Fitting radiators in new concrete floor

As part of our house renovation we had new concrete floors poured. As part of the renovation we are also having a new gas central heating system installed.

The plumber had all the heating pipes in place before the new floor went in but left the pipes under the floor with the connections to be made at a later date.

Holes have been left in the ground below where the radiators will be and he intends making the connection below ground when the radiators are being fitted.

I would have expected that the pipes should have been in place and protruding from the ground before the floor was poured. This would have ensured that the connections were above ground and also that the floor construction would be more sound.

As it is the earth within the holes that have been left is damp and I am not sure if the damp proof course and insulation will be as uniform or robust as they should be. I am also worried about problems in the future with water leaks at the pipe joints within the concrete.

I would appreciate information from people who are more experienced of how such pipes should be fitted on a new build / renovation in conjunction with a new concrete floor.