Hi,
I have been designing retaining walls and abutments following chapter AASHTO LRFD 10.6.3.4.
I am using equation Rr= Phi*Rn= Phi1*R1 + Phiep* Rep
Rn : Nominal Load
R1: Nominal sliding resistance = V * tan(angle of friction) ----- V is total vertical Load ( Dead Load, Live Load, etc)...