Base: Bull RUshing into a wall deals damage equal to your unarmed attack's damage
Improved Bull Rush: Deals an extra 1d6 for every 5 feet you're pushed them. That gives it a base of (normally) 2d6 + 4 (average fighter). Weaker than a normal attack, but with the added benefit of moving them, which is powerful enough.
Greater Bull Rush: A successful bull rush into a wall stuns an opponent for 1 round (fort save dc = damage dealt)