Maybe if I had called it a spit as in bbq. the column will eventually have more vertical steel added to it to beef it up and to hold a carriage with a spindle on this a rotating assembly will be added which can be used as a welding positioner or the head piece to attach one end of a semi trailer or truck frame to in order to turn it on its side or upside down. As you can imagine this will require some substantial amount of steel as the rotator will have to elevate up and down to a certain degree.
What I am calling bluish white clay is almost a shale like substance. hard enough that if you want to dig it by hand a pick is helpful. If I was only concerned with moisture inclusion, frost heave or settling due to weight loading I could have stopped digging at a foot possibly 2 feet deep. I have all of the columns in the building embedded between 6 and 8 feet to make them have roots more like a giant redwood than those of a palm tree