Unfavorable soil for walnuts

We began preparing a plot of land for planting 450 walnut seedlings. When drilling holes, we encountered the following problem: in the area, the thickness of the chernozem fluctuates 25-50 cm, and below there is a very solid and dense layer of brown clay. What should I do? What can be done for the normal development of trees and is it possible to grow them in such soil? Thank you!