Existing Pipe

[Ryan H]

I have a condition where there is an existing water main in a condo slab on grade house. I am doing work that requires a new footing over the pipe. depth of pipe is unknown. I am proposing an 18"x18"x10" concrete footing flush with the top of the existing 4" slab on grade. I have a note stating now pipes within 12" under bottom of proposed footing. and proposed footing to sit on 12" thick clean stone.

the pressure from my proposed footing at 12" under the bottom of it is 400 psf. not knowing where the pipe is, do you think I am safe with 12" min. clear under the footing to any pipes. pipe could be 20" down, I don't know. is 12" enough?

 

[JedClampett]

I wouldn't guess on the pipe depth. Pot hole its location.
If someone put a gun to my head and asked me if they could do what you're suggesting, I'd say yes (among a lot of other things if I had a gun to my head), but I'd mostly be worried about breaking the pipe while excavating, not the load on it.

 

[Ryan H]

JedClampett,
they will be determining the pipe location for sure. it's just right now they need to T off of the existing main line for a new sink connection and they want to set up the structure in such a way that the new sink line, trenched into the existing slab, is not under the new footing(s). and the note I will provide is the new footing(s) need minimum 12" under them to any existing pipes. this is an SK for the job so that's why there are unknowns at this time

 

[SlideRuleEra]

...new footing over the pipe. I am proposing an 18"x18"x10" concrete footing...
...the pressure from my proposed footing at 12" under the bottom of it is 400 psf.

If space allows, in addition to locating the pipe, I would enlarge and/or reshape the footing. Going to, say, 24"x24"x10" cuts load on the pipe (per unit length of pipe) by more than half. If needed, reshaping the footing to, perhaps" 18"x30"x10" (18" parallel to pipe axis) should help, too. See what other size/shape options are practical for the location.

http://www.SlideRuleEra.net

 

[BAretired]

What's an SK?

BA

 

[dik]

BART: Site Kondition?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BAretired]

The whole concept of placing a footing over an existing pipe is flawed. Much better to wait until the location of the pipe is known, then, if necessary, span a beam across it to support whatever it is that needs support, with a compressible, waterproof plastic fill between beam and pipe.

BA

 

[Liam Nesson]

I would locate the pipe first.

If it is 12" below and compacted gravel is put on it. I feel like you have created a rigid point under your foundation, although i could be wrong. Everything else will settle and full pressure goes to the pipe. If it were 12" below i might sleeve it, if it was within 6" i would put void form over it.

The load as a udl is not alot, you just dont want a concentrated point load.

One comment though, future maintainence on this line will be painful if you dont sleeve it. Client might prefer it sleeved so it can be repair/replaced easily in future.

 

[Liam Nesson]

Any chance the slab on grade can take the 900 lb force?