inspection shaft

Hello. We need to make an inspection shaft. 1m diameter. There is a store near us that has concrete elements 1m diameter. I just have to make a base slab first. 20cm thick. dumb question, but I am new to this field. What do I need to make a base slab: gravel, concrete .... how much of what. So base slab 120cm diameter and 20cm deep. And afterwards I want to make about 20cm high a ring on the base slab, so no ready-made element. Cast it myself. Formwork. Can I use the same mix as the base slab for this or do I need something like screed or so? Thanks for the answers.

Hello donkey,
first inform yourself about the function of a [Kontrollschacht].
Why do you need a [Bodenplatte] for it?
Open or closed [Gerinne]?
How do you want to form a round shuttering?
How do you make the passage of the [KG Rohr] through the shuttering?

I need a base slab for this because the concrete rings will be placed on it. They weigh quite a bit. I wanted to form a square on the base slab at the height of the pipe. I also wanted to ask whether I should do something around the pipe at that point or not at all, and if so, what? Please also tell me the mix for the base slab—and if I can use the same one for the formwork. Thanks