tunneling under slab -- safety issues?

Are there any issues regarding tunneling under a slab? I live in a remote area, dry, with mostly clay soil (very stable).
Difficult and expensive to get "pros" to work out here, and for personal reasons I won't go into I need to save my pennies.
I'm kind of jealous of people with raised foundations who have such great access to their plumbing. I posted awhile ago about my plumbing woes, and sure enough, one of my drain lines has been broken under the slab and is full of mud and roots.
The break is about 22" under the lip. Measured at the perimeter, my slab is 12" thick...it thins to about 4" moving under the slab. There is a 40 ga. hot water heater above the area I want to excavate, but most of that load is spread outside the proposed tunneling area.
I'd really like to dig a tunnel under the slab, making a crawlspace for accessing the plumbing. All of the house plumbing joins up in this one area.
My idea is to excavate it, pour a footing around the perimeter, brick it in, then put an access cover outside. When I'm done, the tunnel would be about 3' wide and extend 5' under the slab.
There are a few cracks in the slab elsewhere, but this entire area is free of cracks. House is 50 years old.
Crazy to do this? If I don't I'm guessing my water heater, oven, and three cabinets would have to be pulled to cut directly through the slab and down to the break...not a pretty job.
I've been digging along the sewer line and have a 2' wide tunnel, about 13" under the slab, so far...so I have to ask, is this dangerous or truly inadvisable? Would a pro consider this, and if so, would they be shoring it up as they go, etc?
The digging doesn't bother me, actually it's kind of relaxing, but I don't want to trash my house or kill myself or anything along those lines.
Please, unless you are familiar with the engineering/safety issues, don't just automatically post negative. This is a real project, and I'm moving forward with it, unless someone (knowledgable and experienced) explains why this should not be done...and I would be very grateful for that explanation, BTW!
You're welcome to email me at the reply-to account if you want to keep your comments off the list.
Thanks, Flash.
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How far are you going to tunnel? Are you planning to break out the floor to make the plumbing repair? How deep is the line? Are you working where the line comes out from under the house?
The problem will be void(s) under the slab after you finish. If your tunnel is only 2 feet wide don't worry about shoring the concrete floor while you are working, it would be no different than digging out under your driveway or sidewalk. You will need to get compacted fill back around your pipe. Your crawl space idea would solve this, but it sounds labor intensive to me. If I could not clear the pipe, I would break out the floor on the inside, dig down to the pipe, cut and replace, refill the ditch, compact, repour the floor.
Cast iron? You should be able to grind and clear almost any version of small roots. If the root has grown the full diameter of the pipe it will need some replacing. It sounds as if you need some tree removal. We have backfilled the outside of the pipe with several 50# bags of copper sulphate in some problem areas. It seems to have stopped the problem where it was used. Landscapers and tree huggers seem to have an uncanny ability to locate new plantings directly over sewer lines.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing. . . . DanG

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DanG wrote:

Actually I found the break right after I sent the email, it was much closer to the edge of the slab than I estimated.
The sewer line runs out to my backyard at nearly a right angle to the outside wall, right where my kitchen and bathroom meet.
It runs at a slight angle out to my septic tank, so I won't be bugging any neighbors with my work, everything is on my property.
The sewer line is 2 ft. down at the edge of the slab. The broken collar (with root) is about 12" below the slab, maybe 32" from the edge of the slab (under the house). I've now basically exposed all the plumbing up to the main going out to the septic.
I've already cut/yanked the root out of the line, and everything is draining great. Now it's just a matter of figuring out how to patch that cast iron joint--it's a collar type join and a big chunk is torn off.
Maybe I can clear the soil around it, clean it up and strap something over it? Not sure.
Someone has worked on this before, they just didn't go under the slab. I found a rubber collar splicing the cast iron main to ABS, which runs out to the septic. It looks new...I bought this place 4 yrs. ago.
I have no intention of breaking out concrete, but as you say I need an effective way to deal with the void I've left below my slab...no idea what to do. It's 32" wide by 32" deep right now, but it narrows under the slab because I had a good idea where the problem was.

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|I've already cut/yanked the root out of the line, and everything is |draining great. Now it's just a matter of figuring out how to patch that |cast iron joint--it's a collar type join and a big chunk is torn off.
Professional plumbers have things made for just such situations. I would consider calling around. You have already done most of the work, so the actual repair should be relatively inexpensive.
Alternately, find a plumbing supply house, take photos of your problem, and ask what they have to address the problem. Rex in Fort Worth
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