We had have piers made from 2×6 wood in a box format, anchored in concrete and filled with sand. They rotted in about 10 years and were never really stable! So this time we were going for fullon concrete. Since our soils is about 3-5″ of peat then into solid fractured rock, we opted to get a bigfoot concrete form and then a sonotube to extend it upward to the height we wanted.
Height was a very difficult thing to calculate as there were many variables with nknown error bars: an uncertain depth of hole, our sonotube was 8.5″ and would fit inside the bigfoot after it was filled and it was uncertain if the sonotube could go all the way down. The deck would be raise quite a ways up off the rock but it also had crushedgravel, a patio stone and then two stacked deck blocks. So we tried to design with a lot of give/slack/room for errors.

So the BF28 bigfoot went into the ground. It was weighed down with deck blocks to make sure it did not float away.
7 bags of concrete (just add water) 30kg each were mixed one at a time in this tray.
It was tiring work and about 1 hour of time. We had prepared three threaded rods with a wooden adapter and nuts and bolts to and placed them inside the concrete and placed a 2×4 across the top to hold it in place.

This was the end result of the pier (ignore the deck.. that is decsribed in the next part).