I just read through a very very long thread on gardenweb that had an incredible number of creative ways to kill gophers. "Caddyshack" is making more sense to me now! Nobody on that thread mentioned anything about changing the soil in any way, other than the hardware cloth and cages. The traps that had the most success were called Black Box and Cinch Traps. They are available at Home Depot. The cages are sure to prevent problems, but wow what a chore. Sorry you have to go through that! Has your extended drought caused the gophers to seek gardens with juicier roots?
I do think the expanded shale will help with moles, voles and mice, though. They are a different critter from your gophers.
I thought rats were supposed to be the most intelligent rodent, but it sounds like gophers are much more difficult to trap.
