The formula is thus seen to give a thrust greater than the true one, or what may be called an upper limit. To realize the hypothesis more clearly, it may be said that if a vertical crack in the earth is assumed along
, the resulting value of
will be the same as that given by [Equation (7)]. It is a fact of observation that sometimes earth which has been saturated and then dried out, cracks along one or more vertical planes. This indicates tension in the mass, which is overcome, however, at certain points (only) and thus vertical cracks appear.
In the construction of [Fig. 11], the full friction and cohesion which can be exerted on the length,
(of [Fig. 24]), is supposed to be exerted. This construction then gives a lower limit to the thrust. As to which hypothesis will lead to the most probable value, it may be observed that the broken line of rupture,
, [Fig. 24], is nearer the true curved line of rupture (which is assigned both by theory and the facts of observation) than the straight line,