How Would You Fix This Bridge?

[thankyouTBN]

Oh my God, I feel for you!

The bridge was built in 1913 by dumping fill over a 6' diameter steel pipe, which is rotting away. The span over the pipe is around 20' to get to solid ground on either side. There is 36" of fill on top of the pipe, topped with 6" of modified gravel. Water is running beside the pipe and undermining what's left of it. As a result, the downstream side (left in pics 2 & 3), is subsiding. The aerial photo is north up orientation and the stream flow is top to bottom.

It's a private road, used by only a few property owners with limited resources. Contractor estimates to replace it with a 6' poly pipe are upward of $50K, which we can't afford. The price is elevated by DEP regulations which require the stream to be diverted during construction. The only way to do this is with large pumps. Stream flow and environmental impact studies prepared by a certified engineer are also required. Construction is made more difficult by overhead transmission lines.

The "cheap" fixes we've been looking at include using flatbed rail cars to span the pipe, or perhaps a 20' reinforced concrete slab. To avoid DEP regulations, anything we do can't disturb the stream.

Any ideas or suggestions would be greatly appreciated.

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Oh my God, I feel for you!

The bridge was built in 1913 by dumping fill over a 6' diameter steel pipe, which is rotting away. The span over the pipe is around 20' to get to solid ground on either side. There is 36" of fill on top of the pipe, topped with 6" of modified gravel. Water is running beside the pipe and undermining what's left of it. As a result, the downstream side (left in pics 2 & 3), is subsiding. The aerial photo is north up orientation and the stream flow is top to bottom.

It's a private road, used by only a few property owners with limited resources. Contractor estimates to replace it with a 6' poly pipe are upward of $50K, which we can't afford. The price is elevated by DEP regulations which require the stream to be diverted during construction. The only way to do this is with large pumps. Stream flow and environmental impact studies prepared by a certified engineer are also required. Construction is made more difficult by overhead transmission lines.

The "cheap" fixes we've been looking at include using flatbed rail cars to span the pipe, or perhaps a 20' reinforced concrete slab. To avoid DEP regulations, anything we do can't disturb the stream.

Any ideas or suggestions would be greatly appreciated.

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Oh my God, I feel for you! Years ago, I knew a woman who had a bridge with two really large (8’?) diameter corrugated galvanizedculverts that were encased by concrete and rubble fill. The pipes rusted out at the bottom, and the water was causing the rubble fill to fall into the water. I told her “you have GOT to get that bridge fixed!” ended up costing $30,000 and I’m pretty sure her father paid for it. I really feel for you. In her case, she didn’t have any such onerous DEP stipulations.
I’m praying for you. I hope you can get it done as inexpensively as possible.

 

[JJT]

One thing I learned from watching the high water flow through the two 5' concrete culverts on the creek behind our place is that once you add flow restrictions you have to WAY over size things due to various forms of flow restriction. I've seen the tailwater out of them be at not much more than half full when it's already topping the road over them due to inlet restrictions/friction. I found the disparity to be real interesting and did a little reading and it's a significand design challenge. Bridges can have similar problems once you hit peak flows if there are restrictive abutments, etc.. but in general you need less slack than you do with a culvert system.

In light of that I would add that if you can pull the culvert and put in a bridge that would have a lot less flow restriction overall and might be easier to size/engineer the thing.. especially something prebuilt like one of the railway flatcars (I see those for $10-15k BUT you'd also need to built abutments, etc.. so the final total cost would be quite a bit more .. but it'd drop in pretty fast anyway).

I'd also be a bit wary of what would happen moving from a 6' to a 5' opening by sleeving it.. you already have overtopping problems and IMHO (I'm not a hydrologist.. I just like looking at the creek in the rain and observed some things..) that could make that a lot worse.

"I'd also be a bit wary of what would happen moving from a 6' to a 5' opening by sleeving it.."

The OP already said it's partially sleeved with a 5' pipe. The 6' pipe is now limited to a 5' opening. Sleeve the rest of the pipe and be done.

Bill the neighbors for the work you've already done and see what happens.