn't rest on anything. It passes out of the trench and into the surrounding soil, via two circular holes on both sides of the formwork. The pipe is currently air filled and runs a couple of metres about 700mm below ground before surfacing and terminating. The pour is standard dense concrete (reckon 2400kg/m^3 or similar) and will be slowly poured, so there's no dynamic force, just hydrostatic pressure on the tip/bottom/sides of the horizontal cylindrical pipe.

My question is, suddenly, a concern that the pipe isn't internally braced or anything. Will it be crushed by the concrete? I know a cylinder will generally resist pressure well, but there's also a differential between the top and bottom.

If crushing is a risk, what can I do to mitigate it? It would be tricky but not impossible, for example, to:

  • Sand fill: Fill it with dry "kiln" sand before the pour, then vacuum the sand out after it's set. A lot depends how much a sand fill will compact before resisting.
  • Water fill: I could also fill it with water, cap the open end, and brace the cap, so that the water resists the pressure and braces it internally, rather than just being forced out of the open end as the pipe crushes it (if that were going to happen). But water doesn't resist movement like sand does - I suspect it could be squeezed out through a joint by the external concrete pressure, rather than staying in situ to resist it. Instinctively apprehensive about this one, due to risk of water being squeezed out.
  • Expanding foam fill: Possibly not practical, as, due to pipe layout, it would be hard/very hard to hack the foam out (even if it was expanded into a plastic bag or something, so it didn't stick to the inside of the pipe). But otherwise ideal for resistance purposes.
  • Encase in boxing: I could also entirely encase it in 50x170mm profile timber "boxing" sealed at both sides to the formwork, so it doesn't experience the pressure at all. That should also be pretty easy. But if the boxing leaks, or cracks along its length, the pipe will be at greater risk.

Best of all would be if I don't have to worry about it, or there is an easier "fix" than any of these.

The pour is due in a couple of days. How can I address this?

(NB, I've checked similar questions - they generally refer to a pvc pipe run in shallow concrete. Nothing specific about crushing risk and a comparatively large bore in a deep pour)

  • I don't know the wall thickness so it would be hard to guess but filling with sand would keep a thin wall pipe from collapsing. Foundation vents I use in stem walls sometimes have up to 4' of mix above them, these are fairly thin plastic. on pours over 2' above the vent holes I have Styrofoam blocks I fit inside the vents prior to setting them in the forms and this keeps the plastic from buckling and if a leak into the box it is very thin because of the Styrofoam and chips out easily. – Ed Beal Sep 16 '18 at 16:10
  • Just sanity check here, are you sure all the utilities you want to put through that pipe are allowed in the same pipe? Common mistake. – Harper Sep 16 '18 at 16:46
  • @Harper - yes:) the main one is incoming water main, which the local code requires to be insulated *unless * it's run 750mm below ground level, and 700mm under the building footprint before coming up into the building, in which case it can be run in a bare pipe (much easier to fix a leaky valve or joint if needed in future!). The other services are things like fiberoptic cable which doesn't carry electricity and is safe sharing the same overall conduit, and perhaps the natural gas pipe if allowed by code/desirable/in its own sub-conduit for protection. – Stilez Sep 16 '18 at 18:22
  • @Ed_Beal - I hadn't considered Styrofoam. Presumably dense foam blocks not easily compressible ones? Sand is sounding like the easiest overall, if anything were needed. – Stilez Sep 16 '18 at 18:25

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