More on turret bunkers types and configurations
Basic Type: tank stripped of the useless stuff (engine for example) and put in a concrete lined emplacement.
See this as example:
http://img284.imageshack.us/img284/3401/230200033ln.jpg
Note the concrete shelter for the crew on the right
Slight variant on the above: buried hull (I think at least)
http://img221.imageshack.us/img221/1...nanpiph5kx.jpg
Note the open hatch in the left, I bet that the engine space was converted in storage space for additional ammo and what not.
A more radical conversion: the hull is discarded and the turret is put in a purpose built concrete bunker. See here one built using an early T-54 turret.
http://www.fortification.ru/uploads/Polt_UR_3_a.jpg
Also note the bunker entrance.
Another one
http://www.shatoon.ru/gallery_krabbe...h/DSC01387.JPG
Note the concrete base is shaped in such a way to protect the turret ring.
In regard to concentration typically you will not see walls of them but that does not mean that they are absolutely never grouped together
http://img134.imageshack.us/img134/518/02dv4.jpg
All these pictures were found on tanknet