Culvert on Unyielding Foundation

[CoryChamberlain]

AASHTO 6.2.2 states that a culvert to be designed on a unyielding foundation requires a special analysis. I have looked through numerous references and contacted several people on this issues with no answer to what is required for this special analysis. Has anyone dealt with this? The thing that comes to mind is the distribution of loads through the bottom slab may be different.

 

[dicksewerrat]

Are you talking about a rc box culvert? If so the load is spread over the entire area and should be fairly low. I don't see a problem here. Pipe may be aproblem because you now are putting the pipe into a two-point load situation.

 

[CoryChamberlain]

Yes, I forgot to mention that I was looking at RC box culvert cast directly on sound rock.

 

[ghghghgh]

My 2 cents,
Since culvert bottom slabs are generally designed with a uniform bearing on soil, I think the intent of the code is to caution against a situation that may change the support conditions. We had a culvert that longitudinally had one end on rock, the other on soil, so we had to consider soil settlement in the design. I guess the same situation could occur in the transverse direction too.
In our case, "special analysis" was a few engineers talking about it and some quick calcs, but I imagine it could easily turn into a 100 page STAAD model if you so choose.

 

[wiktor]

Special analysis required by AASHTO for culverts on unyielding foundations are related to the increased overburden soil loading. As the culvert will be a "hard point", the settling soil in the proximity of the culvert will exert a down drag on the column of soil lying directly on the culvert, thus in effect increasing the loading..

 

[BigH]

of course it depends on how much fill is placed over the culvert. If your structure is all on rock and your fill heights are modest - say less than 3m above the culvert (usually are much less) I don't see you having any problems in practice.

 

[LeoSC]

If the bottom of the culvert is on sound rock, I suggest that you consider a rigid frame rather than a box. This, of course, assuming that the entire culvert is on sound rock and that the rock will be competent against scour.

 

[dicksewerrat]

I guess I'd call the box culvert supplier, give them all the info and see what they say. But I don't see any major problems unless you are putting a freeway 6 inches above the culvert. Even then more reinforcing can be added to the culvert to counter the loads. you may want to clean off the rock and put a leveling course of neat cement grout down before placing the culverts.