Jim Timber
Veteran Member
Mine are 24". I don't dig out double, but I do make sure there's enough room to "flow" material under the sides of the pipe as it's being placed.
The first scoops are poured directly over the top of the tube so it has some weight on it to hold it down. Then I manipulated the stuff that's spilled off to envelope the bottom of the trench on either side. Once that's been done, I continue to fill up over the top of the tube and once it's sufficiently covered, I start packing it in with very little top dressing over the culvert itself. Once that's good and firm, I pile more on and start doing 3" lifts until I get my top layer established.
I only have about 6" of fill over the second culvert right now because of how much we displaced when re-setting the repair. You want a minimum of 12" compacted base over the pipe, but 18" would be even better.
Your fill also needs to be sufficiently coarse to prevent erosion from hollowing it out. If there's a way for water to go around the pipe, it'll do it when flow rate increases. The OP mentioned not being able to stop the water to place the pipe, but it could be a situation where you'd be better off with geotextile and laying another 6" of coarse base down over that. That'd prevent the culvert from carrying the road above it and would reduce the chances of mashing in the replacement.
The first scoops are poured directly over the top of the tube so it has some weight on it to hold it down. Then I manipulated the stuff that's spilled off to envelope the bottom of the trench on either side. Once that's been done, I continue to fill up over the top of the tube and once it's sufficiently covered, I start packing it in with very little top dressing over the culvert itself. Once that's good and firm, I pile more on and start doing 3" lifts until I get my top layer established.
I only have about 6" of fill over the second culvert right now because of how much we displaced when re-setting the repair. You want a minimum of 12" compacted base over the pipe, but 18" would be even better.
Your fill also needs to be sufficiently coarse to prevent erosion from hollowing it out. If there's a way for water to go around the pipe, it'll do it when flow rate increases. The OP mentioned not being able to stop the water to place the pipe, but it could be a situation where you'd be better off with geotextile and laying another 6" of coarse base down over that. That'd prevent the culvert from carrying the road above it and would reduce the chances of mashing in the replacement.
