Figure out the active coefficient using whatever method you want and multiply it by 1.5.
Rational: For a friction angle of 32 degrees, Rankine Ka is 0.31 and Ko (using 1-sin(phi)) is 0.47. 0.47/0.31 is 1.52, which is close enough to 1.5. I used this in graduate school (JM Duncan) as rational for the sloping backfill problem and he had no problem with it. Your milage may vary. . . .
For Ka from sloping ground, you probably need Coulomb (sliding wedge) rather than Rankine (shear failure in whole mass of backfill). Rankine assumes level backfill and frictionless wall. (The two come out the same for that condition.)
I take it you need to know Ko to determine the pressure exerted against your wall. Why not look at it from the viewpoint of elastic theory since the movements will be so small elastic theory should hold. Use Poulos and Davis to determine the horizontal pressures exerted by a triangular loading at the 'base' of the loading at that particular plane. Then superimpose this on the pressures exerted by the level backfill.
pertaining to this quote - " rather than Rankine (shear failure in whole mass of backfill). Rankine assumes level backfill and frictionless wall."
I agree and did not mean to imply otherwise. If you refer to my post, I said to determine Ka using any method that you chose. The justification of using a 1.5 factor is based on the simple observation of how the pressures vary under a horizontal backfill scenario. I realize this is different from the problem statement, but the use of a 1.5 factor would apply whether it's horizontal or sloping. Maybe I'm not making sense, but I agree that Rankine is for horizontal. You can take the Ka log spiral and multiple it by 1.5 to get an at-rest pressure for your design.
