LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
(OP)
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?






RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
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-engineeri...
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL 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?
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
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.
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
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.
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
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).
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
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?
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
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).
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
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.
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
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?
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
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.
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
Yes that is what I believe...that one or more hangers have broken, most probably due to corrosion.
I assume that elastic layer analysis would not by itself be able to take account of the effect of pipe deflection between hangers and how that may affect the load on the pipe. I am given to understand that there may be published tables of hanger load for different types of soil and pipe size and depth and hanger spacing, etc. That might be the fastest way to find the hanger load, if I can find such tables. Maybe I should try checking with a trade association such as the cast iron pipe association if such exists. Perhaps the deflection of the pipe is too small to affect the soil load, so I am going to make soe assumptions and see if I can get a feel for how mcu the pipe deflects. I will also look at elastic layer analysis. Thanks.
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
If the pipe deflection is 0.02" and the soil weighs 100 pcf, the additional weight due to the deflected pipe is 0.16 psf. To say that this is the load that breaks the camels back is not realistic....the weight of the soil is not even accurate to within 0.16 psf.
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
The cast iron pipe is in 10 foot lengths, with bell and spigot joints that are filled with oakum and then lead. That was the method used in 1965 when this was built. Today it would be mechanical joints (MJ). The Code required hangers at 5 foot centres. I would expect that there would be a hanger beside each joint (plus another mid-length between joints). I do not think that I would rely on the joint to take any significant moment.
It seems I am getting conflicting advice, so I better give it some further thought.
Hokie66 and Kootk: Under the scenario you suggest, how would you calculate what load comes onto the hanger? The soil is a sandy silt till with gravel, about 10 blows per foot, but changes to a very dense silty sand till of 60 blows per foot, at a few feet below the pipe (see attached 1965 soil boring).
(By the way, from the original design drawings, it does not look like the slab where the pipe is suspended was designed for anything more than the slab self weight and 50 psf classroom live load, but that is another issue. There is no sign of excessive deflection or any other problem with this 50 year old slab, so I am a bit doubtful that a very large soil load actually comes onto it from the pipe hangers, although admittedly it was designed by working strength design and I cannot see the actual slab surface to look for cracks).
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
I'd go with a frustum as SteinvW recommended. Honestly, I don't see ever obtaining a really accurate value.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
If you had no slab in the way and you tried to pick up the pipe, what load would the pipe place on the soil above it. For the soil classification you gave, you would likely get an inverted frustrum, similar to the cone of resistance in concrete when an anchor pulls out. The width of the top of the soil wedge is not likely definable since it will depend on the lateral confinement of the soil and the shear strength of the soil.
Now reverse that....similar condition.
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
Alternately, the load may have broken the hangers, displaced the pipe, and thus eliminated the demand on the slab.
I suspect that the soil has already settled as much as it's going to and that the pipe just needs to be accessed and the sat corrected. Your comments about the underground water flow make me a bit nervous though. If soil moisture is still fluctuating, you could continue to see problems if the pipe isn't isolated from the soil movement.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
Thanks all for you help to-date.
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB
a 2 m ±(6'-6"±) deep trench type excavation? The top 1 m ± is a loose fill/soil. I want to add those regulations to a set of documents that I am preparing for excavation down to repair an existing drain pipe. Excavation is about 2 m long along the pipe. Please see attached for section.
Also, if no shoring to the sides of the excavation is used, then what are the rules for the side slopes?