This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
There are four distinct ways in which failure of a retaining wall may come about:
(1) A retaining wall may fail by sliding bodily on its foundations or on any horizontal joint. This may occur when the wall is resting on a soft soil, and especially when the foundation is not sunk sufficiently deep into the subsoil so that it is anchored. The failure of the wall on a horizontal joint is very improbable when the masonry work and its bonding are properly done. Perfectly flat, continuous joints should be avoided.
(2) A retaining wall may fail by crushing the toe of the wall. This may occur provided that the resultant of the weight of the wall and of the overturning pressure comes so hear the toe of the wall, and the intensity of that pressure is so great, that the masonry is crushed. The method of calculating such pressure will be given later.
(3) The wall may fail by tipping over. This may occur provided that the resultant pressure (described later) passes -outside the toe of the wall
(4) The same effect occurs provided that the subsoil is unable to withstand the concentrated pressure on the toe of the wall, and yields, while the masonry of the wall may nevertheless remain intact and the wall itself be properly proportioned.