Connection shaft for district heating as precast concrete part or cast in place?

Hello everyone,

sometimes I feel that when I call about various things related to construction, I am the only one in the country building a house, ignorance as far as the eye can see...

I suspect I have a special problem but still hope you can give me some advice.

For the connection of the district heating, I need a 1x1x1.2m deep connection shaft in the base slab either

a) to be installed as a prefabricated part

or

b) to be poured during the base slab work or afterwards (probably the more expensive option)

For option a) I imagine that this shaft is installed by the earthworks contractor during the earthworks and gravel is compacted around it for the base slab, then the base slab is poured and that's it. Although I probably have to insulate the shaft separately to avoid thermal bridging.

For the second option, I am a bit confused about how this should work. Do you first make the base slab and leave this opening for the shaft? What happens to the compacted gravel next to it, doesn’t it get loosened?...

Maybe you have had to do something similar?
Which option should I choose with regard to tightness and reliability?
Where can you even get such shafts from?


Thanks in advance for the tips!


Regards
Robert

At our site, the shaft was realized as a recess in the floor slab. The gravel can still be compacted - the shaft walls are made of shuttering blocks.

Ok, so option c) so to speak. The shaft was not cast, but built with masonry. Were the formwork blocks also insulated on the outside and sealed with bitumen sheets?

Most of the houses here are built with frost skirts made of shuttering blocks. The shaft is then built together with the frost skirt and practically costs nothing.

When the pipes were in place, the shaft was cast up to the height of the floor slab. On top of that came the bitumen membrane, insulation, screed, tiles. At that time, the screed was otherwise already finished. It is important that when welding the floor slab, a rim protrudes all around, where you can then work again when closing the shaft.