I remember, back in the late '60s I was working as part of a cable TV construction crew in Foster City, California. That was when Foster City was just being built. All utilities were underground and our cable lines were required to be between 12" and 18" down, while gas lines were required to be between 24" and 30" deep. We set our trencher for a depth of 15" to be pretty much within our depth requirements and I lost count of how many times we had to halt our trenching because we hit a gas line! It always meant we got an hour or more break waiting for the gas guys to come out and fix the break, and then rebury their line at the correct depth!