Construction of a culvert presents its own unique challenges. In this article, I will outline the basic culvert installation and construction techniques. I am referring primarily to larger, highway culverts but the same principles can be applied to smaller drainage culverts.
Let’s establish the basic structural concept first. Corrugated Steel Pipe (CSP) has very little strength on its own. The loads from above are simply transferred to the backfill, thus if the backfill is not solid, the pipe will fail. Concrete pipe is the opposite. The American Concrete Pipe Association has a neat little video showing an army tank driving directly over the top of some concrete pipes. Concrete pipe is indestructible and the biggest issue is the settlement of the fill around the totally rigid pipe (i.e. dips in the road on either side).
Firstly, you need to excavate the site. Excavation should be down to the elevations specified by the engineer, or for small drainage culverts below about 6 feet (1.8 m) in diameter, about 2 feet (0.6 m) below the inverts. There is likely to be water seeping into the excavation, which can be controlled with one of the following methods:
- If pump(s) can control it, pump it out.
- If it’s too much for pump to handle, the next thing to try is a bit of water control within the excavation. Dig some trenches on either side of the excavation, fill it with washed rock and try to pump out from the trench.
- You can also try large sump holes in the corners
I have seen some pretty extreme water control issues but rest assured there is always a way to handle it. By the time you build sump holes and trenches, you’re at the point where you can control 90% of what nature is likely to ever throw at you. The next step is the culvert bed. It should be compacted to 95% of proctor density. If a densometer is not feasible for your project, follow these instructions and you should have the necessary compaction:
- Use only gravel that is not wet or saturated
- Place gravel in lifts not exceeding 150 mm compacted.
- Allow for 6-7 passes with small hand-operated compaction equipment like jumping jacks, vibratory plate compactors, etc.
- Do not neglect the hard to reach “haunch” areas directly beside the pipe at the lower levels. As a minimum, fill the haunches with loose gravel and fill in the corrugations using shovels.
- At the ends of the culvert, use clay for the backfill with the same compactive effort as the gravel. This will stop seepage around the culvert.
After seeing many culvert installations, I can take a walk on the gravel backfill and tell you if it has enough compaction. This might seem vague, but the backfill must be “hard.” Backfill that is not compacted to 95% will give you a macro depression around your foot as you step down. Like the bending of a piece of plywood. Directly around your foot doesn’t really matter (it is just loose stuff) but if you don’t see the wider movement, you are probably good.
The gravel should go about 3 feet (1 meter) wider and higher than the culvert. Outside of that “backfill envelope” you can place whatever road embankment material you need.
Rock riprap should be placed on both culvert ends to prevent erosion, as detailed in our article Erosion Control for Culverts. Placing geotextile underneath the rock will help it stay in place.
Then you can finish building the rest of the road embankment above the culvert.
Coincident with the publication of this article, we have introduced the pdf document Construction Installation Procedures into the Resource Center. Don’t forget to look at it and browse through the entire Resource Center as we have almost everything you need to design a culvert at CulvertDesign.com.
Culverts are not a difficult construction project (from and engineering point of view), and we hope that this answers some of your questions. Cheers.