Like to thank those with advice on my query last week re. burying a drainage pipe deep enough to be safe (i.e., not bent or crushed when my tractor/Fel/boxblade rolls over it). Last weekend I placed a 6 inch PVC (thick, schedule 40) 9" under, from the top of the pipe to the surface. I packed dirt around the sides of the pipe, dumped fill on top, and then packed it down again with my FEL. No problems thereafter when my tractor, and then loaded pickup, drove over the area. Note that a middle buster (potato plow) digs a trench large enough for a 6" pipe one heck of a lot faster, and straighter, than does gouging it out piecemeal with a FEL.