Stresses on buried water utility pipe due to crane walking over

[nagatalluri]

I am trying to evaluate the stress on a buried utility pipe (water) 6' below ground surface due to a crane walking over it. Can anyone suggest how to do the analysis?

 

[oldestguy]

Assuming all ground, including trench backfill, as uniform medium density. Put half the total crane load on one track. Compute the pressure at a depth of 6 feet on an area of crane track spread out in all directions at 60 degrees to the horizontal. Chances are it would be a very small fraction of the natural pressure from earth above the pipe. Peanuts.

 

[fel3]

nagatalluri....

A good treatment of this subject is found in "Buried Pipe Design" by A.P. Moser. See

https://www.amazon.com/BURIED-PIPE-DESIGN-P-Moser/dp/007147689X.

The 3rd edition is current. If you run across the 1st edition (which I have), please note that Equation 2.3 (shown twice on page 12) has an error: the K term should be Ku' (where u = the Greek letter mu). A friend told me a while back that this error had been corrected in the 2nd edition, so it should be correct in the 3rd edition also. BTW, a quick internet search indicates that .PDFs of the 2nd edition are available on some file sharing sites.

Developing a solution isn't difficult and the math isn't hard, but there are a lot of pieces to put together. So, I offer the following to jump-start your work.

First, however, you need to know if the buried pipe you are concerned with is rigid (e.g. concrete, vitrified clay, etc.) or flexible (PVC, HDPE, ductile iron, corrugated steel, etc.), because it matters. BTW, almost all utility water pipes are flexible.

Second, you need to know the burial condition of the pipe (trench condition, embankment condition, etc.), because that matters, too. BTW, almost all utility water pipes were installed in a trench condition.

Now to the jump-start: several years ago, I created Mathcad Prime 3.0 worksheets to solve this problem for both rigid and flexible pipes in a trench condition. You can find these worksheets on the PTC website at the following links:

https://community.ptc.com/t5/PTC-Mathcad/Lusk-Flexible-Pipe-Design-zip/m-p/449427

https://community.ptc.com/t5/PTC-Mathcad/Lusk-Rigid-Pipe-Design-zip/m-p/449424

Even if you don't have Mathcad Prime 3.0 or later, I suggest you download these two .ZIP file anyway. The .ZIP files include .PDF versions of the worksheets, so at least you can see static versions of the calculations. It should be a simple, though perhaps tedious, exercise to translate these calculations to Excel if you need to. If your pipe was installed in any other manner than in the trench condition, then you would need to modify a couple things in the procedure. Moser's book covers these differences.

Also, many years ago I developed an Excel spreadsheet specifically to evaluate buried ASTM C76 RCP in a trench condition, which I have attached to this post.

One final point: because construction vehicles use large tires (typically at low pressures) or caterpillar tracks, they typically have much lower ground pressures than road vehicles. This is why the typical design vehicle for buried pipe design is H-20/HS-20 and not construction vehicles. In my experience, the only time construction vehicles would govern is when depth of cover is less than the ultimate because the soil cover is a work in progress.

So, I doubt that a water pipe buried six feet down is going to be a problem. BUT, run the numbers to be certain. Also, if the water pipe is pressurized during the time the crane rolls over it, I would be even less concerned because the internal pressure supports the pipe from collapse. All of the calculations I am sharing assume an empty pipe because that is the critical condition for failure due to an imposed vertical load.

I hope this helps.

Fred


==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[oldestguy]

fel3: A very complete response for the poster. However, do a quick review of this poster's past questions and my bet is you are way, way over his/her head. Some items I suspect are not available to him/her such as the weight of the vehicle and the track dimensions and how to find them.. Again was the pipe laid in a trench or a simple fill, as in a roadway?

To the poster, try this link:

http://www.mlcranes.com/wp-content/uploads/2015/03/3612_brochure-.pdf

It will get you in the ball park.

 

[Ron]

If the direction of the crane is perpendicular to the pipe placement, OG has it exactly right...no worries. If parallel and on top of the pipe analyze as OG noted.

 

[Okiryu]

I would check the distribution of stresses using Boussinesq theory which will create a moment in the pipe and compare the bending stresses against pipe manufacturer’s catalog (allowable bending stress of the pipe).

 

[robyengIT]

compaction chart under construction loads : may be it can help

 

[robyengIT]

Here You can find complete theory for any laying condition

https://www.concretepipe.org/pipe-box-resources/design/design-manual/

 

[nagatalluri]

Thanks everyone for your excellent feedback. The type of crane is a heavy one used for installing Wind Turbines, the model is Manitowoc 18000. I haven't got much information other than that but a feedback that the crane will be tracking on a crane mat (12" thick) which will lead to a ground bearing pressure varying from 3800-5300psf. I checked guidelines for design of buried pipes and using their equation for a Cohesive soil, I came up with no resulting pressure due to overburden of soil above. I am trying to check and put together some simple calculation I can share with the utility

 

[robyengIT]

at a depth of 10 ft the live load is always negligible. You are at 6 ft , so the live load is reduced a lot

 

[oldestguy]

An unusual situation would be with the crane lifting a big load causing the tracks to be carrying most of their load on the "toes", then the spread out of load might be much less than using the full track area, meaning some computations for a "spot" load of the crane and its load on two places.

 

[nagatalluri]

Thanks but the crane will only pass over the pipeline between the turbine locations, it won't be lifting anything

 

[fel3]

OG…

I agree about the OP's track record, which I checked this morning after seeing your comment. However, I never worry about such things because the audience for our responses is much larger than just the OP. Other members of this website stand to benefit from a good answer whether or not the OP does.

Fred

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill