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Stresses on pipes 11

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pelelo

pelelo

Geotechnical
Aug 10, 2009
357
Hello,

I am working on a project which involves placing fill about 50 ft high on top of a pipe.

It is my understanding that the total vertical stress acting on the pipe will not be the full height (50 ft) of fill. At some point there will be arching and the stresses on the pipe will be less that then ones at full height.

The fill is SP material. The pipe is CMP type.

I was wondering if anyone knows a reference that can provide information about how to compute the actual stresses on top of the pipe.

Please let me know.

 
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to calculate loads on top of pipes, not stresses on pipes(free download):

1 - American Concrete Pipe Association (manual for hand calculation and software "Pipecar")

2 - software "Pipeclass" : similar to previous but from Australian pipe association

3 - software FEA (but you need mathcad pre-installed) CANDE (Culvert ANalysis and DEsign)

4 - software : Google "buried PVC pipe design" (
 
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The US Dept of the Navy has manuals for engineering. One manual, possibly updated with another name is "Design Manual" NAVFAC DM-7. Chapter 14 has info on loads, etc on pipes. I'd also look at pipe associations and their design info. See George Sowers. See his text for other info. "Introductory Soil Mechanics and Foundations". Also look up Spangler's theory on the"Imperfect Trench". That idea works well where you add more fill on an existing pipe. What it calls for is a zone of compressible material on top of the pipe to cause the loads above to split and load the soil beside the pipe, not the top. There probably are many other references out there, generally in text books.
 
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Now let's see if we have design guidance for triple-box culverts (17') beneath 300 ft of rock fill?

Bed the boxes (i.e., undercut the alluvium), not the overall embankment (i.e., the ground below the embankment will settle more than the ground below the culvert). Consider arching and stress transfer?

More than gamma*H?

How does arching factor into this, if at all?

I'll take any thoughts on the matter - thoughts that don't include Plaxis. . . for now. . .

f-d

ípapß gordo ainÆt no madre flaca!
 
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Why oh why would you place a CMP under 50 feet of fill? A CMP has a much more limited life than other pipe types. Replacing a pipe under that much fill will be very expensive to fix.

As for the load on the pipe, it can be more or less than gama*H depending on the specific installation conditions and the stiffness not strength) of the soil on either side of the pipe.

Mike Lambert
 
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Thanks for your reply.

This is a project being done between different departments. It seems the water department what knows only is how to work with CMP. I just want to prove to them that the CMP under such fill will have problems.
 
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A rough estimate for stresses around a circular opening in a semi infinite continuum is Kirsch's solution derived from the elastic theory.
He gave solutions for:
1) considering only vertical stress
2) hydrostatic conditions (Sig_v = Sig_h)
3) general 2D stress state with different horizontal and vertical stress

Whichever you choose you just have to insert the required parameters in his formulas and you will get the resulting stresses around the pipe in a polar coordinate system (Sig_r and Sig_theta), where "r" is radius and "theta" angle in the polar system.
EDIT: Link - Here is a link I found that explains the solution and, more importantly, gives the equations.
 
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Thanks,

FYI, a good source I found is the "Corrugated Steel Pipe Design Manual" (2008), by the National Corrugated Steel Pipe Association.

The soil load acting on the pipe will depend on the degree of compaction of fill (the looser it is or the less compacted it is, the more the load that will be applied on the CMP". They use factor (K) as load factor. For example, if fill on top of the pipe will 90% compacted per Max Standard Proctor, the load factor will be 0.75. That means 75% of the full load height will be applied to the CMP.

The manual is available online.
 
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pelelo,

Your last post is not really correct for all situations. I'm not familiar with specific reference, but as I posted previously the load on the pipe will depend on the conditions beside the pipe. Your post seems about right if the pipe is embedded in a trench and the soil on either side of the trench is stiffer than the pipe since this will allow load to arch away from the pipe.

However, the soil on either side of the pipe is less stiff than the pipe, then load will arch onto the pipe resulting a much higher load.

So the real question you have to look at is load path and the load path depends on the relative stiffness of the pipe relative to the soil on either side of the pipe.

Just remember that nothing bad happens to the pipe if you over estimate the load on the pipe. However, if you under estimate the load; the pipe fails. Your choice.

Mike Lambert
 
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Handbook of Steel Drainage and Highway Construction Products is helpful. Out of print, but available at Amazon for a few dollars.
Be sure to have good compaction all around and beside the pipe, better than the book says, especially if the soil may become nearly saturated someday.
 
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CMP as storm pipe lasts about 30 years. How long has been in the ground? Calculate the cost to replace it or tunnel in a new pipe. Then calculate the cost to replace with RCP before you fill over the pipe.
 
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