Culvert Advice -- Plastic vs. Galvanized ?

[CurlyDave]

I need to put in a 20' culvert to extend a road to an RV pad I am building. I need to be able to drive both a 14,000 # motorhome and my 8000 # tractor over the culvert, not at the same time.

I have seen both metal and plastic used, the costs are close in my area.

Which is the preferred type & why?

Second question:

How do I tell what size to use? The county says 12" minimum everywhere, but for the difference in cost I would rather do it right (maybe some overkill) the first time than replace it next year if it is too small.

From the topo map, it looks like the water from 10-20 acres of 15%+ slope drains through this spot. The culvert will have about a 10% slope.

I am sort of leaning to 24" diameter, which only costs twice as much as 12".

The culvert is going into a natural gully and not a ditch. I looks to me like there will be over 3' of fill over the culvert at one end, and over 5' at the other.

Third Question:

Do I need to fill around the culvert with gravel or can I just use the dirt I excavated for the RV pad?

Forth Question:

Should I have some kind of grate to prevent debris from entering the culvert?

 

[dmccarty]

CurlyDave,

I like plastic since it won't rust.

You need at least 12" of material over the culvert to get the maximum strength. Mine is covered maybe 6"-8" but it handled 55-60,000 pound dump trucks just fine.

I used ABC to cover the culvert since that is what I was using to build the driveway. Most people would use whatever soil they had available.

I think mine is 18" in diameter. I got mine for free so I used it. I don't get much water in mine since the culvert is only about 50 feet from the top of a hill. I would want 18" or larger culvert since I think it would be harder to stop up in the first place and if it did get clogged it should be easier to clean out.

10-20 acres can generate alot of water. Since it sounds like you have a deep ditch a bigger culvert would be better. Bigger than you need is better than not enough culvert. You might want to contact you county extension office and see if they can give you some guidence to size the culvert. 1 inch of rain on 10 acres is alot of water....

Later,
Dan

 

[Mickey_Fx]

I'm a long way from being an expert here but with the weight on the MH (possible 8-9k# on the rear axle) and not knowing anything about the "dirt" I'd be a little concerned whether the plastic culvert could handle the stress. Does the galvinized steel have spiral flutes? This adds to the strength considerably. The trade off is that somewhere down the road the galv/steel may rust out as zinc is water soluable.

As for the grate, on a 24" culvert one might not be needed depending upon what gets washed down the gully. With a grate you could find yourself having to clean at least once a yr. I'd do some asking around from others in the area that have done it both ways and see what their feeling are.

Up the road a piece at MP #272.

 

[EddieWalker]

Plastic is better than metal for several reasons. First, it will never rust out on you. EVER. Second, get the type with the smooth inner wall. It stays cleaner and doesn't hold silt and debri. As for strenght, both rely on preasure from the fill material being equal on all sides. You need a certain amount of fill on top based on the size of the pipe you install to reach maximum strength. For an RV, you will need this. For a car or light tractor, the amount on top isn't as important.

Since you want to create a crossing over a ravine, go as big as you can. That ravine was created by allot of water. The size of it shoud tell you the amount of flow you have to deal with. 24 inches is a huge pipe and should be able to handle most situations, but there is a reason they make bigger pipes and/or install two side by side.

Putting grates in front will cause more problems than it would solve. Small leaves will wash right through it, but with a grate in front, they will create a dam. It's very rare to see one along highways or anyplace that they are put in profesionally, so I'm thinking you don't need one. I don't have any on mine and so far haven't had any issues.

Backfill is usually with clean soil, but sometimes sand is used instead. Rock isn't any good because of the voids and dificulty in getting it all compacted. At the entrance to the pipe you will need to put something down to stop erosion. I've used sacks of cement for this with good results. Of course, it's not as attractove as rock. If you use rock, be sure it's big enough not to wash away or let the water through it. Small rock is useless during a heavy storm.

And always plan for the very worse that can happen.

Eddie

 

[BB_TX]

I am with Eddie. Go as big as you can. 10-20 acres draining thru a 6' or 7' deep ravine can be a lot of water. And you want it to go thru the culvert, not over the top.
And as Eddie said, don't use loose rock or gravel for fill. You don't want the water seeping around the sides of the culvert.
I have a 24" culvert in the ditch coming in off the road. I had to have some fill dirt hauled in but it was similar to what I have on my land. Once it is filled and packed, the circular nature of the pipe is very strong. Weight on top tends to push the dirt down around the sides of the pipe. That applies extra pressure to the sides adding support to the top. Due to the depth of my ditch, I have only about 1' fill on top of mine. But when I built my metal building, they brought in all materials on a flat bed semi. No problem at all driving it over the culvert. Oh. mine is metal.

 

[RobS]

I used plastic. Check out this pic of my culvert in it's early life. This was four years ago and all is well to this day. Plastic is lighter, easier to handle, won't rust, cheaper and by getting the double wall you won't have mosquito problems (it's smooth on the inside).

I've since added landscape block endwalls which really dressed it up /forums/images/graemlins/smile.gif

 

[RobS]

I guess I should have added that the strength of the culvert isn't in the pipe itself but in the soil around it. For either steel or plastic to work well you need enough material on top (at least six inches but a foot is better) and it should be well packed all the way around.

And of course, be sure to post pics before, during and after your project /forums/images/graemlins/smile.gif

 

[Highbeam]

Or just use both. The pic shows a 24" CPEP (smooth PE inside, corrugated PE outside) beside a 36" CMP pipe. Both full 20' sticks. I wanted plenty of flow through the flat bottomed channel. I scrounged these pipes or they would have both been CPEP. It is just a better material. You can cut it with a chainsaw, clip two together with these zip tie connectors, it is lightweight, won't rust, is cheap, etc.

2' burial depth with CPEP or CMP for full H20 loading. 3' with PVC, and one foot with ductile iron. H20 is the standard for most every road and bridge.

 

[Clyde]

RobS - Your thought about the smooth inside walls of a double wall plastic culvert eliminating a mosquito breeding site really caught my attention. That thought had never entered my mind but it definitely is a factor in deciding galvanized versus plastic in my opinion. Thanks - Clyde

 

[Henro]

<font color="blue">
Putting grates in front will cause more problems than it would solve. </font>

No doubt in my mind that Eddie's words are true! Don't ask me how I know... /forums/images/graemlins/blush.gif

However, my experience is that you can get a similar benefit from some kind of grating that is placed AHEAD of the pipe opening, to catch debris that might block the pipe entrance.

In my case, I have an 18" pipe that goes a distance, then drops 45 degrees, and then goes about level again. I certainly did not want some sticks to jam up in there and plug the pipe, so I put a grating made from a couple pieces of rebar over the front of the pipe...

Well, a stick jammed in there, then some leaves, then some other stuff, then gravel and then it was totally blocked, so the water ran over the top and eroded out a bunch...

When I saw what was happening, I got in the ditch, scooped out some of the blockage at the pipe entrance, started flow into the pipe, pulled out the sticks, and let all the stones, gravel and small pieces wash down through the pipe.

Lesson I learned was to put a grating in the ditch about 20 feet or more from the pipe entrance. Bad stuff is caught there, and when the grating startes to block up, the overflow still runs through the pipe carrying the small stuff with it.

You can clean out the ditch at your convenience, and not worry...

The larger the pipe, and the shorter the pipe, the less the worry. I have about 100' of 18" plastic pipe, corregated on the outside, smooth on the inside. Good stuff for all the reasons mentioned above.

I do need something to catch potential pipe blockers...and my grating in the ditch seems to be the winning solution for me... /forums/images/graemlins/cool.gif