Corkscrew Hazel - Suckers

I have a corkscrew hazel that has been in the ground for about 10 years. It sits in the middle of a small bed surrounded by slate chippings over weed membrane. I am constantly having to remove straight suckers from the rootstock (I guess the corkscrew is grafted onto standard rootstock) and now some have grown laterally under the slate and are coming up some 3-4 foot from the original.

Have I planted this tree too shallow/deep or is this just a natural problem with this plant? I don't want to have to disturb the tree now that it is well established but ...

 

I have one in a pot and it does this too. I just cut them off, but I appreciate when they're spreading that far with you, it's more of a problem. Hope someone can be of more assistance.