Adding More Detail / Custom edits to Corridor Models

Good Afternoon all,

I have a corridor model here:

I am trying to add drainage catchpits, shown in orange (there are more from a topo survey in the actual model), that go say 500mm deep into the grey concrete. I also want to add channels, that are 200mm wide, from the catchpits towards the edge of the concrete (towards the centre of the corridor) so that the bottom of these channels will then be flush with the dark grey/black surface.

Is there a way I can do this without having voids in the mesh (this is for visual/rendering purposes). Example of void shown below.

As you can see the mesh then loses its "material" and other properties. What would the best way about tackling this problem be?

Thankyou,

Jake