Replacing culvert at a crossing that's never dry

[Boondox]

I've studied the threads on installing culverts, but none appear to have the same challenges I face. I have a 10 foot long 24 inch diameter culvert that is (a) a tad too small to handle the seasonal volume of water that flows in the stream, and (b) too short to permit a sufficient amount of fill to drive my tractor or excavator safely across. Unfortunately, my woodlot is on the other side of the stream.

I'm going to replace it with a 20 foot long 36 inch diameter double walled culvert that will allow me to build the crossing up with fill, which I have plenty of. I have two challenges:

1) With water constantly flowing (on a dry day it's two feet wide and a couple inches deep), will it be possible to spread and pack down a stone base?

2) The approach is very narrow. In a perfect world I would be able to use the tractor bucket to hold the new culvert in place while adding fill, but there's barely enough room for the excavator as it is. Is there another way of preventing the culvert from shifting as the fill is placed around it?

Advice welcomed.

Pete

 

[s219]

Pete, you might have to forgo the stone base. I know I did a 4" crusher base on mine, but that was because it was going to be inspected and that's what the highway department wants. I've seen many other culverts installed in the area with no base or bottom grading/prep done at all. The existing base under your culvert now might be OK as-is.

If you determine that you really need a base, crusher run is the best, but might be messy when dumping into water. So I'd maybe up the size to #78 or #8 gravel, which still have small pieces but are washed of all dust and won't become muddy. Could maybe even use #57 gravel if the pipe didn't have fine corrugations. I think the main goal of the base is to support and conform to the pipe. If the ground is really mucky, I'd start with #3 then the smaller stuff on top.

As far as holding in place, is the new pipe concrete or steel, or plastic? If it's heavy enough, you can use shovel-fulls or small piles of crusher run or fill on each side, at about 4 foot intervals, like "chocks" to wedge it into position and keep it from rolling (that would be the only major motion on a heavy pipe). You can tamp the little piles down side to side as needed to fine-tune centering the pipe. Build up those little piles as high as needed to make it fairly solid. Then start dumping fill over the top, aiming for the exact top of the pipe to keep the fill from pushing the pipe one way or another as it falls. Work the fill around the side and under the pipe with a shovel or stick/pipe, and then tamp that down once it's fairly uniform. Then add more fill, tamp more, etc... Do it in layers to get good compaction, which is critical to support the pipe and help it only see hoop loads.

That said, I have see a lot of construction culverts that are nothing more than a pipe dropped into a ditch with fill dumped on top and then driven over. The tend to look lumpy and lopsided eventually, but they do hold up just fine in most cases. So I think you don't have to be 100% by the book.

Good luck! It's a fun project.

 

[dave1949]

Maybe some sacrificial rebar stakes and cheap tie-down straps? Drive the stakes in beside the new culvert, run straps over the culvert from stake to stake and bury them in-place. Probably need to form a hook in the top of the stakes before pounding them in.

 

[Sysop]

If there is much water moving very fast where you need to work, you might run a spare trench along where you are working to temporarily divert the flow.

 

[Gator6x4]

A 20 foot long 36 inch diameter double walled culvert is some serious weight. I don't think the two inches of water will move it. Remember the culvert will be setting in the center of the stream and water will be flowing through it, not against it.

 

[Sysop]

I think his concern was more about having trouble getting the base material in place so the culvert has proper support across the bottom and lower sides rather than the culvert floating away... The depth and width do not matter as much as the rate of flow which wasn't mentioned. It would simply be more difficult and time consuming to work in the water if the water is moving fast enough to make placing the base material difficult.

 

[Boondox]

Some good advice, thanks. The flow is significant only after a rain as the entire cirque drains via that stream, but between storms it's just a low volume babbling brook. And Sysop is right; my concern is the fill displacing the plastic culvert. But I have a channel that's pretty deep, so if I go slow and tamp things down I should be fine. BTW a parallel trench is not an option due to granite boulders on either side that outweigh my excavator by several tons.

Thanks again.

 

[CurlyDave]

Is it possible to do this a different way?

For instance, I would strongly consider leaving the existing 10' x 24" culvert in place. Build small coffer dams on either end to keep the water flow through the culvert.

Then add a 20' x 24" plastic culvert next to the original one. Then re-arrange the coffer dams to send water flow through the new culvert and add a 5' section to each end of the existing culvert.

The final result is two parallel 24" culverts, which will carry ~88% of the water a 36" culvert will, and the advantage is that you never have to lay a culvert in running water. There is a second advantage of a lower profile with two 24" culverts than one 36" one.

I suspect 30' of 24" culvert will cost just about the same as 20' of 36" culvert.

 

[bcp]

I like fords. A 14 foot wide ford 6 inches deep will be about the same volume of water as a 36 inch culvert. It doesn't get plugged, or wash away in a storm.

Small version:



Larger version:



Bruce

 

[crazyal]

If the ground under the pipe is solid then you would be fine putting down something that will pack well that's stone with fines. If it's muddy then larger stone that will sink into the mud. It doesn't have to be perfect. Once you lay the stone put the culvert in place and then put more solid fill around the inlet to the new culvert. That will force the water through the pipe. You can give the muddy ground time to dry and you can pack fill solidly around the culvert. I would use something like 3/4 crusher run around the culvert.