LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

[ajk1]

How to determine the vertical soil load on top of a 6" diameter buried cast iron drain pipe which is locate below a 6" thick reinforced structural suspended slab? The slab spans between drilled concrete piers at about 10 to 16 feet centres. The soil is very weak so the pipes are suspended from the structural slab above. I expect that the weight of soil that comes onto the pipe should be determined based on a wedge configuration, but are there any established procedures for this calculation?

 

[BigInch]

It should be based on the deflection of the slab and resistance of the soil to that deflection at the location of the pipe below. If you say the soil is as weak as it is, there should be very little, or no load on the pipe. To be conservative, assume that the soil is as strong as possible and base your resulting slab deflection and pipe load calculations on that.

 

[SteynvW]

I would use the angle of repose (can vary depending on type of soil, see link below)
of the soil going from the widest part of the pipe all the way up to the suspended slab.
You can then calculate the area and you can assume a density (see second link for typical densities) and thus calculate the weight(load) on the pipe.

http://www.structx.com/Soil_Properties_005.html

http://cereference.com/book/geotechnical-engineering/unit-weights-and-densities-soil

 

[ajk1]

Thank you both very much. I will look at the links and see what they say. Seems like they should provide what I need. I am not sure why a very weak soil should produce very little load on the pipe, but I will give that concept more thought.

 

[SteynvW]

Just a thought, A weak soil does not necessarily mean it is not a heavy soil, clays for instance can have quite a high density and add a more significant load to your pipe but under it, it will still be considered a weak soil in terms of its bearing capacity.

 

[MotorCity]

Your slab sounds rather thin to span up to 16' supporting itself, any live load, and hanging pipes. To answer your question, why not simply take the weight of soil above the pipe as: soil unit weight*6"*distance between top of pipe and bottom of slab?

If the soil truly cannot support a 6" pipe, are you also responsible for checking the ability of the pipe to span between hangers?

How are they going to install pipe hangers in the bottom of a cast in place, supported slab w/ soil below?

 

[ajk1]

To SteynW: Good point.

To MotorCity:
I did not mention it specifically, but this is a two way flat plate. Minimum required thickness by Code empirical method is L/33 = 16x12/33 = 5.87" < 6". OK.
I have yet to check that there is sufficient rebar, but that is my next step after I confirm that the slab has adequate rebar to support the weight of soil brought onto the slab by the pipe hangers.
I think the pipe will have to support much more soil load than just the weight of soil directly above it. SteynvW has provided a link about that (which I am about to read).
This is a 50 year old pipe. A camera put into pipe indicates a sag in it. Blockage at the sag was recently snaked out and a camera put into the pipe and video made and recorded. Sag was found in pipe at the location where blockage occurred.

I as the structural engineer for the pipe repair project am responsible for the safety of the slab during the period the temporary openings are cut thru the suspended slab to access the pipe, as well as for the slab after the temporary access holes are filled in. I would expect to be responsible for the pipe hangers and the pipe to span between hangers. If not me as structural engineer, who then? Anyway , we are perhaps straying from my question about how much soil load goes to the pipe and then up to the slab, so I can check that the slab reinforcing bars are adequate.

 

[jayrod12]

Why not replace all of the removed soil above the pipe with cardboard voidform, then the load on the pipe hangers is negligible. and you also don't have to worry about the uplift on the bottom of your slab repair in those areas.

 

[ajk1]

To Jayrod:

Excellent idea. In fact a mechanical engineer told me the other day that they fill it with styrofoam, but Voidform sounds like perhaps a more economical way to do it. I could give both options on the drawings. Thanks.

 

[MotorCity]

"I think the pipe will have to support much more soil load than just the weight of soil directly above it".

Nope. The angle of repose mentioned above affects the lateral soil load imposed on a vertical surface. It does not affect the vertical load (i.e. weight of soil above the pipe). You are not supporting a "pyramid" of soil. If you wanted to include the additional weight of the soil on the deflected pipe, I suppose that would be technically correct but probably negligible. If you replace the poor soil with stiff, well compacted soil, the pipe will be soil supported (no hanger required).

 

[ajk1]

Hi MotorCity: I value your advice and appreciate your taking the time to respond. But your advice seems to be contrary to what others have advised and to what I had always believed, namely that the pipe would be supporting an inverted pyramid of soil and the weight of the inverted pyramid is substantial when the pipe is down 8 feet, unless there is something else acting of which I am not aware to significantly reduce that weight. But if you are right, it would simplify things. Can you suggest any published paper or soil mechanics book or the like, that I can read that supports what you say?

