3 Single Precast Box Culverts

[edward1]

I am designing 3 single precast box culverts that will be placed side by side with variable gaps between them from 3" to 1'-7" depending on fill and varying wall thickness. The question asked by DOT engineers is would the lateral pressure be different on interior walls from exterior walls if the gaps are filled with porous gravel?

 

[GeoPaveTraffic]

Depending on the size of the culverts, i expect that the pressure on the interior walls be essentially zero.

Mike Lambert

 

[cvg]

you will have leakage through the joints which will saturate the porous gravel. so you might expect lateral water pressure loading on the interior walls

 

[edward1]

These boxes are 7'span x 7' high and fill is from 50' to 310'. Isn't vertical pressure essentially gamma*height of fill? Lateral pressure is Ca*gamma*height of fill?

 

[PEinc]

Ignoring base friction, it could be argued that there are equal and opposite lateral forces on the walls. Pressure from the far sides of the first and third boxes could be assumed to apply force to the other four walls. I would design all walls for full earth pressure, water pressure (if applicable), and surcharge pressure. After all, it is possible that, in the future, two of the boxes could be removed and backfilled, thereby subjecting the "weaker" wall to higher pressures.

http://www.PeirceEngineering.com

 

[edward1]

PEinc,
You are correct sir!

 

[GeoPaveTraffic]

But if you don't ignore base friction, then you have a different situation. In the narrow space between the wall, you will have very little pressure due to silo effects.

If it were me, I would place concrete, flowable fill if I couldn't get concrete, between the culverts instead of gravel. That way you know the water will stay in the culverts and that the sides of the culverts are adequately supported.

Mike Lambert

 

[PEinc]

I agree with you, Mike, but I still would design the walls for the same, full, lateral pressures.

http://www.PeirceEngineering.com

 

[BridgeSmith]

Unless there is a reason other than the box design to ask the question, I'm not sure why anyone would even ask it, considering that the precast box sections will have the same reinforcement in both walls anyway. To do otherwise would be risky (what if the the sections aren't installed in the right places) and pointless (welded wire fabric is ridiculously cheap).

I'm a design engineer for a DOT. I've laid out numerous multiple barrel precast box culvert installations, and I've never been asked by any of the precasters who do the designs for our culverts what we were putting between them. I fail to see how it could possibly matter.

Just as an aside, we typically put them an inch apart with a cement slurry in the gap.

 

[fattdad]

There are many references that support loads other than gamma z for the vertical. Boundary conditions also govern - i.e., induced trench v. embankment installation. Refer to Quigley and Duncan. That citation also provides reference to horizontal pressures. Design case may be sigmaV*Ka, which would instill greater design stresses than sigmaV*K-naught. As a pre-cast element, there is no opportunity for rotation at the top, so it's unclear how an active wedge could develop, actual loads are more likely the latter, but a design case study of the former may be good?

with intimate contact between the boxes, statics would require loads be conveyed from one to the other.

Let's just leave it at that for now.

kind regards,

f-d

ípapß gordo ainÆt no madre flaca!

 

[Doctormo]

310' of fill might require a conservative analysis?

 

[Okiryu]

If the fill is 50' to 310' (is this a typo??), will the culverts move to mobilize active pressures? I do not design box culverts but perhaps the vertical load due to fill may be controlling the design of the interior walls?

 

[BridgeSmith]

"I do not design box culverts but perhaps the vertical load due to fill may be controlling the design of the interior walls?"

With separate single cell box sections, there are no interior walls, only adjacent exterior walls.

Design of the top and bottom slabs will have to consider a minimum pressure condition on the sidewalls. This is why I mentioned/suggested grouting the gap between the sections; that way the adjacent walls push against each other, minimizing the moments on those walls. A granular material between the boxes could reduce the lateral pressure on those walls, creating a worse condition for the walls and the slabs.