I need to bury a 4 inch PVC drain pipe under my gravel driveway. It will span about 10-12 feet. However, in the future I'd like to pave the driveway with asphalt. How deep does the PVC pipe need to be to avoid crushing it from an asphalt installation?

I probably can't go farther than 12 inches in depth which might not be enough. If that's the case, is there a proper way to reinforce the pipe?

 

The top of your 4” PVC pipe will only be down about 8”. Much depends on the weight of vehicles using your driveway. I think you’ll be okay for typical cars and pick-up trucks . . . but try to dig down at least 12” if possible. I might be concerned if a fully loaded cement truck or something large and heavy were rolling over it. However, I have two HDPE drainage pipes on far end of my property, and one is down a bit more than your depth, and cement trucks and tractor trailers having to get to rear of my property rolled over them w/o any movement or problem . . . no longer concerned. Without getting into an overblown and IMO unnecessary reinforcement project possibly encasing the pipe in concrete, the only other more practical option I can think of is to use 4” sch. 80 vs. sch. 40 PVC . . . however at larger diameters, it starts to get more pricy. I would steer away from various thin wall PVC.

One other issue possibly arises given your future plans. When doing the asphalt driveway, I assume you’ll need to create a more stable base than gravel. If that turns out to be the case, I’m again assuming you don’t see any reason for lowering existing ground level so that depth of pipe becomes shallower at some future time.

When redoing the front portion of my concrete driveway, I installed a drainage pipe and also a spare pipe for unknown future use (below ground, and capped off). The extra pipe cost me a bit more but didn’t want to have to go through the aggravation down the road should I identify a need for going under the driveway . . . make sure you have no future outdoor lighting or other type project where an extra pipe might be needed. Once you asphalt the area, you probably won’t want to tear into it again.

 

I'm not an expert, but I don't think it's going to be a problem burying it in a 12" deep trench. I doubt the base for your asphalt driveway will be 8" deep anyway. I would install it as deep as you can and see what happens while you still have gravel. If everything goes well and it comes time to install your asphalt, ask your contractor, they should be able to tell you what you need to do if anything.