Foundation for granite steles 3m

Hello,
for the design of the terrace, a border stone made of granite slabs 30cm high will be set as an edging and granite pillars will be placed as a privacy screen. Between them, there will be privacy panels made of wood.
Now my actual question.
The columns are 3m long, I want them to protrude 2.20m above the terrace surface. So I want to set them 80cm into a foundation. How deep should the foundation be in total? Can I make the foundation in two parts so that I have the exact height for the base, then place the columns on it and pour concrete all around? Because the columns are extremely heavy and not easy to lift to adjust their height.

Hello Framkos

Of course you can pour the foundation in 2 parts. Consider the substructure as a "clean layer".
80cm deep is definitely enough. That is below the frost line. Make the clean layer 5-10 cm thick, let it cure for a few days, place the posts on it, align them and fill with rammed concrete. That will hold.

Steven

Maybe you can also incorporate a bit of "reinforcement" into the cleanliness layer, protruding upwards. Preferably a few stainless steel wires, air layer anchors, or the like.

Thank you for your answers. The reinforcement sticking out upwards will be difficult, let's see how I handle it. Can I put gravel under the [Sauberkeitsschicht] as drainage or do I not need that? And how to mix the concrete, more like rammed concrete or poured concrete.