I just have to echo the issue of safety when you are digging your own trench. This is an issue that I know about all too well.
About 10 years ago my brother was digging a small tench in his back yard to try and improve drainage. It really didn't even go own that far. But he didn't call the utilities to check if their were any underground cables and he ended up putting his spade through one of them. He was electrocuted and died immediately. It was such a senseless accident that it took our family a very long time to come to terms with.
So learn from his tragic lesson. Call before you dig. The commercials are right no matter how silly they sound. This is a serious issue and you could be risking your life.