WES Theoretical Manual, Part IV: Slip on Vertical Center Plane in Fill

This part covers the shear check on a vertical plane through the center of the fill, the idea Terzaghi introduced and TVA engineers had used earlier. The cell is treated like a mass that must resist the overturning effect of the lateral loads without shearing along that plane.

The resisting forces come from the shear strength of the fill and the friction in the interlocks. The manual notes that for some fills, interlock friction can account for 30 to 40 percent of the total resistance, which is why it must not be ignored. It discusses the choice of lateral earth pressure coefficient, including the argument that overturning compresses the fill on the inboard side and raises the coefficient, and explains why the manual still recommends the Krynine coefficient, because the safety factors in current use were calibrated against lower values.

It also warns about the assumed straight-line pressure on the base. That picture is valid only while the base stays in compression over its whole length, meaning the resultant falls in the middle third. If it does not, the whole vertical-shear analysis is questionable.