CONSTRUCTION NOTES 283 



The width of the base not only varies because of the lateral pressure 

 of the soil, but there are many instances where it is necessary because of 

 poor subsoil to increase the width of the base in order to make a more 

 stable wall. It is not advisable to construct a brick retaining wall to retain 

 soil of a height exceeding four feet. Brick retaining walls should only be 

 used when non-porous brick are obtainable absorbing less than two per 

 cent of their own weight of water. Under such conditions brick should be 

 laid with a waterproof mortar (Portland cement plus 10% by volume of 

 hydra ted lime). Mortar mixture should be one part of the above cement 

 and two parts sand. 



Gravity walls of concrete should usually be constructed with a mix- 

 ture of 1 12:5, although a mixture of 1:3:6 with excellent aggregate may be 

 sufficient. Such walls may take the general section shown in Plate 115, 

 figures 5 and 6. If the surface of a gravity concrete wall is to be veneered 

 with either brick or stone (Plate 115, figure 6), wall ties should be installed 

 as specified under Reinforced Concrete Walls. Stone masonry walls may be 

 constructed with the general section as shown on Plate 115, figures I, 3 

 and 4. If constructed similar to figures 3 and 4 the bottom of the foundation 

 of stone masonry or concrete should extend below the line of normal frost 

 action. Brick masonry walls and stone masonry walls of the type shown 

 on Plate 115, figure i, will have the same approximate section. 



(c) Gravity walls laid dry. All dry walls must be gravity walls (see 

 Plate 115, figures 2, 3 and 4). It is sometimes necessary to construct a dry 

 stone retaining wall of greater height than five feet. The section shown on 

 Plate 115, figure 4, with the stone masonry or concrete foundation on an 

 inclined plane, provides greater stability and is more desirable for the greater 

 heights of wall. Dry stone walls constructed similar to this section ought 

 not to exceed, over the best soil conditions, more than eight feet in height. 

 In the construction of dry stone walls it is sometimes desirable to intro- 

 duce freely within the structure of the wall a cement mortar which does 

 not show on the surface and which adds to the stability of the wall. While 

 the average width of the foundation of the footing for a gravity wall approxi- 

 mates from .35 to .40 of the height of the wall above grade, it is advisable 

 that the width of the foundation for dry walls should approximate .50 of 

 the total height of the wall. The average dry stone wall should have the 

 stones superimposed in direct contact with each other, although it may be 

 advisable and sometimes practicable, with great care, in low walls, to intro- 

 duce a thin sandy loam joint. The depth of the foundation for dry stone 

 walls should approximate from 12 inches in sandy soil to 18 inches in the 

 heavier soils, below the finished grade at the face of the wall. A more stable 

 wall is produced by constructing footings with stone masonry or concrete. 



