Glenn -
Sounds like you're on your way to some experiences I can relate to. From what you've said, though, I feel compelled to share some tidbits I've learned from my own research (anybody with
actual experience, feel free to intervene).
You're certainly right about plastic pipe floating, and from what I've read, that's a
big problem especially with the larger pipe (greater than 12 inches).
Big loud caution here -- it's
notjust an installation problem! Once the pipe is buried and the creek level rises with the rains, the buoyancy factor is tremendous and it can burst right up through your roadway. /w3tcompact/icons/shocked.gif
Somewhere I saw mention of using proper "anchors" on HDPE pipe where floatation could occur, but I never saw a picture or description of what they were talking about. Might be worth a phone call or two.
Also, with plastic
orsteel the pipe strength is derived from proper backfill. Only reinforced concrete pipe can support vehicle-sized loads with only "casual" backfill. The rule of thumb is to have
at least one pipe diameter of fill on either side of the pipe (I'm cutting it a little close on that score myself). If your pipe is butted up against a "normal" soil or clay, it may eventually deform. Boulders and rocks should be kept away from the pipe, since they will cause localized pressures and cause detents into the pipe, degrading your water flow.
I didn't find any specific information on laying side-by-side pipes, but again, the strength of the pipe is a factor of how well it is supported, particularly below the centerline. That's why the backfill material must be evenly compressed in layers by tamping. I'm concerned that if your pipes are "squished" together (I love technical jargon), they will not have proper support in the most crucial area.
As for the fill itself, I think it was right here on this board in one of the culvert/colvert threads that I learned about the crushed limestone. It seems to have different names depending on who you're talking to, but the important thing is to make sure it contains
plenty of fines (sand) in the mix. That's what allows it to compact so firmly. I can tell you that as I tamped it down I could see and feel it compacting down to a very solid material. I placed layers of about two inches and tamped each one down to little more than one inch, at which point it started to feel like I was walking on concrete. Amazing stuff.
One of the early posts on this subject referred to a page with a diagram that I used as part of my guidline (see attachment).
The last factor for a successful culvert, from what I've heard local experts say around here, is the "overflow". That is to say that you
must assume that sooner or later the creek flow will be more than your culvert can handle. The secret then lies in providing a non-destructive path for that water to take. I haven't completely dealt with this part yet, but my plan is to provide something like a swale at just the right level, leading off from the upstream end of the culvert. Here's a
sketch of what I had in mind.
To tell the truth, folks, I'm sure my culverts will be okay 99% of the time, but I'm worried they aren't going to survive the inevitable torrential rains that come once or twice each year. Any words of wisdom for me and Glenn?