Bridge Abutment Settlement

Hello. I have a question regarding settlement calculations for a small bridge abutment. 20’ span with 6’ of soil above the footing bearing elevation at the toe and 16’ at the heel. Would you consider the 6’ to be Df or 16’? Certainly the 6’ would be conservative, but that Df results in settlements being a little too high. I think the reality is that the stress the soil is feeling is somewhere in between. Note this is for settlement calculations not bearing capacity failure. Thanks!