Take a look at the following US Army Corps of Engineers publications:
Publication Number: EM 1110-2-2104
Title: Engineering and Design - Strength Design for Reinforced Concrete
Hydraulic Structures (Shear Criteria)
Publication Number: EM 1110-2-2902
Title: Engineering and Design - Conduits, Culverts, and Pipes
Web Address: http://www.hnd.usace.army.mil/
You did not give much information on the use of this culvert such height of
fill and where it will be used, etc. Have you considered that there maybe
another alternative such as a bridge? A 30 m X 30 m double barrel box
section is really extreme.
Bob Conley, P.E.