What a challenge! By creating that moat you may have halted any further outward growth. V-shaped moats are what my dad used around our trees in NY. I think the roots were halted at that line and essentially air root-pruned. I'm no fan of chemicals, but short of digging out the whole mess and sifting the soil to remove all the remaining root bits, you may not have a choice but to try more Roundup or call a landscaper.

If you would be interested in creating a planting bed in that spot, you could smother the stuff by repeated layers of wet newspaper, shredded leaves, grass clippings, and peat moss. This method is currently called lasagna gardening and is based on the old farming idea of sheet composting. That's how I've set up all my flower beds. No digging is involved. You just lay the materials on top and keep them moist so that they can break down. Maureen