Precast box culvert on inclined slope

[JC93]

Hi all,

I am currently detailing a precast box culvert, I know I could do it as a bottomless culvert with insitu slabs but would prefer to keep the full box unit to avoid scour issues. The proposed gradient to suit catchment study is 1 in 12 which means the end units lean about 5deg. I know it's not a lot but to me it affects how the wingwalls and the header beams are positioned, has anyone came across this? and how have you tackled the lean? See screenshot for more details.

TIA!

 

[DoubleStud]

You cast the end unit tapered using styrofoam or plywood. The headwall (if not too tall), will be poured second phase when the end unit (tapered end) is sitting flat on the floor.

 

[JC93]

Thanks for the reply. Okay I can cast the end unit as tapered, just thought that was going to be a bit of a faff with the rebar.

 

[DoubleStud]

If it is a big job, the precaster will use wire mesh. Then at the tapered unit they would cut the mesh to fit, then add additional rebar. This is assuming the precaster is a good company and have an engineering staff in house. but 5 degree is not much for a short unit. You wont lose that many longitudinal wire reinforcements if they didn't add additional rebar. You can demand a shop drawing and review the end unit shop drawing from them.

 

[JC93]

Yeah we also have a note on our drawings that say fabrication drawings are to be provided for review.

 

[DoubleStud]

The other option is just to leave it leaning including the headwall. The wing walls can go on the side like you drew it. It is all depends on where the project is. Sometimes aesthetic is the least of the client's concern. Or sometimes the wingwall is cast in place. Sometimes the first few ft of the wing wall is cast in place.

 

[JC93]

Yeah its one of those, if we leave it leaning are people in the future going to be concerned because it looks like its overturning. I don't want that for the project and it is early enough to detail out.

 

[BridgeSmith]

Precast box sections of any size pretty much always use welded wire reinforcement (WWR) mesh.

so, if I understand correctly, it will have a 5 deg slope across the bottom slab from one side of the opening width to the other? How many sections of precast (i.e. what length of the culvert) will have to have a slope?

 

[JC93]

There will be about 13 sections, the whole thing is on a uniform 5deg slope.

 

[BridgeSmith]

the whole thing is on a uniform 5deg slope.

Well then, the solution is simple - just use standard sections and place them on a sloped base, and slope everything else to match. That's what we typically do.

If the parapet, end wall and wingwalls are CIP, then you could make the transition to level there.

I would definitely call a precaster in the area who does box culvert sections, and inquire about the cost premium for doing special end sections. I would anticipate that adding significant cost to the fabrication.

If this conveys water, your bigger problem is going to be preventing a swimming pool from forming at the outlet due to the ridiculously high flow velocity. The last time I had a culvert with that much drop, I created a drop structure within the culvert, which essentially meant it was a 10'x10' box at the inlet, with a 5' drop of the invert in the middle, and a 10'x15' box to the outlet. It reduced the scour potential significantly. We still needed stone-filled gabions at the outlet to prevent scour and dissipate the energy.