We are going to drive down some rods into the hole base before pouring. We are using pre-mix concrete and a mixer, the observatory site is in my backyard - probably a 50m windy walk, up at least 7 metres and 6 steps. I don't relish carting sand and gravel up that - prefer dry mix, convenient too.
The tech specs from the cement co (Blue Circle) state for a "more structural concrete" - add 2kg of cement to each 20kg bag of concrete when mixing, this bumps up compressive strength from 20 to 32MPa. Sounds like what we want.....
At least the rain around here in the past 6 weeks or so has left the ground easy to dig - smooth sailing down to about 450-500mm, then moist sticky clay....
Stay tuned for a new obs building thread....
