Concrete in WPC fence post

I see it similarly! What might work, but is not really a clean solution:

1. On the garden side, dig down accordingly at the spots and then "hollow out" under the stones. The problem here will likely be that especially the gravel keeps slipping again and again.
2. If you manage to do it somewhat cleanly, you could pour the foundation with a small formwork and insert the iron and posts from above.

But it's all improvised, and I suspect that you can't neatly "undermine" the wall with a clean boundary.

I don't know what the 25 dimension is. But if that is the width, which to me looks like it from the pictures, then it has a clear width of about 17 (-8cm for the 2x wall thickness). That is the diameter of a typical earth auger. If he really left a hole like that every 1.8m, he can easily pour 80cm deep footing foundations there. Reinforcement inside, also reinforce the stone rows horizontally. In my opinion, it should hold, but I am not a structural engineer.

Oh, a small addition. I wouldn't use WPC for the posts, if that was the idea. They should be made of steel, or at least aluminum. If WPC absolutely has to be used (because of the look or something), then don't embed them in concrete but use post shoes and then anchor them onto the stones.

Thank you very much for the many ideas and suggestions. Exactly for this reason I asked here because I don’t want to put together something I will regret later.

For the WPC fence, aluminum posts are used, which I can even order up to 300 cm in length, so there should be enough material for concreting in place. The holes in the masonry bricks have a diameter of about 15.5 cm. Here I wanted to get a 130 mm drill bit and try to drill at least 50-80 cm below the ground surface. This would mean that up to 1.2 m of the post would be embedded in concrete. Shouldn’t this actually be sufficient?

We also had the idea to get a WPC fence that has 2-5 mm gaps between the individual elements. Would this change the wind load at all or would it hardly make any difference?

The idea with the drill is not a bad one in itself and could work, but 13cm is really not much diameter and accordingly also little pressure in the soil.

Whether such narrow gaps really provide serious relief, I don't know. That probably causes turbulence on the other side, and that is pressure again.