The soil pressure formula P/A +/- M/S (or re-written as P/W +/- 6P/W^2, where
W is footing width and P is total axial per lineal foot of wall) is only true
if resultant is within middle-third (e<W/6). If outside the middle-third
(arguable justification), since the soil interface can't take tension, an
easily derived formula for max pressure at toe is: P/(.75W - 1.5e), where e
is the eccentrictiy of the resultant.
Hugh Brooks, S.E.