223. A retaining wall is a wall built to sustain the pressure of a vertical bank of earth. The stability of the wall is a comparatively simple matter when three quantities have been determined:

(1) The intensity of the earth pressure;

(2) The point of application of the resultant of the earth pressure;

(3) The line of action of this pressure.

Fig. 64. Combination of Pneumatic Caisson and Open Well Methods.

Fig. 64. Combination of Pneumatic Caisson and Open-Well Methods.

Unfortunately, earthy material is very variable in its action in these respects, depending on its condition. It is not only true that different grades of earthy material act quite differently in these respects, but it is also true that the same material will act differently under varying physical conditions, especially in regard to its saturation with water. On these accounts it is impracticable, even by experiment, to determine values which are reliable for all conditions.

It is also comparatively easy to formulate a theory regarding the pressure of earthwork which shall be based on certain theoretical assumptions. One of these assumptions is that the so-called plane of rupture is a plane surface - or, in other words, that the line a b (Fig. 65) is a straight line. There is considerable evidence, and even theoretical grounds, for considering not only that the line a b is a curved line, but that the curve is variable, depending on the physical conditions. It is also assumed that the earthy material acts virtually the same as a liquid with a density considerably greater than water; but there is ground for believing that even this assumption is not strictly warranted. Theoretically the problem is also very much complicated by the question of the earth pressure which may be produced by a surcharged wall. A surcharge is a bank of earth which is built above the height of the top of the retaining wall and sloping back from it. It certainly adds to the pressure on the earth immediately back of the wall itself and increases the pressure on the wall.