Just to expand a bit about the PGE requirements: "The Power Company will not install electrical conductors in a common trench with non-electric utilities such as water, gas, and sewer unless unusual conditions such as adverse soil or route restrictions exist. All such installations require the prior approval of the Power Company. -- pg.28 Oregon Electric Service Requirements (applicable to work by PacifiCorp and PGE)
Joint use trenches for electric lines, like transformer secondary service lateral conductors, communication, and signal conductors, telephone, Cable TV are ok if there is 12" separation horizontally (edge of conduit to edge of conduit...not center to center)
Anyway, although this stuff isn't directly applicable to your query Mike, it might serve to indicate that gas and electric might not be combined in a "joint trench", as a general rule.
[This message has been edited by Elzappr (edited 02-23-2003).]