is the angle of friction of the earth on the wall. As the earth settles more than the wall, this friction will always be exerted. Again, as the wall, from its elasticity and that of the foundation, will tend to move over at the top on account of the earth thrust, the earth, with its frictional grip on the wall, will tend to prevent this, so that the friction is exerted downward in either case, and the direction of the earth thrust,
, on
is as given in [Fig. 1].
However, if
, a thin slice of earth will move with the wall, and the rubbing will be that of earth on earth, so that