Looking for advice - can I fix a partial belly by strapping the sewer line to my slab and poly jacking the slab?

Submitted by basic_hydronium t3_1223xn6 in DIY

In my basement I have a settling issue where the slab has sunk about 2-3 inches. My main sewer line goes through this same area and has a 2-3 inch belly in the middle of it. I’ve had the sewer line inspected and was recommended to manage through the belly with regular cleanings with a bladder as a trenched repair would be very expensive and disruptive. The foundation specialists found no problems with the structural foundation and the issue was a localized sag. They recommended polyjacking the slab (injecting a polymer foam in to lift and level the slab, while filling the void space created) to fix the sag. The location of the sag and the belly are near identical and the same relative depth, probably a common root cause.

My idea is to drill holes in the concrete and fish a high strength hose clamp (like this: http://surl.li/fuhbo )through to attach the sewer line to the slab. This way when the slab is lifted the ~2inches the sewer line is also lifted and the belly is gone. The poly foam would then expand under the sewer line and support both the sewer line and the slab. This seems like a good opportunity to minimize the disruption of a trenched repair and solve both problems. Is there anything I’m overlooking? Would this work?

Here’s a drawing of the layout and the idea to try and make it clearer: https://imgur.com/a/pYl9iGK

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allangee t1_jdrx9wo wrote

  1. Do you know what caused the settling in the first place, and have you remedied it? Otherwise, you'll be repeating the process.

  2. How do plan to fish down through one hole, past any gravel/fill, under the pipe, and then back up through gravel/fill to a relatively tiny hole?

  3. Is the pipe plastic? If it's cast iron, any strange stress could crack it. Or perhaps the pipe is already leaking somewhere causing the settling in the first place?

  4. If your basement is unfinished and you have clear access to the concrete, you can do most of the work on your own. (I've replaced entire sections of sewer pipe.) Get a saw and a couple of diamond blades -- a thrift store circular saw is ideal. Cut a line either side of the trench you need and break out the concrete. Raise the pipe and pack fill under it. Add a little rebar to the edges of the trench and fill it while also bringing up the floor to the correct level.

But again -- if you haven't figured out why it happened in the first place, it's likely to happen again.

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basic_hydronium OP t1_jdrzkal wrote

These are good questions, thanks.

  1. There’s never been water in my sump pump and no drainage issues around the house. No evidence on the sewer cam of a crack line and no evidence of a leak on my water supply. I’m on flat ground with substrate of primarily coarse river gravel. In short no clear cause. Two hypothesis are poor installation under the slab and there was a flood a few years back (prior to possession) that did not leak water into house but would have been close, could have washed away a little. Sewer inspection team with the city says they see this relatively commonly in my area, it usually doesn’t progress, usually installation related. So you’re right, could happen again but there’s nothing obvious to fix. I suppose it could be a crack in the sewer line we can’t see on camera, but would become obvious once poly fill.

  2. Tricky for sure. Plan was to drill 1.5 or 2 inch holes so there’s a little space. Pull out and loosen what you could immediately below the holes. The curvature of the hose clamp will want to hug the outside of the pipe and either hook it or powerful magnet on the other side to pull it back to surface. The pipe should be very close to the slab, it’s relatively close to the stack with relatively long way to go to the tie in and not a huge drop. In short the installation guys would need every inch they could get.

  3. plastic. Ran cameras through it, pipes looks fine as best you can tell from a sewer line. 4” ABS

  4. basement is finished, would prefer not to rip it up. I haven’t checked location of the line with a line finder, just best guess based on camera and tie in points. While the main part of the belly is down a hallway, the lateral from the second stack to the main is also slightly dipping and under my hot water tank and furnace. Would prefer not to be ripping everything out if I can solve more creatively. Breaking out and replacing a couple tiles is a lot less work than redoing a big chunk of the basement.

Thanks for your response. I’m seeing it a bit as a ‘no lose’. If it works I’m golden. If it doesn’t I have to cut into the slab anyway which is where I was at before

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allangee t1_jdtf77a wrote

I love the concept and the attempt. If you make a narrower adapter for a shop vac, you might be able to suck up some debris as you loosen it, if need be.

I would be thrilled to see this work for you. All the best!

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basic_hydronium OP t1_jdtgmsz wrote

Appreciate the comments and well wishes. Will report back if I give it a go!

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HyperSource_ t1_jdyg3i6 wrote

As someone who went through something similar, I recommend not trying to take the cheaper "easy" way. I had a sagging basement slab (4.5" elevation difference at the lowest point in the middle) and had three separate lifts by regional pros. After almost $17k and months of planning/inspection/consulting, it still messed up my sewer lines, and I had to jack out everything from the first 4" wye back to the vertical stack, replace it all, and repour it.

Save yourself the trouble..trench it and replace.

Edit: Jacking it all out identified the culprit of the sagging...a leaking laundry drain line. It wouldn't have been able to be identified without jacking out the concrete.

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basic_hydronium OP t1_jdygsrw wrote

Can you tell me more about your experience? Jacking the slab didn’t level it for you because the void was too big?

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HyperSource_ t1_jecv134 wrote

TLDR: IMO, don't take the "quick and easy" way out. In retrospect, I should have bit the bullet and had the entire slab demoed out and poured new with rebar. I could have inspected the entire plumbing & water systems/foundation wall footings, and it would have been done in a month at roughly the same price.

Jacking the slab didn't work for a number of reasons:

  1. My house has three support/jack posts cast into the slab supporting the main floor beam. The way the support posts were incorporated into the structure, they had been set into a unified concrete footing (lazy pouring approach of the early 80's). This created a problem when attempting to lift the slab, as isolating the post footings from the slab without compromising the footing or jacking out sections of the slab wasn't possible.

  2. Line locating. The sewer and water line locating completed by three different plumbers was not accurate. I was told accuracy within 6", and it was out in some spots by almost 18" by two of the three plumbers. One drill went through a line when the slab jackers were installing their ports, and they ended up pumping enough 4lb foam into the line that flushing toilets or running taps into sinks was no longer an option.

  3. Oversold/Underdelivered. The structural engineer and unaffiliated slab lifting company I used were both specialized in residential/commercial foundation repair. Engineer convinced me that it would be cheaper/easier/faster to do poly jacking than demoing the entire slab and repouring. The slab lifting company swore up and down that it could be done and I'd have level within 3/4" over 20', which I was good with. When all was said and done (three separate lift appointments), I was still 3-1/2" out at the lowest point and $17k poorer, in addition to a bunch of sewer line that I had to replace at about $1200 in material for concrete and pipe/fittings due to standard waivers that were part of the job contract. Once I jacked out the slab around where the sewer lines were, I saw some issues with poly lifting. The slopes of my pipes had been messed up by the sagging slab, and then messed up differently by being lifted. There was a lot of sheer stress on the system as I cut it apart.

Ultimately, it was a good thing that they messed up the sewer lines, because jacking it out was the only way that I would have discovered the culprit leak.

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