Illinois does not allow for any type of backflow preventer to be placed underground or in a pit. Even if the pit has a sump pit/pump, floor drain, etc.
Section 890.1130 Protection of Potable Water
g) Installation of Devices or Assemblies.
5) Double Check Valve, and Reduced Pressure Principle Backflow Preventer Assemblies. No in-line double check valve backflow preventer assembly (DCV) or reduced pressure principle backflow preventer assembly (RPZ) shall be located more than 5 feet above a floor, or be installed where it is subject to freezing or flooding conditions.