Culvert Installation Basics

[Currmudgeon]

Driveway culverts are pretty much limited by the grade of the existing roadside ditch. The elevation of the culvert should match the ditch, even if it doesn't make sense. You can do a little ditch grading to clean things up, but you are pretty much locked in to meeting existing grade at both property lines, and you don't want to regrade the whole County.

If the line of the culvert is higher than the ditch flowline, the ditch will silt in to match the culvert. If the culvert is lower than the ditchline, the culvert will tend to silt in to match the ditchline.

You can probably get enough accuracy with a string and a line level. A good operator can grade it out from the seat, but I don't claim to be able to do that.

If the existing soil is gravelly, you may be able to simply shape and compact the bottom to accept the pipe. If it is soupy, you may want to undercut it a foot or so, and put down some crushed stone for a workable base.

A corrugated matal pipe is a flexible pipe. It gets all its strength from the backfill. When load is applied, the pipe squashes a little and the dirt picks up the load. It is important to compact the backfill well around the sides and under the haunches of the pipe. Loose backfill will rob the pipe of most of its strength.

For automotive loads and an occasional truck, a foot of compacted granular material on top of the culvert ought to be enough. If this creates a hump, then taper it out a bit so small cars don't high center on it.

Find out what the County or the State DOT uses for road stone, and use that. The terminology varies all over the place, but you want a well graded, crushed aggregate material. The County Engineer can tell you what to ask the gravel pit to deliver.

If you have an erosion problem in the ditch, a stone apron at the outlet end may be necessary. Undercut the ditchline by about 1-1/2 feet, lay down some landscape cloth or geotextile, and bring the grade back up with 4 - 8 inch stone or broken concrete. The apron should match the culvert flowline at the end of the culvert. I'd make it maybe 10 feet long for an 18 inch pipe, and spread the sides out on about a 2:1 angle so you have a long narrow trapezoid. Fake everything in to meet existing grade at the edges, and the contour of the ditch.

 

[rdam]

This project is now underway. The problem is this. My estimate of the swale depth was fairly accurate; 12". The pipe is 18" diameter. In order to have a level driveway with 12" of material on top of the pipe, The entire pipe would be under the swale grade !!!!

What I have done so far is dig down an even 12" below the grade of the swale. This will leave 6" of pipe above grade and allow me to put 6" of fill/lime rock on top and still have a level driveway. I'm thinking that may be too deep. I want to back fill in another 2 inches before putting in the pipe leaving 8" above grade and creating a small but manageable "hump" if I put 6" of fill on top.

Nobody in my subdivision has more than 6" of fill on top of their culvert. Most are only 3 or 4".

There is little if any problem with water flow off my property. I have never seen flowing or standing water in the swales. To those with experience, How does this plan sound?

 

[DVerbarg]

In my opinion you have dug down too deep, unless you intend to build a foundation to bring the base back up.

4 years ago I put a culvert pipe in for my house. My pipe was only required to be 15" and the swale was only around 10" lower than the edge of the road. The only digging I did was to cut the high/lumpy points from the swale to make it straight. Dropped the pipe and then backfilled with what we call CA6 in this neck of the woods.

Only have 1" to 2" or so on the top of the pipe due to the same problem you are facing. If the fill on the side of the pipe is packed adequately the pipe will not crush. In order for the pipe to crush the sides will have to push out. Due to the pipe being higher than the road I had to add more gravel to create a higher flat area to avoid a peak that would be a problem for low cars.

This configuration of the culvert pipe withstood the barrage of construction traffic for our house, including more than a dozen concrete trucks and dozens of the semi dump trucks (our driveway is more than a quarter mile long and we dug it out around 8" to 10" for a good base, took a little bit of rock to fill it back in).

The only problem with any crush of the pipe was due to a sloppy truck driver running over one of the ends.

Hope this helps.

 

[BB_TX]

With your pipe below grade you are going to have standing water inside the pipe for a long time after every rain. In a humid environment, it may never dry. Eventually it will fill with dirt and you will have very little drainage area left.
Maybe they will let you put in two 12" pipes in place of the one 18". Two 12s are not quite the same total opening as an 18 but close.

 

[SPIKER]

With the pipe below grade like you stated I'm with others that water will stand and creat moscuito heven in there, as well as help rust out the pipe quickly. if you are affrade that the 4" or so fill on top would not be enough then I suggest back filling with some concrete whith a little reinforcing over the top and gravel on that ... but like others said the pipe will not crush unless the walls let it crush. if it is packed in aginst the pipe well it can't crush.

Mark M

 

[Phred]

You have alot of good advise here, but one question in my mind is are you going to pave over this culvert?

If not, which is always the case for me since I live in the sticks, why go to a lot of trouble?
If your going to pave over it then it needs to be right, and by all means spend the money to get it done right the first time. Otherwise, so it lasts only 10 years? But what if it costs 20% of the fancy methods?

What I do is use a minexcavator, or similar, which you can rent. Clean up and deepen the trench as needed. Drop the pipe in and back fill with clay, then spread gravel over the top.
Fancy, no.
Cost: 1 pipe, and the gravel since I have plenty of the clay. I would use the gravel any way for the road so the culvert really only costs the pipe and the labour.

I have tried all sorts of fancy fill gravels that you can read about all day long on this site and others, and IMHO nothing works half as well as clay. It packs like cement and water does not easily penetrate through it. And best of all its free in many parts of the country.
I can't tell you how annoyed I was the first culvert I put in with crushed limestone that did not last 6 months. Granted it was on a pretty steep hill, but after a few very hard rains not one grain of that expensive limestone was to be found. The clay filled replacement is still there.

Just my two cents....

Fred

 

[BB_TX]

Just a second thought. With the culvert that much below grade, it will probably not meet the county specifications for installation. Depending on your code enforcement person, he/she could make you dig it up and start over!!

 

[GrayBeard]

As others have said, you probably won't be satisfied with the pipe lower than the invert of the ditch line.
One of the basic rules of "culvert maintenance" is , if you take care of the discharge end; the inlet will take cake of itself.
That meaning, in the installations in my area it's a very good idea to dig a "sump" at the discharge. That allows good flow and she won't plug. Now and then you have to clean the "sump" but that's better than a plugged pipe.
If the local codes would allow, there's a much better product (imho) than corrigated metal (we call it wrinkled tin) pipe . That is a rigid corrigated plastic with a smooth interliner.
It's just as strong , won't rust and the smooth lining allows for much better flow. The cost is not that much more than metal.
The contrs. here having been using it for years with no ill effects.
All my comments are reflective of local conditions. It sounds like you may not have much water to deal with.
We have alot of water (at times) and some big hills.
GrayBeard

 

[Soundguy]

Down south we call that corugated pipe 'squash' pipe.

Also on the outlet side he may want to put a flume or energy disipator ( used to be just called rip rap.. etc )

Soundguy

 

[cj7]

I am surprised no one has mentioned using a smooth-bore pipe. This is what my excavator is recommending to me. We have to wait 'til spring to install it. It is a black curagated pipe that is smooth on the inside just like your pvc or dw pipe in your house. This helps to insure that no silt builds up and is kept clean provided that you have proper pitch on the pipe. It is more expensive but should last longer. You may want to look into this pipe for your project. You may not need the type of backfill that steel requires. I am sure someone knows the answer.