API 1102 - Pipeline Highway Crossing Calcs

[BCG1]

The application involves installing a concrete slab over an existing pipeline to protect it from loads due to a new road (highway) crossing. It currently fails the combined stress limits as calc'ed per API 1102

If anyone is familiar with the API 1102 calculations for pipeline stresses, I am looking for some assistance in determining how the installation of a concrete slab (to further distribute loading) can be incorporated into the equations.

AASHTO indicates that the surface load distributes at a rate of 1.75 ft per every 1 ft. of depth (earth). By theory, if we install a wide concrete slab above the pipeline, we can further distribute the load over a greater area than soil alone.

My problem arises because while AASHTO provides this nice table to show how (w) decreases with depth, the API 1102 calculations seem to ignore depth. (w) is taken as the surface loading (single or tandem axle) and appears to retain that value regardless of depth. I looked at the pavement factors (R & L), and they don't seem to address this either.

Am I missing something here? I fear that in combining specs, I have somehow mixed methods, but I don't see a clear way to incorporate slab protection in the API 1102 calcs. Do I need to back-calculate and fudge in a new valve for (w)? BTW, I only have an excerpt of AASHTO, so hopefully the next page isn't a disclaimer...

Thanks,
BCG

 

[unclesyd]

Not my area of expertise but here is some design criteria along with a few different references than you mention that you might want to look at.

http://www.acipco.com/aswp/pdfs/ASWP2-Design.pdf

 

[GregLamberson]

BCG1

I have run into the same situation before, but we always lowered the pipe. Is that an option in your case?

Greg Lamberson
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[BCG1]

Thx for the feedback...

Unfortunately, lowering may not be feasible. The pipe is at 3 to 4' of cover, and the stresses fail until we get near 10' depth. It's an 18" OD pipe, so we'd probably have to dig back further than we've got.

The link posted above is re-affirmation of the AASHTO guideline for the load distribution decrease through the soil. I am still trying to figure out how a slab would be incorporated into API 1102. I think I can account for it by adjusting the soil modulus (E), somehow.

I may end up recommending a concrete span over the pipe, which is in essence like a box culvert. Trying to avoid too much overkill though....

Thanks again

 

[BigInch]

Treat it as a beam on an elastic foundation. Use your existing soil modulii springs for beam support, calculate the combined beam and soil spring loads. Convert spring loads to PSF imparted to the soil.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com