My first shot at these holes are that you are correct in the positioning of the terminal columns. The loading on these columns would be quite high necessitating in a piled foundations. Piling people do not like obstructions I.e. Concrete Tarmac in the way of their boring machines. So you predig the pile location with a conventional excavator to remove any obstructions, if any. Once you reach virgin ground the digging stops. The hole is backfilledt so as the piling rig can stand adjacent to the pile position and down goes the auger. The standing costs of a piling rig used to be in the order of £120.00 / hour whilst any obstruction was cleared. Better to clear the pile position in advance.