I am working on extending my patio. A large portion of our patio does not have a cement slab but white yard rocks. Underneath that is pea gravel then a layer of sand. I want to replace it with bricks. While digging out the rocks the bottom of the pit consists of all three (big rocks, sand and pea gravel). What is a good material to lay ontop of this mixture so I can level the ground and set the bricks in. This material will need to be able to keep the bricks in place.