Sub-Foundation Excavation

[1SEngineer]

Subject: Preliminary Research

Problem: There is a sewer pipe drainage problem in a two story condo. The condo hired a plumber to investigate the problem, and he believes the only solution is to remove and replace the sewer pipe. The plumber wants to create a tunnel(3'x3') below the slab to install the new sewer pipe (3" diameter).

Structure: We don't have much info yet about the structure but we are fairly certain that the building has the following components: shallow foundations, possibly spread or monolithic footings, 4" slab on grade, bearing walls are reinforced masonry.

Possible Solutions:

1. No tunneling allowed. The plumber has to saw cut the slab from above and all plumbing work will have to take place from the upper side od the slab. In order to do this, (3) condo units will have to be torn apart. Kitchen, Bathroom, Flooring, Etc. Estimated cost is $125K.

2. Tunnel underneath the shallow foundations and slab on grade. Estimated cost is $25K. The tunnel would be 3'x3' approximatley. I believe this is very feasable but of course there is always the risk of some settlement and cracking to occur in the slab and masonry and the development of additional stresses in these materials. If this route is taken, preliminary considerations would be to excavate in small lengths and widths and intall P.T. 4x4 or steel tube shoring as the excavation progresses, perhaps 4' on center. Arching action in the bearing walls will assist somewhat, but shoring would still be place underneath these footings.

I am a bit uncertain regarding backfilling as there is no way that this can be done and the bearing capacity maintained. I have thought about using flowable fill but I'm not sure if this will provide adequate bearing capacity due to voids. And it creates other problems if the pipe needs to accessed again.

I am curious to know how others on the forum would approach this challenge.

Thanks!

 

[kslee1000]

Sounds a lot of money for a 2 story condo. Can the sower line be routed though the side of building? Also, I remotely remember there was technic to jack-in the pipe segments rather than excavation. I don't think it is cheap then.

 

[DRC1]

How far do you need to go?
How deep below outside top of ground?
How deep below bottom of slab?
Where are footings?
Are they all wall footings or any interor column footings?
Outside soil inside soil and grondwater conditions?

 

[civilperson]

Flowable fill can be specified up to 100 psi, (14.4ksf), and with suitable techniques will have no voids or air pockets. The value is that future excavation can proceed with out extreme effort by hand techniques, (pick and shovel).

 

[dgillette]

How would you do the 3'x3' tunnel? Seems to me it would require hand excavation (on hands and knees, under unaesthetic conditions if the sewer pipe has leaked), and mucking with little sleds or carts. Brings to mind Charles Bronson tunneling under the fence in "The Great Escape." The slab would probably make a fine roof, but you would have ventilation, lateral support, and such to worry about for worker safety.

I like the pipe-jacking idea if you can make it work, since it would keep people out of the small tunnel. I would also look into pipe bursting if you have the right material in the existing line, and those formed-in-place plastic linings that are slipped in folded lengthwise, then inflated. Bear in mind that my expertise on those subjects consists primarily of having read ads in Civil Engineering magazine or ENR.

 

[1SEngineer]

"Sounds a lot of money for a 2 story condo."
(3) condos-(3) kitchens, (3) bathrooms, tile, etc., etc. It will be very costly.

"How far do you need to go?" 50'-60'

"How deep below outside top of ground?" 3'

"How deep below bottom of slab?" 3'

"Where are footings? Are they all wall footings or any interor column footings?"
We'd be tunneling under 1 to 3 footings. More research is required here.

"Outside soil inside soil and grondwater conditions?"
sand, sand, water table at +/-6' below slab


"Flowable fill can be specified up to 100 psi, (14.4ksf), and with suitable techniques will have no voids or air pockets. The value is that future excavation can proceed with out extreme effort by hand techniques, (pick and shovel)."
Thanks


"How would you do the 3'x3' tunnel? Seems to me it would require hand excavation (on hands and knees, under unaesthetic conditions if the sewer pipe has leaked), and mucking with little sleds or carts. "
That's pretty much it...it is done quite often. It's not a problem when the building is on piles. But shallow foundations...?

"Brings to mind Charles Bronson tunneling under the fence in "The Great Escape."


"I like the pipe-jacking idea if you can make it work, since it would keep people out of the small tunnel. I would also look into pipe bursting if you have the right material in the existing line, and those formed-in-place plastic linings that are slipped in folded lengthwise, then inflated."
I spoke to a pipe jacking contractor. He wouldn't do it as he said it'd be likely that he'd break some pipe or ultility in the process. It would cost about $15K-$20K for a 50' run, though.

 

[kslee1000]

I am a 2 flat owner in Chicago, you have opened my eys and am worried about my sower line now. It is a lot of money for me to come up with, so I will try to get it down in the safest way (as you are doing now). If there is no land right issue, re-route could be cheaper, have you looked into it? Good luck, like to know your decision and final result.

 

[1SEngineer]

kslee,

Rerouting would still involve sub-foundation work.

I was hoping there would be a better dialogue from other engineers on this challenge. I guess no one knows what to do in this situation.?

The problem is to offer the client the cheapest and best solution possible without causing other issues.

Tunneling would be the cheapest, but tampering with the soils could cause settlement.

Cutting up the slab from above would be the safest, but this would be very costly and would make difficulties for the owners.

 

[kslee1000]

If you have a heated space, does the pipe needs to get that deep into ground? Can you divide the pipe into segaments inter-connected by collectors to maintain slope required for flow? Can you drain to a sump area than pump out. There maybe other cheaper, feasible, solutions out there. I suggest to talk to a few contractors experienced in the drainage works. I would want 100% guaranty on a 3'x3' tunnel running beneath and across my house.

 

[DRC1]

You could probably dig and post the floor slab as you go. The wall footings could be handled, but interior footings could be problematic. Your best bet is to hire a reputable utility contractor. They are the guys who get the lines down the street and to the buildings, so they are more familar with how to do something like this than most plumbers.

 

[Patgeotech]

1SEngineer,

If one was to repair the pipe at cost to your client, what happens if the sewer pipe breaks again and you are left with the same problem or your client phones you to discuss paying for the ineffective work. Why not re-route the pipe to another sewer manhole/system. If connection is a problem then connect into existing system at another point and bring sewer line around the house (may be cheaper and less disruptive to the homeowner). Just an idea.

 

[dicksewerrat]

Call a couple contractors that do trenchless pipe jacking or pulling. Look online at Trenchlessonline.com for vendors of the equipment. TT Technologies is one. They are located near Chicago. Vermeer makes one. Hdqrs is Pella,Iowa. Have you or others looked at the pipe witha tv camera yet? why does it need to be replaced?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[1SEngineer]

I spoke to one of these contractors. He didn't think it was a good idea for this case. He was worried about breaking other pipes underneath the building.