I'd at least lay the pipe in sand, then cover it over with sand. I was on the gangs on the water board for a while, it wasn't uncommon to dig out a leaking pipe, and find a stone had punctured it.
That blue alcethene (spelling?) will also split without any encouragement from a stone. Admittedly, I don't suppose it is that common, we used to see a lot of water pipes.
In your favour, the pipe is not under any pressure, so should be ok. Otherwise ,I'd say run it in a duct. You dont want to have to be hacking concrete or patios up to find a leak.