Preventing spread of blackberries

Some years ago I planted 2 small blackberry shoots along the rear fence of my yard. While I love the blackberries I get, I now have 50+ plants and they are migrating forward into my yard. Last year I dug and removed the plants that had come too far forward from the fence. My goal is to do something permanent to keep the grass out of the blackberry area and the blackberries out of the grass.
My idea is to dig a narrow trench (30 to 40 feet long) parallel to and a few feet away from the fence (basically an arms length away) and bury something like alumninum that is sold in rolls of various widths to prevent the roots from migrating across that border, then maybe later putting down rock inside the border.
My questions:
1) How deep does material need to go? I am guessing 6 to 8 inches or will the roots eventually go deeper and come under? I would like this solution to last 20 years or more.
2) If I put some rock down where the blackberries are will I kill them off accidentally? I want to keep the blackberries, just contain them and have an area that still looks reasonably attractive in the winter when the canes have been pruned back to the ground. I am guessing a thin layer of small landscape rock might be okay or should I go with a mulch?
3) If the alumnium is a bad idea, why, and what would you suggest instead?
Thanks in advance for any and all help or humor.
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You didn't say what kind of blackberries and where you are, so this only goes for western Washington. Please don't grow blackberries, the western half of the state is overgrown with them. Nothing you do will contain them beyond using plutonium, napalm, or morning glory. I've seen morning glory/blackberry wars before - it wasn't pretty.
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