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I'm currently in the process of digging a hole in my front yard to install a flag pole. The dimensions of the hole are 16" deep and 24" in diameter. Once I reached my 16" depth, I discovered my PVC sewer pipe that runs from my house to the street. There is no damage to the sewer pipe thankfully.

Can I pour the concrete over the PVC pipe without damaging it?

chicks
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Randy
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  • If not, does anyone have any recommendations on how to proceed? is off topic here because it is asking for an opinion – jsotola Jun 13 '21 at 21:29
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    @jsotola if it were rephrased to "If not, what is the best practice to secure a flag pole without damaging sewer pipes underneath ?" it would definitely on-topic however... – zakinster Jun 14 '21 at 08:09
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    Can you? Yes. Should you? No. – MonkeyZeus Jun 14 '21 at 13:59
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    Obligatory mention that most major cities (and probably the minor ones) have a hotline to call before any sort of groundwork to have them come out and mark all of the pipe locations for free. It's always a good idea. – Sean Duggan Jun 14 '21 at 14:45
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    @SeanDuggan, they will mark the incoming utility lines, but I've never seen them mark drain or sewer lines, at least in my area (Iowa, United States). – Milwrdfan Jun 14 '21 at 15:55
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    @Milwrdfan: Hmm... I had one marked in my backyard, but I think that was, in part, because it was part of the neighborhood network. If it were a personal sewage line, you're right, that probably would not be noted. That said, I think it's worth rementioning because I've seen a lot of people start ground projects without checking, and either run into trouble, or narrowly avoid it. – Sean Duggan Jun 14 '21 at 16:01
  • Sewer laterals (the line between your house and the pipe under the street) were all marked around here a few years ago. That was when my community was having it's ~century old sewer lines replaced due to storm water intrusion overwhelming the treatment plant and resulting in raw sewage being discharged into the river though; so I'm not sure if it's standard practice here or not. – Dan Is Fiddling By Firelight Jun 14 '21 at 19:00
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    Suggestion. Project the line of the pipe back to your house. Get a small metal plaque made with "Sewer Line" and a downward facing arrow. Screw it into your foundation a few inches above the ground line. If you ever sell your house, point it out to the next buyer. – Flydog57 Jun 14 '21 at 22:31
  • "Backfill shall be free from rocks, broken concrete and frozen chunks until the pipe is covered by not less than 12 inches (305 mm) of tamped earth." – What do I use for backfilling septic pipe leaving my home? – Mazura Jun 16 '21 at 19:43
  • I agree with, well, all of the answers. One thing to note: When you backfill over the pipes, install some warning tape (plastic tape like crime scene tape) about 6" above the pipe. Red will certainly get attention because it usually indicates electricity. So maybe yellow is better. Just something to make it obvious that whoever is digging should stop. – dougp Jun 16 '21 at 23:44

4 Answers4

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Move the hole.

You know its in a bad place. If you break the pipe with your flag pole you will curse your own stupidity and hate the flag pole and that is how bad things start. If you do get away with it then someday in the future someone will try to remove the flagpole and break the concrete, and they will drive the concrete into the PVC and then track you down in the nursing home and hide your teeth for putting a flagpole on top of a PVC pipe.

No, leave the PVC in peace. Move the hole a ways to the side, clear of the pipe and try again. Be happy you did not dig into the pipe like I did recently planting a new little tree. What a mess.

