Hi,
The way I would handle this is by using a program like Slope/W or Slide to model the berm and estimate the passive pressure. My colleague has a nice example on his website.
Link
There is a rational method in Bowle's 5th Edition textbook (1995) that I use (Chapter 4-9), theoretical. It was referenced in the 6th Ed. of AASHTO. The 4th Ed. of the textbook has the computer code if you wanted to write your own spreadsheet. Mr. Bowles was very generous.
AASHTO 8th Ed...
