I'm currently in the process of digging a hole in my front yard to install a flag pole. The dimensions of the hole are 16" deep and 24" in diameter. Once I reached my 16" depth, I discovered my PVC sewer pipe that runs from my house to the street. There is no damage to the sewer pipe thankfully.
Can I pour the concrete over the PVC pipe without damaging it?
Move the hole.
You know its in a bad place. If you break the pipe with your flag pole you will curse your own stupidity and hate the flag pole and that is how bad things start. If you do get away with it then someday in the future someone will try to remove the flagpole and break the concrete, and they will drive the concrete into the PVC and then track you down in the nursing home and hide your teeth for putting a flagpole on top of a PVC pipe.
No, leave the PVC in peace. Move the hole a ways to the side, clear of the pipe and try again. Be happy you did not dig into the pipe like I did recently planting a new little tree. What a mess.
Never, ever, mess with any pipe underground.
No matter if it is any kind of plastic, steel, brass, copper, ceramic, concrete, whatever.
No matter if it is used for sewer, water, gas, electricity, Internet connection or underground smuggling of rocket parts (it happens!) or vodka (it happens, as well).
Pipes sometimes break after a while even when left alone, just because the ground settles unevenly. A bulk of concrete is known to settle in a different fashion (it sinks as a whole) and to break pipes even not directly in contact with it. 20-40cm separation is probably OK-ish, but aim for a meter (3ft) if possible.
When a pipe breaks, you get a great deal of both annoyance and expenses.
TL;DR
You shouldn't do that.
If for no other reason then consider this simple fact.
If that pole gets knocked over via wind or a rogue vehicle then the concrete will tilt and puncture a hole in the PVC. Now you have two problems.
I would move the flagpole to a different location so you do not disturb that PVC pipe. Also, I would contact the sewer company and ask how deep their piping is since most sewer piping is much deeper than 16". That PVC pipe may be for something else.