Willk
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    Yes! Someone that knows by experience! Never knowingly place concrete fill and vertical pipe or fence posts of any material over drainage pipe. Even without others blunders later, ground settles, gravity affects weight and that flag pole is just a huge lever waiting to dislodge and break your PVC.. –  Jun 14 '21 at 03:46
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    Also planting invasive trees like willow. My parents did and after a few years its roots wrecked the drains, cracked them open and grew inside to get at the water. I gather grapevines will do this too. – RedSonja Jun 14 '21 at 05:51
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    All plants try to get into sewer pipes and those that have deep enough roots invariably do. They crack the pipes open, no matter how innocent and vulnerable the plant looks above the surface and no matter what material the pipes are made of (well, stainless steel will probably stand for a while). Gas/electricity pipes are safe only for a while, a thick root will crack them even without the water stimulus. No trees, no vines and no bushes over the pipes, period. – fraxinus Jun 14 '21 at 14:07
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    If you DO decide to move the hole, then it might be useful to leave a note in the old one when you fill it in. Fill it in half way, then put your note in a heavy duty ziplock bag "suspected sewer line 8" below this note" and put that in, then the rest of the soil. It might save the next guy some hassle if he wonders why you didn't put the flag in the 'best spot' and he wants to move it back. – Billy left SE for Codidact Jun 15 '21 at 07:47
  • @fraxinus Can you provide some reference that shows tree roots can crack open a PVC sewer pipe? I've never heard of such a thing. They get into steel and clay pipes at the joints or existing cracks. PVC is chemically welded at the joints. I would expect the roots to go around an intact pipe. – JimmyJames Jun 15 '21 at 17:40
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    @JimmyJames big roots break plastic pipes just by growing. – fraxinus Jun 15 '21 at 18:30
  • @fraxinus And claims without proof mean very little. I see nothing that suggests that roots can 'break plastic pipes just by growing'. I've poked around a bit and this seems to be the consensus of exports: "The reason PVC pipes are more impenetrable than other pipe materials is because they have fewer and tighter fitting joints and they are less likely to crack and leak. These two factors make them a superior piping product." – JimmyJames Jun 15 '21 at 20:00
  • I'll try to find the pictures I sent to my landlord when I had the issue personally. A cherry tree roots vs 110mm PVC sewage pipe. The tree won (short term, because we had to eradicate it and fix the pipe). – fraxinus Jun 15 '21 at 20:06
  • Here's an academic article on the subject: "On first appearance then, it may appear reasonable to conclude that tree roots can easily crush and certainly dislodge pipes. This, however, is simply not the case and the explanation can be readily provided with some basic physics and supported with regular observations such as those contained in the video footage provided." It explains the real reason that roots get into pipes i.e. cracks or gaps due to other causes. – JimmyJames Jun 15 '21 at 20:15
  • @JimmyJames Roots do displace things, though--our neighbor planted a palm too close to a block fence. There's nothing about the fence that would attract a root, but the tree cracked it anyway just by lifting it. There are multiple houses on this street where people put a cast concrete ring around a tree--and I believe all of them have been cracked by roots by now. – Loren Pechtel Jun 16 '21 at 02:55
  • @BillyC. Nice idea, but I would use something more resistant, like a slab of thick material, maybe with the notice embedded in it (two slabs of plexiglass or policarbonate?) or engraved on it (some brass alloy?). This way someone digging in that spot will hit something hard and stop well before hitting the actual pipe. The bag could go unnoticed if pierced by some digging machinery. Maybe put the ziplock bag just below the slab of hard material. Another solution would be to place an engraved brass/bronze/aluminium note on the surface (bolted to the ground), if this is acceptable. – LorenzoDonati4Ukraine-OnStrike Jun 16 '21 at 07:36
  • Another thing to consider is drain-waste-vent (DWV) PVC is much thinner than schedule 40 PVC, and may even be foam core, which can be easily broken by careful hand digging. Placing the concrete could easily collapse that type and you'd be none the wiser until your sewer backed up into the house. – Colin Young Jun 16 '21 at 13:10
  • @LorenPechtel The sentence directly preceding the last one I quoted above was "... They also provide calculations that demonstrate that roots can readily lift light structures such as driveways and pathways." Once the roots get in to the pipe, they cause more damage. They just don't have any way to pierce, crush, or crack the pipe except in rare circumstances. Most likely, if you get pipes in PVC, it's because the ground shifted and opened a pathway for the roots. This is surely becoming more of an issue in places like FL and CA where the aquifers have been severely drawn down. – JimmyJames Jun 16 '21 at 13:25
  • @LorenPechtel On the subject of concrete: I guess it's possible that the roots cracked it but where I live, pretty much all concrete gets cracks regardless of whether roots are involved. Freeze-thaw cycles are the main culprit and the ridiculous amount of salt put in the roads makes it worse. Once the cracks are there, plants inevitably move in. – JimmyJames Jun 16 '21 at 14:26
  • @JimmyJames No meaningful ground freezing here. The sidewalks and driveways are fine, the rings around the trees get shattered. – Loren Pechtel Jun 16 '21 at 14:38
  • "can roots break pipes?" sounds like an excellent new question. – Criggie Jun 16 '21 at 19:54
  • @LorenPechtel I just stumbled across this: "... immense turgor pressures have been observed in their hyphae. The study showed that they could penetrate substances like plant cells, and synthetic materials such as polyvinyl chloride." Fungus on roots is possible mechanism I had not considered prior. – JimmyJames Jun 21 '21 at 19:33
  • Fraximus is right, I had tree roots pinch a LDPE pipe closed recently if it had been PVC that amout of deformation would have split it open. if the pipe is distant from the tree there is less fisk – Jasen Jun 22 '21 at 03:09
  • @LorenzoDonati--Codidact.com, yeah etch the note into a paver or scrawl in in wet cement. – Jasen Jun 22 '21 at 03:12
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Never, ever, mess with any pipe underground.

No matter if it is any kind of plastic, steel, brass, copper, ceramic, concrete, whatever.

No matter if it is used for sewer, water, gas, electricity, Internet connection or underground smuggling of rocket parts (it happens!) or vodka (it happens, as well).

Pipes sometimes break after a while even when left alone, just because the ground settles unevenly. A bulk of concrete is known to settle in a different fashion (it sinks as a whole) and to break pipes even not directly in contact with it. 20-40cm separation is probably OK-ish, but aim for a meter (3ft) if possible.

When a pipe breaks, you get a great deal of both annoyance and expenses.

fraxinus
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TL;DR

You shouldn't do that.

If for no other reason then consider this simple fact.

If that pole gets knocked over via wind or a rogue vehicle then the concrete will tilt and puncture a hole in the PVC. Now you have two problems.

MonkeyZeus
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I would move the flagpole to a different location so you do not disturb that PVC pipe. Also, I would contact the sewer company and ask how deep their piping is since most sewer piping is much deeper than 16". That PVC pipe may be for something else.

d.george
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    Where I live, incoming water pipes need to be under the freeze limit but sewer pipes not - they are not prone to freeze. On the other hand, they need the right slope. Some of my pipes are almost at the surface. – Peter Tölgyesi Jun 14 '21 at 11:50
  • Depends on the sloping. I have seen some PVC drain lines that shallow because there was a slow drop to the street lines – Machavity Jun 14 '21 at 17:01
  • I doubt there is a standard for septic lines, since they are designed to not run full and always drain. Although, good to check now that the pipe is exposed. – dougp Jun 16 '21 at 23:41