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LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

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

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.

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

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-engineeri...

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

(OP)
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.

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

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.

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

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?

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

(OP)

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

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.

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

(OP)
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.

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

"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).

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

(OP)
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?

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

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?

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

(OP)
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).

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

You're more than welcome!

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

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





RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

(OP)

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

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!

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

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.

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

(OP)
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?

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

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.

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

(OP)
Ron:

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

(OP)
My calculated deflection of the 6" dia cast iron pipe, with 0.18" thick walls (giving I = 16.2± in4), Young's modulus of 15,000,000 ± psi for cast iron, and hangers at 5 feet centres (as per Code), soil 6"wide x 7' deep above pipe, is 0.02" or 1/2900 of the span, based on the conservative assumption of single non-continuous span of pipe for deflection calculation. From this it seems that the pipe deflection has negligible effect on the soil load, and the weight of soil on the pipe is as MotorCity and Ingenuity indicated. So a good thought Hokie66, but it seems that the pipe deflection is not a factor in this particular case. Agreed?

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

No, I don't agree. I think the shape of the pipe is dependent on the settlement of the soil mass, not on the weight of the soil. The pipe wants to go where the soil goes, but the hangers fight against this movement, thus imposing bending on the pipe. This could only be alleviated by providing compressible material above the pipe.

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

If you want to design for the worst case, neglect any support that the soil offers and treat the pipe as if it were suspended in the air by the hangers with whatever the soil load is above the pipe. This is the only approach that I would take given the unfavorable/unpredictable type of soil you have.
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 echo Hokies's opinion with regard to the loading. Soil settlement will effectively have the pipe plowing upwards through the soil mass. I'd expect much more load than just that associated with the vertical column of material above the pipe.

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

How is the cast iron pipe jointed? I think at a minimum, you would need a hanger at each joint.

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

(OP)

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

I don't see this as a "very weak" soil condition, so see no reason to suspend the pipe. It probably just needs to be accessed in order to correct any sags. Any remaining hangers would be cut loose.

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

(OP)
The original 1965 design suspended the pipe as per the soil consultant's recommendation at the time. Part of the site was very poor (some people have described it as a swamp), with an underground stream which to this day still runs thru part of it. The nearest borehole to the area of pipe with the current sag was #3, but that is about 50 feet away, so I cannot tell you and no one knows what the actual soil conditions are at the location of the pipe sag. But the soil is not likely to be all that great if there is a large sag in the pipe. Anyway, irrespective of whether the soil is good or not, I think we are inadvertently wandering away from the asic question, which is how to determine the hanger load. I have a feeling that I should be on a geotechnical web site to find this answer. It is quite common to suspend pipes due to poor soil, so there must be an accepted and simple way of determining the soil load that goes to the hanger. I will look at geotechnical sites but if anyone can point me in the right direction, I would be grateful.

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

Quote (ajk1)

Under the scenario you suggest, how would you calculate what load comes onto the hanger?

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

I doubt that geotechnical engineers will be able to give you a better answer. Maybe plumbing/pipeline engineers could help.

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

Think of the soil load pattern this way....

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

(OP)
To Kootk, Hokie66 and Ron: I understand what you are all saying, and in fact that was my first approach before I posed the question on this forum, but the problem with that is that at 7'-8" depth down to the pipe, the amount of soil load is huge and the slab would have long ago collapsed. Since the slab seems in perfect condition as far as I can tell, I know that the soil load is not anywhere near that large. I will follow your suggestions and see if mechanical engineer might know (the first one I spoke to did not) and if soils engineer might know. I have a feeling that slabs are not generally being designed for the suspended soil load where pipes are hung. Since I must get on with the job, I will have to make some assumptions if I cannot find a recommended procedure. It seems to me that it depends a lot on the relative stiffness of the soil above the pipe to the soil below the pipe. If the soil above the pipe is relatively flexible compared to below the pipe, then I suspect very little soil load goes to the pipe. If the opposite, then a lot of soil load goes to the pipe. Like you say, I will never know the real answer. Thanks for taking the time to respond.

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

Quote (ajk1)

but the problem with that is that at 7'-8" depth down to the pipe, the amount of soil load is huge and the slab would have long ago collapsed.

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

(OP)
I now have the Concrete Pipe Handbook which gives procedures for calculating the soil load on a pipe. The procedure does not use a soil wedge...it seems rather complex, but I have not read it all yet. It includes the effect of friction on a prism (not a wedge) of soil that is equal to the width of the excavation for the pipe. I will try to read it all (goes or many dozens of pages covering different conditions, etc.) in the next few days.

Thanks all for you help to-date.

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

(OP)
I have done some reading up since my last time on here, and find that according to the Concrete Pipe Handbook, the earth load on concrete pipe when installed in a trench is the weight of a prism of soil equal in width to the width of the excavation (not the width of the pipe), less the friction acting on each side of the soil prism. Assuming that the trench excavation must be significantly wider than the cast iron pipe in order to fill the bell and spigot joints of the cast iron pipe with oakum and lead (as was used when this building was constructed in 1965) this will give much greater load on the pipe than the equation given in the ASCE/FEMA document that Ingenuity sent, which is based only on the weight of the prism of soil directly above the pipe. Is there really that much difference in the soil load for a steel (or cast iron) pipe and a concrete pipe, and if so, why?

RE: LOAD ON BURIED PIPE HUNG FROM STRUCTURAL SLAB

(OP)
Can anyone tell me what Ontario regulations govern the case of a worker going down into
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?

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