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sengr (Structural) (OP)
14 Nov 06 16:52
Hi all. I am specifying a 18" reinforced concrete pipe culvert under a road with HS 20 loading. The pipe is barely under the base and there is no sub-base. In other words, the pipe is right below the base, with minimum thickness above the top of pipe. Is there a requirement for a minimum "cover" or distance before we could put the base above the pipe? The thickness of wear course is 3", base is 8". Thank you in advance.
SlideRuleEra (Structural)
14 Nov 06 19:26
The American Concrete Pipe Association has information that will help. See "Design Data 1 - Highway Live Loads On Concrete Pipe" (free .pdf download) at this link
http://www.concrete-pipe.org/pdfs1/DD_1.pdf

Also, you may want to download the "Concrete Pipe Design Manual" at this link (see Chapter 4)
http://www.concrete-pipe.org/manual/START.pdf

Finally, if it turns out that you do need more cover, perhaps an equivalent elliptical shaped pipe can be substituted for the 18" circular RCP.

www.SlideRuleEra.net idea

gbam (Civil/Environmental)
14 Nov 06 19:32
All tables available, that I am aware of, assume 1-foot of cover to the sub-grade.  These tables identify .01 crack for a concrete pipe vs pipe classification.  The loading on the pipe can be manually computed and then the required classification can be identified.  These procedures are availble in the Concrete Pipe Design Manual.  Check out www.concrete-pipe.org for information.  I set up a spreadsheet to automate the procedure.

Good Luck & Have Fun!
  
BigInch (Petroleum)
14 Nov 06 20:35
The only way that will work as a long term solution is to place concrete all around the culvert.  I don't care how well compacted the base is on either side, within 1 or 2 months at best, there will be a nice speed control device there.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

blueoak (Civil/Environmental)
15 Nov 06 9:42
Seems like a pretty small pipe.  I wouldn't be as worried about the pipe as the road.  As BigInch says it is probably best to lay it in concrete.  I would also look at adding some subbase next to the concrete so that if you have movement it is spread further.
gbam (Civil/Environmental)
15 Nov 06 10:23
If you plan on using concrete encasement, remember that over time the subgrade next to the concrete will compress creating a little depression and mound of asphalt.
BigInch (Petroleum)
15 Nov 06 10:42
Right, It won't do to have a square bump in the road either.  I wasn't clear.  I meant to say make a ramp up of the concrete in the subgrade until its even with the top of the pipe.  

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

saruman26 (Civil/Environmental)
15 Nov 06 22:35
Also, it would be wise to specify a greater strength class of concrete pipe since the cover is so shallow.  Class III pipe is 'standard' RCP, while Class V would be the highest strength class for pipe before a custom pipe design is required.
ACtrafficengr (Civil/Environmental)
16 Nov 06 7:47
You probably already considered this, but can you raise the profile for more cover?

     "...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

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