You could use old tires for a hillside but you would need to use a "U" shaped re-bar at each of the 4 corners touching another tire. For not too much money, you could use, what I think, is the finest erosion control product ever invented. It's call Presto-Geo-Web. Installed correctly, it will never erode and you can plant whatever will grow in your area on top. Check them out at http://www.alcoa.com/alcoa-geo/en/home.asp . It is a cellular confinement system.