 

[MotorCity]

ajk1,
Your method is certainly on the conservative side, as it considers more soil above the pipe. I don't have a copy in front of me, but there is a publication by the Concrete Pipe Association that has several diagrams and examples of soil load on a buried pipe. For large diameter pipe, the vertical load is not uniform, but this is a special case for projects such as tunnels, not drain lines below a slab. (In this case, the load profile resembles 3 adjacent "peaks", not a pyramid)

The only poster above advocating the use of the angle of repose is SteynvW. I clicked on the links he provided and did not see how or where it correlates the weight of soil above a given depth to the angle of repose unless you are trying to figure out the geometry of a conical, free standing, heap of soil. (if I missed it or if Stenynv can chime in, please let me know). As an example to reinforce my point, say you were considering the soil load above a 1'x1' plate 10' below grade and the soil weighed 100pcf. Would you not calculate the weight on the plate as 100pcf*1'*1'*10" = 1000 lb?

Can you please explain your rationale regarding the inverted pyramid?

 

[ajk1]

MotorCity:

Another engineer in the office has the book to which you refer, at home, as he was recently checking a culvert-like structure. I was just speaking to him at lunch time, and he says he does not think that the book requires as much soil weight as what I had been considering. So looks like you are right...I will see what that book says.

Thanks again for taking the time.

(Actually I should have said a wedge, not a pyramid, as the pipe is continuous).

 

[MotorCity]

You're more than welcome!

 

[Ingenuity]

From ASCE/FEMA document entitled "Guideline for the Design of Buried Steel Pipe" dated July 2001:

 

[ajk1]

Thank you Ingenuity. My thinking had been that if the hangers for the pipe corroded away and the pipe dropped down, then the granular soil would start running into the void left by the pipe where it had dropped down, and the soil would continue running into that void until the angle of slope of the soil each side of the pipe equaled the angle of repose of the soil. From that I thought that the pipe should be designed for the weight of the soil that would have run down if the pipe dropped down. It seems that my thinking was not correct, based on what you and MotorCity have told me. Thanks again.

Additional questions:

This is a 50 year old cast iron drain pipe (serving washrooms and a kitchen) suspended from a structural basement slab (the slab spans between supporting drilled piers). The camera investigation of the pipe seems to indicate that there is a sag in the pipe (probably due to broken hanger(s)). Further camera work is being undertaken, but if it confirms a significant sag, we will probably have to sawcut a hole through the classroom floor slab and dig down 7 feet to the pipe to replace the sagging portion. How should the soil be shored? Keep in mind that this is inside a building. I doubt that we can just drop a trench box down. The classroom will of course be shut down during the repair work.

 

[Ron]

If you want to compute the stress on top of the pipe, use elastic layer analysis. Probably no more than 2 layers required and can be done by hand.......otherwise use Styrofoam or voidfill!

 

[hokie66]

I am no expert on this, but the situation of a pipe supported on hangers in soil is vastly different than a pipe supported by the soil. Without the hangers, the pipe is just part of the mass of soil, and takes uniform load like the rest of the soil, as shown in the reference attached by Ingenuity. With hangers, the load imposed on the pipe is much greater, so I agree with SteinvW. Perhaps the pipe is deflecting so much because the true loading was underestimated in the original design.

 

[ajk1]

hokie66:

You raise a very interesting point indeed, about the hangers, which the reference documents may not be accounting for. Be interested in hearing from others.

The pipe is 50 years old, and this is the first problem that we have had with clogging in this part of the building, so I am doubtful that the problem arises due to excessive deflection. Nevertheless I need to know the load on the hangers so I can check if the rebar in the slab is adequate and what temporary support is required when we make an opening in the slab to dig down to the pipe.

Ron:

Can elastic layer analysis be done to account for the fact that the pipe is suspended on hangers? Is it a manual calculation, or software that you use for elastic layer analysis? Where can I find out how to do elastic layer analysis?

 

[Ron]

Elastic layer analysis will only allow you to compute the vertical load on the pipe due to whatever layers and loads are above it. For your application, it would give you a load at the top of the pipe that would be transposed to the tension in the hanger, given no other influences.

Have you considered that perhaps one or more hangers have broken from overload or corrosion?

Elastic layer analysis is commonly used for pavement analysis, either rigid or flexible. For hand calculations, consult Yoder and Witzcak's "Principles of Pavement Design". For software, ELSYM5, CHEV-PC, and EverSTRESS are available programs for elastic layer analysis. Only EverSTRESS is a Windows program.