Water Lines under Heavily Loaded Footing 1

[Martino8]

We're working on a site with a new 3-story building replacing a previous structure. Prior to the footing excavations they had a utility locating service come out and discovered 3 lines (4" + 4" + 3" water and storm lines) running directly under a highly loaded bearing line! The pipes are approx 20-years old PVC spaced approx 10" apart with only 6" to 12" below the bottom of the footings! The bearing line consists of 18" wide lightly loaded footings with stemwalls between heavily loaded spread footings (max = 55kips) spaced at about 25ft oc.

The contractor is claiming they've solved a similar condition on another project by encapsulating the piping back to subgrade with controlled density fill (CDF) and then poured the footings on that. They are highly motivated to deal with the utilities vs relocating them (claiming an added month to the schedule). We haven't made any promises, but said we'd do some homework and look into the matter.

It would be pretty easy to enlarge the footings with no support assumed over the width of the utitlities and verify that the CDF has sufficient strenth to bridge over the width of each pipe. However, we have concerns regarding the intended load path vs the actual stiffness of the PVC lines. If they attract enough load to cause a leak, they could become a major repair risk down the line.

Anyone have an experience to share with something like this?

 

[JedClampett]

We run into this a lot. Small PVC lines like these can take an enormous load. If you can get the "Handbook of PVC Pipe Design" it has allowable loads for buried pipe.
So if you did nothing it would probably be OK. The foundation loads will magically bridge over the pipe and be fine. But if you have the contractor over a barrel, I would encase the pipes in CDF or slurry or whatever it's called in your parts.

 

[phamENG]

I don't like pipes under my footings. I've seen too many break and create voids under there to allow them. If you're only 6 to 12 inches below the bottom of the footings, step the footings to go below the pipes. They'll excavate that area by hand, step the footings down, build a foundation wall up with sleeves for those pipes, and Bob's your uncle.

 

[phamENG]

I also don't like encasing them - repair is nearly impossible if the failure occurs within that encasement. Don't be like all the other scmu...people out there who forget that buried infrastructure requires maintenance and replacement.

 

[jerseyshore]

I don't like pipes below either. If something go wrong and you sign off on it then they'll be all over you about "you said it was okay". Not a risk I take as the structural engineer.

When stuff like this comes up I have the civil & geotech guys tell us if it works or doesn't. Let them be responsible for it.

 

[phamENG]

Another thing to remember - it's not just about the load the pipes can take. It's also about how much displacement they can take. Your new foundation will likely settle, and there will be consolidation in the soil below those pipes, causing them to move down while the segments just up and down stream will not. Even if it doesn't break the pipe, the added stress from a) bending it and b) redirecting the flow through the pipe around the bend will likely shorten the service life of the pipes.

 

[Lomarandil]

I like pham's stepped footing best, but would bridging the pipes with some void form for a reasonable length (4x footing width?) also allow for easier maintenance/replacement down the line?

 

[jerseyshore]

I was reading this like the footing was already constructed. It's a no brainer to step the footing then. Unless you have a really deep or massive pipe we always drop the footings. Like I said above, if for some reason they can't drop the footings, it's the geotech/civil's problem.

 

[Martino8]

Thanks everyone,
Another option we've come up with is to add some pin piles. After speaking with a pile contractor, we could probably make it work with (4) pin piles under each spread footing and potentially another couple under the stemwall footings in between each spread footing.

They said it would run in a ballpark of about $20k and they can hit a relatiely quick timeline. Seems like a much cleaner solution on many levels and avoids the permitting process of moving lines that are tied to the City water system.

 

[phamENG]

Piles sounds like a terrible idea. You have a building on shallow spread footings. You're going to put this one heavily loaded spot on piles. The geotechnical response is going to be very different between the two. You're going to have a 'hard spot' at those pipes. Sure, the pipes may be spared, but you're likely to get some unpleasant cracking of finishes in your building due to the sharp differential settlement. Don't mix foundation types.

Stepping the footing will also cost a small fraction of the cost of piles. It's really just excavating a few more yards of dirt and about 15 square feet of extra foundation wall.

 

[Martino8]

Well,
Stepped footings aren't an option because the pipes are in the same general location and direction that the stemwall or plinths would need to be. I would also agree that it's generally best to avoid mixing foundation systems (although sometimes they're needed for sites with existing fills at one end). As far as differential foundation response goes, we've kept the geotech in the loop and they're looking closely at that.

We're just trying to find the best solution for the corner that the contractor is painted into. After exploring all the options, the best choice might be to just deal with the hassles of moving the utilities.

 

[cvg]

you can certainly redesign the footings, however eventually the pipes will leak. what then? in my mind, relocation is the only rational option

 

[phamENG]

Oh. Parallel to the wall and footing does make it a little harder. Didn't pick that up from your initial post. In that case, relocation is likely the best option.

 

[DoubleStud]

Just oversize the footer each side of the pipe and put foam right above the pipes?

NM, just read pham's post about it being parallel to the footer. Yeah, that is no bueno.

 

[Lutfi]

It is a terrible idea to place a footing over any water or sewer lines especially if they are pressured. If the line ever breaks and the footing is undermined, the structure will be in deep troubles. My advice, based on over 40 years of experience, is to re-route the water line if you can.

Regards,
Lutfi