Here's a much easier one to ponder:
Situation: sloping driveway straight up the "fall line". Want to place a diagonally oriented culvert beneath the driveway from one side to the other.
So far most of what I've read is from the 80's and 90's, so I don't imagine there's much new ground to dig here, but most of it involves the combination of plans (which I wasn't planning on here), and associated staking. Most of it highway related stuff.
That said, do I:
1. Measure everything to the design bottom of the culvert at each end? Top of Culvert? Center of Culvert?
2. Put the stakes beyond the ditch intersection at each end a set distance away, so that they don't get wiped out once the digging begins?
3. If No elevation changes are deemed for either ditch, is it necessary to call out anything at all, other than the beginning and end of the culvert?
1. Yes, generally. You need the bottom of the culvert (aka the "invert") to have a slope sufficient to carry the water and to "scour", and match up with the ditch flowlines at each end. You also need enough cover over the top of the pipe to prevent it from being crushed when you drive over it. BTW, the centerline of the pipe is called the "spring line".
2. Depend on just how long the pipe is. What you propose would work for a single length of pipe. For greater lengths you would set offsets perpendicular to the length of the pipe at regular intervals.
3. Probably not, for a culvert. Unless you are doing some sort of headwall arrangement. The slope of the pipe is something contractors will ask for.
A culvert would pretty much lay into the existing ditch and along the same profile.
The topsoil should be excavated so the culvert would rest on a surface that would not erode quickly, like grass and turf.
A nice safety extension on each end would enhance the structure and make easier slopes for mowing.
Shoot the existing grade and put a grade stake with the appropriate cut on a right angle offset at the edge of pavement or on the backslope.
A good operator can do it without cut stakes......the safety extensions can be tricky. The guy that installed mine put them in backwards from the existing slope, so now my culvert holds water until it evaporates.
Without more specific info, I'd probably use offsets from the centerline of culvert on each end with grades. Put them where they won't be in the way of construction (either parallel with the culvert or perpendicular). It'd probably be easier using perpendicular offsets and make them fall at or above top of bank. If it is a massive box culvert or something you can offset the corners of it in the same manor.
Norman Oklahoma, post: 334341, member: 9981 wrote: 1. Yes, generally. You need the bottom of the culvert (aka the "invert") to have a slope sufficient to carry the water and to "scour", and match up with the ditch flowlines at each end. You also need enough cover over the top of the pipe to prevent it from being crushed when you drive over it. BTW, the centerline of the pipe is called the "spring line".
2. Depend on just how long the pipe is. What you propose would work for a single length of pipe. For greater lengths you would set offsets perpendicular to the length of the pipe at regular intervals.
3. Probably not, for a culvert. Unless you are doing some sort of headwall arrangement. The slope of the pipe is something contractors will ask for.
If the "flowline" is the pitch of the ditch, it's higher on the intake side (almost 15%), than on the outtake end (8%). The pipe will be almost 70' long depending on the angle (which I understand from reading, should not be less than 30 degrees from perpendicular to the drive). At 30 degrees across the drive, the pitch of the pipe would only be 2.5% I'd probably have to get it to 45% to get it to 10%. Still have some topo work to do/learn before I know for sure.
Depth shouldn't be a problem, as the ditches are almost 3' deep, (lined with 7" minus). I can't find a definition for "safety extensions", unless it just means make them longer so that you can widen the shoulder in the region of the culvert. This won't be possible because the ditches are pretty narrow and they're already there.
I was actually referring to the elevations of the ditch at each end of the pipe rather than the slope of the ditches.
But you have suggested an issue. You don't want the slope of the pipe to be so great, and so different from the slope of the outlet end ditch, that water will cascade out of it (during storm events) with such force that the ditch would get eroded. It doesn't sound to me like you have much to worry about, except that you wouldn't want the pipe aimed at the side of the ditch at the outlet end.
A few counties in Oklahoma use to have a "standard" of 1/8" fall per foot of culvert pipe, and went on to call it 1% slope...it's close. Of course there were all sorts of instances where that really didn't fit.
On State Highway work the side drains were usually engineered so the culvert pipes always fit the eventual finished profile. There were a few jobs where we replaced culverts, but did no ditch work. All I can say is an engineer spends way to damn much time trying to apply design hydraulics on things that really don't amount to a hill of beans. All culverts will silt if the slope of ditch isn't proper, it is a function of the entire ditch slope and a little 20' culvert pipe is almost meaningless. It is nice to have a culvert that drains, but sometimes they don't....because the ditch they're in doesn't drain.
As one wise old county motor-patrol operator once noted, "Size is important, slope; not so much...." :bored:
Norman Oklahoma, post: 334380, member: 9981 wrote: I was actually referring to the elevations of the ditch at each end of the pipe rather than the slope of the ditches.
But you have suggested an issue. You don't want the slope of the pipe to be so great, and so different from the slope of the outlet end ditch, that water will cascade out of it (during storm events) with such force that the ditch would get eroded. It doesn't sound to me like you have much to worry about, except that you wouldn't want the pipe aimed at the side of the ditch at the outlet end.
At 10% slope, I'll be right between the slopes of the intake and outtake. There's another culvert on a nearby drive that's set at about 2%, and it's half filled with silt. I've never seen torrents of water coming down the hill, so I'll go with Paden's advice on size vs. slope.
As for abrading the side of the ditch at the outage, I'm sure they make 45's for poly pipe, but they're probably expensive. This isn't a "gubnint" project.:-D