Tree soil depth help

Hi Guys,

I'm new to the forum and require some help please, I have 20 Hornbeams(carpinus betulus) being delivered tomorrow which are around 7/8ft in heights and I plan on pleaching them but I may have a problem with the soil dept of my garden, long story short is my house was built on an engineered hill and as a result our gardens have not much soil depth(which I only found out today) as I went to dig the holes for my new trees I hit the stone only after 6 inches! This is core stone fill!now I was planning to raise this area anyway with more topsoil of about 6 inches which will bring it up tl 12 inches in total depth, so my question is that enough? The trees roots will have plenty of room to spread horizontal tho but only 12inches of depth.

 

Duplicate post https://www.gardenerscorner.co.uk/bookmarks/?type=post&id=1251434