Coulomb step ahead of Rankine

Rankine's theory is limited because it assumes a smooth, vertical wall back (no wall friction), a horizontal or uniformly sloping backfill, and a thrust parallel to the backfill surface. It cannot deal with an inclined wall back, a broken ground surface or a rough wall, and its fixed thrust direction does not match the real direction on a rough wall.

Coulomb's wedge theory removes these limits:

• It includes wall friction ($\delta$) and wall adhesion, so the result is more realistic for rough concrete or masonry walls.
• It allows an inclined wall back and a sloping backfill.
• The thrust direction (at $\delta$ to the normal to the wall) is obtained, and loads such as surcharge and line loads can be included through the wedge weight (and the Culmann graphical method).
• It handles irregular ground surfaces.

Hence Coulomb's theory is more general and gives a more accurate active thrust, so it is a step ahead of Rankine's.