To resist the tendency to slide on its base, the dead weight of the wall, or of the wall and the load it carries, must be sufficient to resist the horizontal pressure without exceeding the coefficient of friction between the material of the wall and the surface upon which it rests.
To resist the second tendency the weight above any joint must be sufficient to resist the pressure above the joint without exceeding the coefficient of friction of masonry upon masonry.
The overturning moment of the earth pressure about the edge of rotation must be balanced by the moment of the weight of the wall and of the superimposed load about the same edge.
The fourth condition applies only to retaining walls supported at their tops and built generally of concrete and steel. A retain ing wall so supported would have to resist tension in one side and, as a masonry joint is not intended to resist tension, such construction involves the use of steel. Such construction is becoming more common on account of the saving in space due to the thinness of the wall. In Fig. 156 is shown such a wall. The tensile strength is supplied by the beams running horizontally and the twisted vertical rods.
The resulting pressure due to the thrust of the earth and the weight of wall and superimposed load must fall within the base in order to give equilibrium, and within the middle third of the base to avoid tension on the masonry joints. Figs. 158-159 show types of retaining walls.
Underpinning Shoring and Sheath Piling. Underpinning is the term given to the processes of carrying down old founda tions or walls adjacent to new construction to the level of time new construction.
it very often happens that footings of new buildings will be twenty or thirty feet below the bottom of the footings of the walls of an adjacent old building. To leave the old footings at this higher level after the excavation of the new building is made, would necessitate making the wall heavy enough to act as a retaining wall, to resist the pressure on the soil back of it. It is generally more practicable, therefore, to hold up the old wall temporarily by timber braces, needles, wedges, etc., and build new work up under it from the level of the footings of the new buildings. This new foundation under the old wall is called underpinning, and the construction necessary to hold it in place, during the process of underpinning, is called shoring. This latter term applies to all bracing of old walls or adjacent construction during the construction of the new work, whether the wall is underpinned or not.
Where a mass of earth is to be held in place to enable new exca vation to be made without disturbing it, heavy planks set edge to edge are driven down as the excavation proceeds, and braced at inter vals by breast pieces or heavy timbers to keep the plank from bulging under the pressure of the earth. This construction is called sheath piling. The planks, generally, are pulled out after the wall, which is designed to permanently hold the earth in place, is built; sometimes, however, it is left in place.