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I have a 4" PVC septic pipe leaving my house on the cinder block wall about 4 feet below my house. There was a crack inside of the part where it leaves the house and I need to replace it (from settling/35 years old). I am going to do the digging and replacing of the soil myself, but was wondering what was recommended for backfilling? Right now the dirt is a sandy/clay like substance with random bricks and rocks in it. I was wondering if it is okay to use the same dirt to backfill or if I should use pea gravel (or something else) around the PVC pipe whenever I backfill the soil. This will be an area about 5 feet long and 5 feet wide. The pipe will go into the house (so backfilling around pipe and next to my house).

Thank you in advance for your help!

6 Answers 6

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I'd use pea gravel, sand, or flowable fill. You want to avoid settling which could cause stress on the pipe and possibly break it or cause it to leak. So fill materials that need little to no compaction are best.

The dirt that you took out will not compact well and it will settle over time. I'd only use it on top of the fill that covers the top of the pipe.

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  • You can limit the good fill to the area under, around, and somewhat above the pipe. Sounds like you have a rather large hole - under the pipe, you want the good fill all the way to undisturbed soil so that it does not shift and is well supported. You'll want to tamp that, even with good fill. 6"/15cm to either side and above is adequate to buffer the pipe from the poor fill. Then again, you may have a minimum delivery size to get a pile of fill delivered, in which case you might as well use it (bagged fill material is absurdly expensive if you need more than a bag or two.)
    – Ecnerwal
    Jan 13, 2021 at 14:02
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    Anything except pea gravel; it never stops settling. "Because of its size, texture, and color, pea gravel is typically used for more above-ground projects and applications." i.e., dog runs 'cause it doesn't - "With crushed stone, it’s important to consider its texture. Crushed stone can be easily tamped or rolled into place, which creates a more stable surface for areas such as driveways." – ozinga.com Crushed Stone vs. Pea Gravel - I prefer 3/4" minus or fines. Also, w/e it is needs compaction.
    – Mazura
    Jan 15, 2021 at 1:33
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    Or rather, 3/4" minus, then fines, then maybe sand, then dirt. - "compacted earth, sand, fine gravel or similar granular material" – Code.
    – Mazura
    Jan 15, 2021 at 1:38
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P2604.1 Trenching and Bedding

Where trenches are excavated such that the bottom of the trench forms the bed for the pipe, solid and continuous load-bearing support shall be provided between joints. Where over-excavated, the trench shall be backfilled to the proper grade with compacted earth, sand, fine gravel or similar granular material. Piping shall not be supported on rocks or blocks at any point. Rocky or unstable soil shall be over-excavated by two or more pipe diameters and brought to the proper grade with suitable compacted granular material.

P2604.3 Backfilling

Backfill shall be free from discarded construction material and debris. 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. Backfill shall be placed evenly on both sides of the pipe and tamped to retain proper alignment. Loose earth shall be carefully placed in the trench in 6-inch (152 mm) layers and tamped in place.

up.codes/s/trenching-and-backfilling

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Once the repair has been completed any native soil that has been unearthed or loosened must be replaced by a compactable fill.

The least expensive and most compactable-type would be what is generically called "fill sand". Fill sand is what contractors use to replace excavated soil from around foundations, when making concrete forms, and when filling trenches that contain utility pipes.

This is what is being compacted when you see a worker using a motorized tamper on a job site. If you'd rather not rent one a similar tool can be employed with out the motor:

Manual tamper

You should layer and then tamp the fill sand. This can be expedited if you wet the layers occasionally. Not to saturated, but enough to start settling the sand. The most important area to compact is beneath the pipe so you may want to plan ahead by determining the location of the new line and placing the fill first.

Compact until the sand is almost above the pipe and than cover with loose fill. Afterwards native soil can be used to fill the remaining trench.

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  • 1
    If you add an m or an s right before the dot of the file extension on an Imgur file it won't be so damn big. It was fun playing the I know what that's going to be scrolling game though :)
    – Mazura
    Jan 14, 2021 at 2:00
  • @Mazura- Thanks. Got it.
    – ojait
    Jan 15, 2021 at 1:13
  • @Mazura wow, that's genius! I had no idea! The m worked, though the s seemed to display just a small section of the middle of the image.
    – FreeMan
    Aug 31, 2021 at 14:03
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This all depends on what kind of sewer pipe you are using. If it that white garbage PVC at Home Depot that you glue together then really all you can use is sand under, over and around. It is so crappy that if you let any 3/4 gravel touch it the stuff will shatter.

