Thanks, to answer a few questions, I'll be digging with a trencher (riding probably). Will be laying several hundred feet (maybe 500-700 before I'm all done) so I don't care to use a shovel. I have a 31hp tractor with box blade, but I don't think that will work well. I have a front end loader but that's a bunch of starting and stopping, the box blade would work better.
I like the idea of ...
"Buy an old, cheap back blade that will pivot around backwards. Cut it down the center and weld it back together so it forms a 90 degree "V". Then when you go backwards the "V" will fill the trench from both sides. No chance of cave-in."
That's probably what I need to do.