, in all the figures, represents the plane of rupture.[Footnote 1] ] In all cases, the earth thrust found as above is supposed to make the angle,
(as shown), with the normal to the inner wall surface.
Fig. 2.
Fig. 3.
In the Rankine theory, pertaining, say, to [Fig. 2], the earth thrust on a vertical plane,
, is always taken as acting parallel to the top slope. This is true for the pressure on a vertical plane in the interior of a mass of earth of indefinite extent, but it is not true generally for the pressure against a retaining wall. Thus, when