A friend is building a driveway over a stream in rural Tennessee and send a photo of the progress. is there anything wrong in the photo?

As a layman it looks to me that the stone wall doesn't go all the way to the top. What do you think?

[enter image description here]

  • Tennessee is a rainy country, I would prefer the gravel pavement over the clayey soil, which may become soft after exposure to the rainwater for a long duration, also, the potential of soil loss due to wash-off.
    – r13
    Dec 5, 2021 at 16:50
  • I would also hope that the recycled concrete slabs are deep enough or anchored in some way that the hydraulic pressure from the dirt and eventual traffic will not blow out or heave the retaining wall. In addition, filter cloth placed against the wall and over the rock at the bottom to keep the fill dirt from silting out over time.
    – Jack
    Dec 5, 2021 at 16:57
  • You don't want the water to pool in the road or between the dirt and stones. You want the water to run downhill away from the road. Dec 5, 2021 at 17:02

2 Answers 2



If the culvert was long enough, there'd be no need for a wall at all - that's how they are typically installed on most public roads - long steel pipe, pile of dirt wide enough for a road, natural slope to either side. More pipe and more dirt is less expensive than building a retaining wall from the road department's point of view, and it also has fewer ways to fail. If the pipe is long enough and the pile of dirt wide enough, it also solves the next issue:

This one is obviously not completed yet, and most people with something that precipitous at the edge of their driveway will end up needing a wall or guardrail above driveway level to keep their home insurance company (if not themselves) happy, as driving off the edge makes for expensive insurance claims.

  • 1
    I don't think the question is asking about a "wall" in the sense of guardrails, but rather about the fact that there is no solid structure above the rather flimsy-looking galvanized pipe - what is going to keep this from caving in when vehicle traffic goes over it?
    – nobody
    Dec 5, 2021 at 15:11
  • 7
    Physics and dirt. That's how culverts work. They are effectively arches.
    – Ecnerwal
    Dec 5, 2021 at 15:13
  • 5
    I like "Physics and Dirt" for the title of your show, @Ecnerwal. It would lay out principles like this in the style of a cop buddy movie.
    – Willk
    Dec 5, 2021 at 16:27

I would have expected that the purpose of the wall is to reduce or prevent erosion of the roadway support. Considering the height of the culvert, one might expect a large volume and high rate of flow of water to be running through the waterway. If the flow rate and volume reaches the current top of the wall, an extra layer of bricks may not matter.

  • If the flow rate and volume reaches the current top of the wall, an extra layer of bricks may not matter. Exactly! My guess (there are probably design guidelines that specify) is something like "large enough such that it takes a once per 100 years (or similar) storm to cause a flow higher than the midpoint of the culvert". Dec 5, 2021 at 16:58
  • Mostly culverts are made that large to reduce the odds of damming/plugging when a branch/tree/dead animal floats down the stream. The actual flow probably never gets above 1/4-1/3 full.
    – Ecnerwal
    Dec 5, 2021 at 17:06
  • The one in the photo appears to be "dead cow" sized and one has to hope such a circumstance does not arise!
    – fred_dot_u
    Dec 5, 2021 at 18:02
  • I believe the culvert was sized for the maximum flow. Judging by the size of the culvert, it looks like providing the headwall and downstream wall is justified.
    – r13
    Dec 5, 2021 at 18:19
  • The culvert really needs to be designed for a 100,000 year storm (which will happen annually at the rate we're going). *If the culvert capacity is EVER exceeded, it will become an earthen dam, and all earthen dams fail catastrophically when over-topped. Dec 6, 2021 at 5:18

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