Will gravel sink to the hardpan over time if put in sand with no other amendments?
Sand will eventually work it’s way into gravel.
If by hardpan you mean a restricted water surface due to a layer of clay, it probably won’t make much of a difference.
What are you actually trying to accomplish? Answering that may help guide us to a more helpful answer!
It depends on the specific gravity of each on which sinks to the bottom.
I’m building raised planters for avocados and they told me down here to do all sand in the lower two feet but I would like to put something in it to improve the drainage since there’s a little clay or silt in my sand. The nurseries telling me the idea is not to put any organic in the lower two feet that could restrict drainage or cause a breakdown or mold which may cause root rot. The upper foot or so is where I’ll add some organic for the feeder roots. I don’t want to waste my River Rock if it’s just going to sink through the two feet of sand to my clay layer.
A bit of gravel in sand won’t improve drainage. A lot of gravel won’t be any better IMO. Gravel is just space taken up in your bed that serves no purpose. It doesn’t hold water or nutrients and doesn’t improve drainage compared to pure sand.
I can’t see or feel what your topsoil is like. But mixing topsoil and sand would be the route I’d take.