Tunnel vs Cutting Slabs


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Old 07-04-09, 04:36 PM
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Tunnel vs Cutting Slabs

After several video inspections, investigating relining, rerouting and other possible methods, I have come to the conclusion that my cast iron sewer line under the slab needs total replacement. It is corroded to the point where it's beyond repair, and the more I snake it the more "metal" from the cast iron pipe I end up breaking and clogging the actual pipe.

The line I need to replace is about 87' long. It is impossible to re-route.

The only option is to cut an open trench 87' long across five rooms and lay in new PVC lines...this will be a very time consuming, back breaking, messy, dirty and expensive process. That's what two plumbers suggested.

One plumber suggested tunneling. Saids we are in south Florida the soil is mostly sand, dig a tunnel under and replace it from below. I said a 90' long tunnel? He said yes it's been done before 4'x4' in size and manually dug.

I asked him how would this tunnel be backfilled he said just "shove" the soil back in. I don't think this would give the same compaction that is needed, however, I have heard of those big truck with flowable fills, I don't know much about it, and don't know if it's applicable in my situation.

Anyone heard of tunneling under a slab to replace residential sewer lines? Would you consider it as a viable option?
 
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Old 07-04-09, 05:05 PM
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It does not sound a bit safe to me. How would they keep the sand from sliding back into the tunnel from the sides? Also sounds like it would take quite a bit of time and money.
Plus without a proper fill and tamp the pipes could settle and leak. Also the slab will crack if not properly supported. Cutting a slot 12 inches wide seems to me the only way to be sure it will be a permanent fix.
 
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Old 07-04-09, 05:14 PM
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But there are companies that push pipe under streets. Might be too expensive but it would require only cutting the slab at the new junction to give you a space to work.

Again may be to expensive but there are companies that reline pipe by inserting a liner. Never seen this done on residential but both are done all the time on municipal lines so there may be a company that will do residential..
 
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Old 07-04-09, 05:27 PM
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Ray beat me, but I'll leave what I posted

A couple of thoughts for you.

I have dug trenches in NJ's sand that were 4' deep and ended up being 10' across the top. Dry sand doesn't stay put very well.

Have you considered a "Tunnel Boring Machine". Not sure if the inspector would allow it, or if the cost would be tolerable, but sand would be really easy.

How about a pump system? One, they use a much smaller pipe, possibly inside what you have or around the house, as distance and elevation changes aren't serious problems.

Definitely need a better option than what they have proposed.

GL
Bud
 
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Old 07-04-09, 06:08 PM
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Relining is not an option. It has been considered, however since my main is a 3" CI pipe, and the inside of the CI pipe has corroded to the point of being remotely close to "circular", the relining will bear the same non-circular shapes with all the bumps.

Second, several spots have already collapsed, meaning there is no pipe. In the video I see areas that are basically soils and they plugged the pipes.

Relining needs access to cut the liner at the tie in points, there are three baths and a kitchen and a laundry to tie in along the way, necessitating may be eight open holes along this 87' length, this is basically cutting half of the trench at multiple spots anyways.

My thinking is if I cut open so many openings and do relining, I might as well replace it all.

and I did talk to a reline expect and he said it's not practical without a straight clear shot and I have collapsed section and pipes are too "out of shape".

The only option left is really total replacement, unfortunately.
 
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Old 07-04-09, 06:11 PM
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Originally Posted by badeyeben View Post
It does not sound a bit safe to me. How would they keep the sand from sliding back into the tunnel from the sides? Also sounds like it would take quite a bit of time and money.
Plus without a proper fill and tamp the pipes could settle and leak. Also the slab will crack if not properly supported. Cutting a slot 12 inches wide seems to me the only way to be sure it will be a permanent fix.
I do have one bathroom slab opened right now for some plumbing relocation. Yes it is sand, but the sand is compacted. Meaning that they are not soft smooth sand that I can poke a finger in and it slides. It is quite hard and will stay in a particular shape...unless it rains and water washes in, then that's a different story.

The pipes will be supported by hanging them with pipe hangers from the base of the concrete slab.
 
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Old 07-04-09, 06:15 PM
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Originally Posted by Bud9051 View Post
Ray beat me, but I'll leave what I posted

A couple of thoughts for you.

I have dug trenches in NJ's sand that were 4' deep and ended up being 10' across the top. Dry sand doesn't stay put very well.

