Called different things – depending on where you live?
We built a rubble foundation using locally available one and one half inch septic rock. (It is lime rock). Footings were dug about 21″ wide and 12 inches deep. Some rock was placed on the bottom of the trench before laying the porous filter covered 4″pipe.
With the clay content of our soil being quite high, the other two domes will have 3″ drain pipes.
Good compaction of stone. 6mm plastic sheeting on the outside of the foundation.
The pipe drains to daylight.
We called a local supplier for crushed concrete (septic tank suppliers are a source). They came out more expensive than “septic rock” and a disadvantage was that their “rubble” is not graded. Fines are mixed with it.