When Jefferson's workmen built the wing, they sliced into the east mound so the wing's eastern end wall abutted directly against the mound.  The ensuing pressure of the soil caused the retaining wall to bulge.

When restoration craftsmen were reconstructing the wing walls, they put a reinforced concrete beam between the mound and two original perpendicular stone and brick walls.  The perpendicular walls will help hold the reinforced beam in place, and the beam will bear the brunt of the soil pressure.  

Photo below shows a mason rebuilding a portion of the retaining wall.