A ripper bar mounted on a heavy pickup truck will get a trench deep
enough,I think.Put it in four wheel drive and mash that go pedal.A piece
of two inch diameter steel bar with a steel plate welded to it and bore
a hole in the plate (a cutting torch can cut out a hole) and mount it
where the hitch ball mounts,jack up the rear wheels,let the bar dig into
the ground,let the rear wheels back down on the ground,good to go.
cuhulin