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?

 

[ajk1]

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.

 

[ajk1]

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?

 

[hokie66]

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.

 

[MotorCity]

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.

 

[KootK]

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.

 

[hokie66]

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

 

[ajk1]

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).

 

[hokie66]

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.

 

[ajk1]

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.

 

[KootK]

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.

 

[hokie66]

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

 

[Ron]

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.

 

[ajk1]

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.

 

[KootK]

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.

 

[ajk1]

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.

 

[ajk1]

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?

 

[ajk1]

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?