I don't know if that's the best strategy. The roots will pretty quickly grow beyond the 1.5' radius (probably within the first), and the feeder roots generally don't go more than a foot deep anyway. So, you'll end up with several hundred cubic yards of rocks to dispose of (roughly 3/4 yard per hole) with little gain.
The X cutting would be better, as it allows feeder roots space to grow (they can follow the X lines). And you don't gain anything by making the holes that deep. A foot to 18 inches would be plenty. Also, I think your best bet is to get soil depth by bringing in tree trimmer mulch.
When I asked Dr Jonathon Crane about this subject a few years ago, he recommended growing sunn hemp and tilling into the soil.
Thanks everyone for all the valuable suggestions. I am going to rent a back hoe and make 3' * 3' holes .
Federico , I will need your help with finding mulch and some guidance on making holes. I will send a message.
Thanks