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Old 08-26-2010, 10:56 AM   #1
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Cover 4" PVC french drain for vehicle loads


I'm going to put in a typical perforated 4" PVC pipe for a type of french drain on the surface next to my driveway, to channel surface runoff from the back yard to the front. The surface runoff drains quite well now, along the surface of typical southern red clay, but I'm also going to cover the area through which it drains with crushed stone so I can park our RV on that spot.

I've done a little measuring of elevations, and I'm satisfied I have about 21" of slope in about 71' of straight run of PVC.

I am not concerned at all about subsurface water. I'm only trying to keep the existing natural surface drainage path working the way it does now, after I cover the entire area with crusher run and then another layer of smaller crushed stone, to make the RV parking pad. I do not need to bury the PVC to any extent at all, in order for it to collect and drain the surface runoff.

But I don't know how much stone I need to put over the PVC so that I can safely back the RV over it when I park it or pull it out to leave home. Can anyone tell me about what the design load of that PVC is, or about how much crusher run I would need above the top of the PVC to keep it from breaking?

This is not the same as driving over it every day. I'm only going to pull the RV over it a half dozen or so times a year, but I want to do it right anyway!

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Old 08-26-2010, 11:33 AM   #2
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Cover 4" PVC french drain for vehicle loads


For schedule 40 PVC perforated you need at least 12, preferably 24 inches of crusher run over the pipe to resist damage from an RV. Less cover if you use schedule 80 (but that is hard to find). More cover if you use schedule 20 or schedule 10 PVC, neither of which I recommend. Make sure you compact the crusher run adequately in 6 inch lifts as you place it. To avoid puncture damage to the pipe, if the crusher run is sharp, cover the pipe with several inches of coarse sand before you place the crusher run.

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Old 08-26-2010, 04:16 PM   #3
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Cover 4" PVC french drain for vehicle loads


...for rev. 2...

Mr. Holzman, thanks for the prompt and helpful answer. I'm a piping guy, so I understand your answer. I've always believed you win by knowing, among other things, what not to do, and now I know I must NOT install a 4" sch 40 PVC pipe under the stone I intend to put down to make my RV parking pad!

I cannot possibly cope with putting the PVC anywhere close to 12-24" deep in the run I need to drain! Whatever I do for drainage--up to and including nothing at all--must be right on the surface, no matter how much stone cover I put over it!

So I'm working on an alternative using a layer of fairly large "river rock" about the size of a softball, on a layer of silt fence below and above that rock, and then covering all that with crusher run and then #57 stone. My sense is that that will still drain surface runoff sufficiently that I'll be ok and of course I can back the RV over the stone with no risk that I'll break anything!

Thx again, and if you have any more feedback, needless to say feel free to share it! After all, all I am is a nuclear engineer turned piping engineer turned project mgr, so what do I know about dirt things!? NOI...
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