buried cast iron pipe

[cmrdata]

Could anyone offer your opinion on the responsibility in ensuring the buried cast iron pipe underneath a structural footing would not fail?
Geotechnical engineers don't think they are the ones, the cast iron pipe manufacturers think it's the responsibility of the structural engineer, and on and on..
Thanks.

 

[TLHS]

If this is going to take a bunch of time to resolve, I'd probably just design the footing to avoid loading the pipe. If it's feasible, either put a break in the footing or throw a void form under it for enough distance so that the load doesn't intersect with the pipe.

If you're the one building the structure over it, it's your problem. It's probably not anyone's contractual responsibility, but someone's got to deal with it. The pipe manufacturer probably isn't the one to be bugging, though. If you have to load it, underground pipe analyses aren't too bad and you could probably whip through it pretty quickly, especially with geotech support.

 

[77JQX]

Since you're building on top of it, I assume it's not in service. Why don't you remove it, or fill it with grout?

 

[JedClampett]

I assume you're talking about Ductile Iron Pipe. Cast Iron is pretty ancient.
How deep is the pipe and what's the diameter?
If it's deep, it shouldn't be a problem. If it's small (4"?), it shouldn't be a problem.
Otherwise, compare you footing load to a vehicle load (is it way less, way more or something in between?) and check the load tables provided by the pipe manufacturer. They should give a buried depth that the pipe works for a vehicle load.
And if you're still at all worried, uncover it and concrete encase it. Better than losing any sleep.

 

[dhengr]

As JedC suggests the question about crushing the pipe is fairly easily bracketed and usually not a likely problem. That stuff is pretty strong. The bigger question might be any settlement which could bend and break the pipe. You can sleeve it through a found. wall or under the footing to allow the two to move independently.

 

[cmrdata]

Thank you all for your suggestions.
To clarify a bit further:

This is a new construction. The 15" diameter cast iron drainage pipe will be buried at about 6 or 7 feet below the bottom of a large retaining wall footing. The overall width of this tall retaining wall footing is about 25 feet and the pipe is running parallel to the footing width. The highest soil pressure at the footing is about 3500 to 4000 psf.

The most sure way to handle this situation is of course to step/lower the footing at the crossing of the pipe to allow the pipe to run through the wall stem. However, that means a lot of concrete when stepping such large footing up and down simply for couple pipes.

On the other hand, I'm not sure if 6 or 7 feet is deep enough for the soil between the footing and the pipe to act in arching. Hence the question.

Curious to see what my fellow engineers would do in this situation.

Thanks.

 

[cvg]

I would be a lot more worried about the pipe leaking. how old is the cast iron pipe? or is it a new ductile iron? sleeve or encase the pipe in concrete or install concrete blanket, or relocate it away from the wall

 

[ztengguy]

25' wide footing? How tall is your wall? How thick is your footing? I can only imagine how much shear steel much be in that bugger. Anywho, use some AASHTO distribution theory and look at pipe charts. 6-7, the load should be minor.

 

[TLHS]

If your footing's really that wide, then your pressure at six feet below is still going to be a very large percentage of the pressure at the bottom of the wall. In the order of 70% or so. Iron pipes are rigid pipes, not flexible, so I'd be very wary of trying to justify any significant type of soil arching. Your pipe isn't going to deflect out of the way to allow the soil to take the load. If anything, it could be a load attractor by not settling with the rest of the footing backfill.

 

[fel3]

TLHS…

Ductile iron pipe is actually considered flexible. It's just that DIP is a whole lot stiffer than the various plastic pipes in use today. Concrete, vitrified clay, and asbestos cement are examples of rigid pipes.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[TLHS]

Yeah, but we're apparently talking about cast iron, which is rigid as far as I know Thanks for the clarification, though.