WES Theoretical Manual, Part V: Slip on Horizontal Planes in Fill (Cummings’ 1957 Method)

This part covers Cummings' 1957 method, which imagines the fill shearing along horizontal planes. The method is widely quoted, so the manual examines it carefully, including its assumptions and the evidence for them.

It concludes that the basic assumptions, a rupture plane at an angle to the base with horizontal sliding in the fill beyond it, are not well supported by experiment. Apart from Cummings' own models, no later studies have confirmed the mode, and the model studies of Maitland and Schroeder showed Cummings' method predicting maximum resisting moments significantly larger than those observed at failure.

The manual also points out that, although the method is described as an internal shear check on the fill, it is really a way to calculate the pressure of the fill on the walls. The factor of safety comes from treating the outboard and inboard walls as a rigid free body. That makes this a part worth reading closely before relying on the method.