When we had our 1200mm footings filled (9m x 5m approx) the mixer used had a conveyor. Very handy for the long part and to get around the back of the house.
The driver/operator was pretty clever and made a wedge half way down using a dryer mix (effectively splitting the footings in two) and...
I had the same problem with the footings for my house extension. I needed to dig down 1000m below ground level. Local BCO wanted clean sides and base to the trench. Ended up buying a pump from Screwfix, draining it out and then digging out the sludge.
Ended up 1200mm deep.
At this point the...