ᐅ Inspection chamber

Hello. We need to build an inspection chamber with a diameter of 1 meter (3.3 feet). There is a store nearby that sells concrete elements with a diameter of 1 meter (3.3 feet).

I need to pour a base slab first, 20 cm (8 inches) thick. A basic question, but I’m new to this field. What do I need to make a base slab: gravel, concrete ... how much of each?
So, the base slab is 120 cm (47 inches) in diameter and 20 cm (8 inches) deep.

After that, I want to cast a ring about 20 cm (8 inches) high on top of the base slab, not a precast element but poured on site with formwork. Can I use the same concrete mix as for the base slab, or do I need something like screed or a different material?

Thanks for your answers.

Hello esel,

First, inform yourself about the function of an inspection chamber.
Why do you need a concrete slab for it?
Open or closed channel?
How do you plan to form a round shape?
How will you pass the sewer pipe through the formwork?

I need a concrete slab for this because the concrete rings will be placed on it. They are quite heavy. I also plan to form a square casing on top of the slab at the height of the pipe. I wanted to ask if I should do anything around the pipe at that point or not, and if yes, what exactly?

Please also provide the mix ratio for the concrete slab and whether I can use the same mix for the casing.
Thank you.