Clay would not be a good material for the base of a berm as it tends to compact and prevent drainage. Trees and shrubs do best with soil that drains well. If you mix the clay with straw, compost and wood chips if will over time (at least a year) loosen up as the clay texture softens from the organic matter. A berm of topsoil at least three feet deep should be enough for our screening hedges. Good luck and Keep growing, Marianne Binetti