Sanitary pipe under raft slab
Hello All,
Maybe not a green building question per se but hoping there may be some wisdom out there. I am about to start construction of an infill building here in Squamish BC. We are near the ocean in an area of soft soils, so as a result the building will utilize a 14″ thick concrete raft foundation. Most all the plumbing is on upper floors but I do have one bathroom group on the ground floor slab.
My plumber suggested simply running the sewer under the slab like one normally would do but there is no way I would ever be able to get at the pipes in the future through 14″ of concrete.
One option I am considering is run the sanitary out just above the slab, and for the 1 bathroom group on ground floor, use a rear flush toilet and above slab shower pan.
Perhaps I am paranoid but idea of running the sewer under thick slab over soft soils that my geotech says may settle a tiny bit has me concerned. We also have to crib the slab with blast rock scour protection which would add yet another complexity.
I have not yet asked local building officials if there is any code requirements surrounding this and perhaps will take this up with them as well.
Thanks in advance.
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Replies
squamishmark,
I'm not sure I'd want to ever have to remove a slab of any thickness to access sewer lines, so if you have concerns, then definitely try to alleviate them - if for no other reason than you will be able to sleep better at night.
If there are too many complications in having the pipes above grade, what about sleeving them below the slab? That's what I do with all waterlines - running the pex in 3" PVC. I wonder if something similar could be done with sewer pipes?
That may be a good option - run an 8" sleeve for the 4" sewer
An above-slab shower pan still needs space for a trap below the pan. Have you considered building a crawlspace instead? You can still use the raft slab system, just at a lower elevation. Or perhaps a crawlspace just below your main plumbing areas.
you could even do a whole basement.
Thanks, without going into long winded explanation - cant drop slab, or add crawlspace or basement - will add a pdf below.
are you worried about the settlement crushing your piping? ask your geotech if it makes sense to put 25 PSI insulation over the top of your pipe run. they can probably estimate what is a little bit of movement is. maybe at the bathroom, they can design a plain concrete portion that doesn't have a bunch of rebar running top and bottom of slab. its hard to get the piping in the exact right spot at that stage of building. better to build in some flexibility.
just googled "blast rock scour protection." it does look complex.
I'd consider using a crystalline additive to the concrete mix design in conjunction with or in lieu of your underslab vapor barrier. block waterstops at the pour joints of the slab would be key to maximizing effectiveness of this measure.
incidentally a 12" slab is more daunting than a 6" slab, but it's still just concrete and steel. worst case scenario horizontal boring is an option.
even with your little description, sound like a fun, intense project
Precisely right - fun and very intense
Yes, am concerned with crushed pipe or differential settlement at slab edge shearing pipe. Geotech report says to allow for up to 1" of settlement. I will look into adding soft insulation on top or sleeving it in a larger pipe. All of your good replies are well timed - we dug up the site this week and that pipe is going in on Monday. Section E-E cuts through the bathroom and also attached my site servicing plan so you can see the sewer line in brown.
Have attached updated plan. Current thinking is to run stronger pipe - plumber advised using System 15 PVC. Use a 6" sleeve under the scour and bed with 2" EPS above and below pipe because I cannot be sure where force on pipe might come from if things settle (above or below)
Also not sure if Option A (run deep) or Option B (45 up to underside slab) makes more sense.