A Contractor's Guide to Installing Culverts - ProfitDig

Culverts might not seem like the most exciting part of a construction job, but if you mess one up, you’ll hear about it for years. A poorly installed culvert can lead to flooding, driveway washouts, and some very unhappy homeowners. So, let’s break it down step by step and cover everything you need to know to install a culvert properly.

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Step 1: Choosing the Right Culvert Pipe

Before you grab a shovel, you need to figure out what type and size of culvert pipe you need. Here’s what to consider:

Drainage Needs:

• How much water needs to flow through the culvert? This depends on factors like rainfall, soil type, and the size of the area draining into the culvert.
• A rule of thumb: A 12-inch diameter pipe is often enough for small driveways, but larger areas may need 18 to 24 inches or more.

Pipe Material:

• Corrugated Metal Pipe (CMP): Durable and strong, but prone to rust over time.
• High-Density Polyethylene (HDPE): Lightweight, resistant to corrosion, and easier to handle.
• Concrete Pipe: Super durable but heavy and harder to install.

Step 2: Calculating the Right Size

To determine the correct pipe size, you can use the Rational Method formula:

Q = CiA

Where:

Q = Peak discharge (cubic feet per second)

C = Runoff coefficient (varies by surface type)

i = Rainfall intensity (inches per hour)

A = Drainage area (acres)

Example Calculation:
Suppose you’re dealing with a 2-acre drainage area with a runoff coefficient of 0.3 (typical for grassy areas), and the rainfall intensity is 2 inches per hour. Plugging these into the formula:

Q = 0.3 * 2 * 2

Q = 1.2 cfs

Once you calculate the flow, check local regulations or refer to charts that match flow rates to culvert diameters.

Step 3: Digging the Trench

Now for the fun part—digging. A culvert should be buried deep enough to provide proper coverage while allowing water to flow effectively.

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Depth:

• The culvert should be placed at least 6 inches below the road surface for small pipes (up to 18 inches) and deeper for larger ones.
• Ensure a minimum cover of 12 inches of compacted material over the top.

Grade/Slope:

• The pipe should have a consistent slope of about 1-2% (1-2 feet drop per 100 feet length) to allow proper drainage.
• Use a laser level or string line to maintain an even grade.

Step 4: Installing the Culvert

Prepare the Base:

• Line the trench with a layer of gravel (4-6 inches thick) to provide a stable base.
• Compact the gravel to prevent settling over time.

Place the Pipe:

• Lay the culvert pipe in the trench with the correct slope.
• Make sure the pipe extends slightly beyond the edges of the driveway or road.

Backfill Carefully:

• Add gravel or crushed stone around the pipe in layers, compacting each layer as you go.
• Avoid using large rocks or debris that could damage the pipe.
• Cover with at least 12 inches of compacted fill material.

Step 5: Finishing Touches

End Treatments:

Consider adding flared end sections or headwalls to protect against erosion and keep the ends from washing out.

Erosion Control:

• Add riprap (large stones) around the inlet and outlet to slow water flow and prevent erosion.
• Seed and mulch disturbed areas to prevent silt buildup in the pipe.

Check for Settlement:

Over time, inspect the area for any signs of sinking or erosion and add fill as needed.

Step 6: Maintenance Tips

Once installed, a culvert isn’t a set-it-and-forget-it deal. Regular maintenance includes:

• Clearing debris like leaves and branches.
• Checking for blockages after heavy rains.
• Inspecting for cracks or signs of erosion around the ends.

Installing a culvert the right way takes a bit of planning and elbow grease, but doing it correctly will save you and your client a ton of headaches down the road. Remember—proper slope, solid base, and good backfill are key to a culvert that lasts.

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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 # 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? 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 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. 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 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.