Have you considered a "Tunnel Boring Machine". Not sure if the inspector would allow it, or if the cost would be tolerable, but sand would be really easy.

How about a pump system? One, they use a much smaller pipe, possibly inside what you have or around the house, as distance and elevation changes aren't serious problems.

Definitely need a better option than what they have proposed.

GL
Bud
I have considered some sort of jetting or boring machine approach, however there are several complications.

Across the line I am trying to bore, there are other underground utilities present including copper water supply lines (4 of them 3/4" size), pool pump and filtration lines (5 lines 2" size), PVC deck drain (1 of them 4" size), underground pool lights (2 of them in PVC conduits). I am not sure exactly where they are because old plans are not available from the city.

There is no way to bore such a line blind. Another complication is there are several baths and kitchen to tie in along the way.
 
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Old 07-05-09, 04:46 AM
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I'm getting a better picture now, so "The only option is to cut an open trench 87' long across five rooms " is the direction to go, except, I wouldn't open up the entire trench unless absolutely necessary. Remove only what you must to preserve as much floor integrity as you can and still be able to compact properly before closing.

It sounds like a good time to take a vacation and enjoy some cooler weather up north. When you return, it's all done, cleaned, and you have avoided the mess. On the serious side, if my wife were to see a job like that in progress, she would never enjoy the home the same again. 72 degrees and sunny today .

good luck,
Bud
 
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Old 07-06-09, 08:01 AM
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Bud9051 thanks for the advise.

I was considering tunneling because it does not create any dust and mess, but the resultant tunnel would cause a void in the foundation support.

If I do make a cut from above, the cut would be 87' long. This essentially splits my monolithic slab into 2 halves. Even if I don't cut a single trench 87' long, but let's say I cut a series of 9-12' long trenches, leave 4' of slab in between each trench, especially where I have interior walls, and assuming I can dig a little from both sides and get the pipe underneath, by doing that, the soil under those uncut slab sections will also be disturbed.

I just don't have a good feel for which one hurts the structural integrity of the slab more...weakened foundation support or a splitted slab.
 
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Old 07-06-09, 09:31 AM
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It partially depends on how they cut it. Diamond saw and the rebar is gone. Jack hammer and the rebar is only removed where necessary. Could also be wire mesh. Sounds like an engineer may be needed to calculate the risks. And someone to determine if the slab can be sealed. I'm out of my field, so will quit guessing, sorry.

Bud
 
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Old 07-06-09, 10:30 AM
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This place in my neck of the woods: Underground Piercing | MN Trenching and Boring Services for cable lines and pipes does all types of horizontal boring. On one job I was on they came out with a pneumatic tool. They dug a hole on both sides of a driveway took a thing that looked like a big bullet and started the tool from a truck mounted compressor. The tool then impacted its way through the clay soil under the driveway. Then they sleeved a 2" pvc pipe in the hole they just made.
 
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Old 07-06-09, 02:11 PM
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Originally Posted by Tolyn Ironhand View Post
This place in my neck of the woods: Underground Piercing | MN Trenching and Boring Services for cable lines and pipes does all types of horizontal boring. On one job I was on they came out with a pneumatic tool. They dug a hole on both sides of a driveway took a thing that looked like a big bullet and started the tool from a truck mounted compressor. The tool then impacted its way through the clay soil under the driveway. Then they sleeved a 2" pvc pipe in the hole they just made.
That would have been an ideal solution if I didn't have many other underground utilties that would interfere along the way. I really don't want these boring machines to damage other pipes as it drills unless it can absolutely guarantee it will drill along the same space occupied by the current cast iron pipe, which I doubt it can.
 

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Old 07-06-09, 02:27 PM
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Just a note..it probably doesn't apply to you. Post tensioned slabs need special care before you go cutting into them. I think they are more prevalent in southern TX, NM, and AZ...but who knows?
 
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Old 07-06-09, 03:14 PM
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Originally Posted by Gunguy45 View Post
Just a note..it probably doesn't apply to you. Post tensioned slabs need special care before you go cutting into them. I think they are more prevalent in southern TX, NM, and AZ...but who knows?
Thanks. I have cut open my slab in two of the bathrooms and the slab is not post tensioned. Actually it does not even have rebars in the slab itself. I found wire meshes embedded in the 4" thick slab.
 
 

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