What is the minimum depth of duct banks in plant area?

"We are trying to determine the minimum depth we can run a concrete encased600 volt duct bank under a plant roadway. Article300.5 and Table300.5 address direct buried conduits and conduits with a concrete slab over top, but does not appear to address concrete encased ductbanks. One of my staff members understands from Table300.5 the ductbank needs to be24inchs deep under the driveway (although there is no column in the table for concrete encased conduit), whereas I had always understood concrete encased ductbanks could even be flush with the ground or driveway."