WES Theoretical Manual, Part XII: Overturning

Overturning is the oldest check, and the manual explains both its use and its limits. The cell is treated as resting on its base, the resultant of the lateral force and the weight of the fill is found, and its point of intersection with the base is noted. When the resultant falls a distance of B/6 from the center, the base begins to lose contact at the heel.

The key historical point is that from 1908 until Terzaghi's paper in 1945, most designs were based on resistance to overturning. The manual explains that this criterion is now recognized as fundamentally incorrect. Many of those older cofferdams did not fail, because the cells were low and cells filled with granular material are inherently stable thanks to the high shear resistance of the fill. Failures have occurred in cells filled with material lacking shear strength.

Overturning is therefore best used as a check on foundation contact, not as the governing criterion for the design.