Slab load on buried pipe 2

[haynewp]

I have a building that will have new mech air ducts running under the slab. They are 3' in diameter made of PVC (schedule 40). The bottom of the ducts will be 8' below finished floor. I am concerned about the weight of the slab and the slab loads applying pressure to the pipes. I have found equations(Boussinesq) that seem to be based more on footing loads being applied to underground pipe than slabs on grade. Anyone dealt with this situation before?

 

[gerrytt]

I'm a bit confused. Are you saying that the vertical distance between the top of duct and finish surface of floor is five feet? What's between slab and duct, ??? thick slab, ??? back fill, is slab reinforced and what live load applied to slab.

 

[haynewp]

Backfill is between the bottom of the slab and the top of duct. The top of the duct is covered with 5' of soil. I would like the slab to be reinforced with #4 bars at about 12" o.c. The live load on the slab is about 60PSF. The slab self weight is 62.5 PSF (5" thick).

I took an equivalent soil height of H=5'+122.5psf/120pcf=6.1ft. I have found in AASHTO a formula for the load on a buried pipe, We=FexwxBxH, I got We=1.2x120x3'x6.1'=2636 lb/ft of pipe. This seems pretty conservative. I am trying to contact the pipe mfr. to see if they have any information on what soil loads their pipe can take, else I will have figure out how to calculate that myself. My slab loads seem to be minor in comparison to the soil itself. The mech. eng. specified this depth of embedment without considering the pipe strength.

 

[stanier]

Suggest you invest in Australian Standard AS 2556.1 Buried Flexible Pipelines Design available as a pdf from

http://www.standards.com.au

or similar AWWA standard. Sharing knowledge is a way to immortality

 

[Focht3]

A 60 psf floor load is pretty light -

Remember that the soil pressure acts around the entire pipe - not just on top. The pressure isn't uniforn; hence the need to look at a design guide for buried flexible pipes.

Given that the soil load is significantly greater than the floor load, I expect that you will find that the pipe will be okay.

 

[mfrad]

Check Marston's formula for flexible pipe. PVC is considered flexible and will ovalize with load. Find out the breaking strength from the manufacturer and divide that load by your safety factor. That is how much vertical load you can place on top. You can reduce the load on the pipe by bedding the pipe and compacting the side wall of the pipe in the trench. Select proper backfill materials will reduce the load to the pipe also. It sounds like you have a slab on grade situtation.

 

[BigH]

If I remember right - in a deep trench - you loosely place the soil (suggest granular) for most of the depth above the obvert - that way it will "arch" during loading. Then compact well the last foot or two (or 1/2m or so). eh?? For such a light floor loading and deep trench, I probably wouldn't have thought too much about this. Could always use a stiffer pvc pipe. What is the crushing strength of your pipe? PS46??? = Schedule 40??

 

[SoilRocks]

Hate to disagree with Biggy H above, but I would never backfill loosely under a slab. I've investigated too many of those. Marsten's formula is going to give you the best estimate of the actual load on the pipe, as mentioned by mfrad above (structural?? way to go!!). The manufacturer should always have specs for the pipe, but lacking that info, you could use the minimum break/burst strength for schedule 40. My guess is that it will be fine. I seem to recall using schedule 40 (or equivalent) to depths of 15 to 20 feet in engineered fills. Using quality bedding and shading materials around the pipe is also an excellent suggestion.

:

 

[Focht3]

Another backfill option: use a lean concrete to backfill the trench. Hard to overstress the pipe this way, no compaction stresses, no settlement issues.

If your pipe runs are long and the cost becomes a problem, fill to a depth of two to three feet, then compact on top of the lean concrete after 72 hours. You - and the contractor - get a lot of benefits to this approach: peace of mind, pipe is permanently protected, and contractor has a firm base to compact against.

You can also use a cement stabilized sand - if locally available, although this isn't quite as advantageous as the lean concrete.

By the way, what is this for?

 

[haynewp]

I have just gotten the info on the pipes. It looks like they are stronger than I thought. These pipes are being used to bring air in from an AHU.

http://www.ipexinc.com/us_download/index.htm

Any suggestions on specifying a minimum cover on the pipes? I just don't want the slab sitting directly on top of the pipe itself.

 

[haynewp]

BTW, it looks like this mfr basis the allowable depth on a deflection of 7.5%. See page 7 here

http://www.ipexinc.com/download/pdf/Series_Pipe.pdf

 

[Focht3]

Treated air from an air handling unit - air conditioned or heated? If so, you need to talk to the project geotechnical engineer about the possible negative consequences on the soil of cracks in your pipe.

I'd also suggest you use o-ring couplings on your pipe -

 

[haynewp]

Sorry, I should have elaborated more on my previous post. Not only do I have the pipes coming in at the 8' level, I also have other air distribution pipes that will be crossing over the top of one of the deep ones. So there will be pipes essentially stacked on top of each other at one location and the top of highest pipe is directly beneath the slab right now.

 

[Focht3]

Then you definitely want to use lean concrete!

 

[mfrad]

A possible solution at the intersecting pipe location is to have a manhole and concrete vault 9' x 9' x 9'deep for ladder access and future inspection of the pipe. The slab above will be a supported slab spanning the 9' area. Hang the pipe from the slab above. The other alternative is to encase the upper pipe in concrete for approximately 20 feet to span over the lower pipe backfilled with compacted soil.

 

[BigH]

Good thread - about loose fill in trench, though, the original was a thin trench under slab. If fill arches, then you will not get the settlement. I like Focht3's comments of using lean concrete (or non-shrink fill - such as kcrete). It places nicely and you don't have the disadvantage of a long "open" time. Seems though that the additional trenches, etc. are making this more complex. Do they have the best "layout" available for all the services? Or are the services becoming haphazardly placed? Some planning to ensure the least number of cuts, etc. for services would be beneficial - sort of like corridors.

 

[SoilRocks]

Got you Biggie. I wondered about the loose fill comment. I just didn't pick up on the original thin trench concept. I agree with the idea of a lean mix; however, I would tend to "lean" towards a lean grout - it's very stiff, but can still be excavated if you need to. Plus, you don't have to worry about compaction control in the middle of all those crossing pipes. We call it a controlled density fill (basically a 1-1/2 or 2-sack grout) with an Fc' of about 150 psi. It never hurts to educate your client on the risks of putting pipes that close to a slab either. That way, if there is any problem in the future, your objection is on record.