I am in the process of getting an excavator and putting in a drain from my foundation to the ditch ( the joys of country living 8) because a birch tree and a lilac bush grew on top of the white PVC and then crushed it. We know this because we can see the pipe at the ditch and when we try to rod it we get stopped at the lilac bush and when we cut that root out we got stopped at the birch tree.

I use ABS only outside. If running conduit I use the gray schd 40 glue stuff. There is PVC you can use, it is quite a bit thicker and uses rubber gaskets to connect but it is harder to get.

With either of those products you can use 3/4 clear as a bed, rake it flat and then put a foot or so of 3/4 clear on top. Back fill with whatever you have to about a foot and then compact it with a jumping jack.

I see guys saying use that 4 inch white stuff with holes in it for weeping tile but the stuff is so fragile you might as well just crush it right away and bury it. Around here everyone uses Big O with the sock and I will put filter cloth around mine as well.

Edit:

I tidied up the answer a bit

Edit.

This stuff https://www.homedepot.ca/product/ipex-homerite-products-pvc-4-inches-x-10-ft-solid-sewer-pipe-ecolotube/1000421825

Very weak, brittle. Easily broken

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    When you say "white garbage" are you referring to PVC DWV, cellular core, or something else? Also, "3/4 clear" and "Big O"? (Jumping jack = vibratory compactor, am I correct?) (The reason I ask is because trade terminology can vary from place to place) Aug 31, 2021 at 3:25
  • I too am wondering what "white garbage" refers to since the only white pipe I'm familiar with is Sch 40 PVC DWV pipe, and that's regularly used, in my experience.
    – FreeMan
    Aug 31, 2021 at 14:04
  • @ThreePhaseEel I cleaned up the answer a bit.
    – BrownPony
    Sep 4, 2021 at 11:47
  • @FreeMan I clarified the answer a bit
    – BrownPony
    Sep 4, 2021 at 11:48
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I've just wrapped up installing a couple of two-way Cleanouts, and now know a few things about PVC pipe.

The strongest is PVC SCH40, that is NOT marked Not For Pressure.

Next is PVC SCH40 DWV Cellular, that IS marked Not For Pressure. The DWV fittings will fit both pipes, and the Fernco couplings are sized for these pipes.

A 4" pipe of either is 4.5 inches in diameter.

There are SDR pipes, ASTM D3034 and ASTM D2729. A 4" pipe of either of these is 4.215" in diameter.

The thinnest is ASTM D2729, the thickest is ASTM D3034. They use SDR fittings. Fernco couplings for that diameter? I could not find them.

However, the wall thickness of D3034 (4.215-3.987=.228) suggests the belled end would be a good fit for a Fernco coupling meant for SCH40 pipe.

Cut off the belled end, glue onto the pipe, and install the Fernco.

4.215+.228=4.443

I would think that .057" gap can be closed up by the Fernco band clamp

On Edit: The Fernco catalog claims D3034 compatibility. On the bell? On the spigot? Or both? I don't know..

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  • This is great information but doesn't answer the OP's question about backfilling the septic pipe.
    – JACK
    Oct 30, 2021 at 17:50
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Okay, about backfilling:

I never leave in rocks, old pipe scraps, or the cookie sheet previously wrapped around the hole in the clay pipe. All that goes in my trash can.

I suspect a lot of pipe isn't backfilled with sand. I am finding sand, but that's a geologic strata.

As I conclude the repair of my pipes, I am in no hurry. I have barely covered the pipes, it's raining all week, and next weekend I'll lower myself into the pit and use a hand tamper to pack the fill under and around the pipes. Old timers will speak of supporting each joint in a cocoon of concrete, and you can do that, but first get that soil compacted.

If the pit/trench is a demoralizing morass of mud, calculate how many cubic yards of goo you need to stabilize, and budget two 90 pound bags of Portland cement per cubic yard.

You can churn the bottom of the trench to mix in the Portland, or use a concrete mixer to mix up your soil/cement mixture before refilling the trench. Each mixer batch just needs a shovelful of Portland for stabilizing. Maybe just 3/4 shovelful.

Cement stabilized soil is still a soggy mess (you can squeeze the water from it), but you can stand ON it, not IN it. The process is hydration, not dehydration.

If you've ever walked on a wet beach, you know that saturated sand can be walked ON, whereas saturated clay wants to remove your boots.

Just decide what is easier for you: bags of Portland or tons of sand.

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