CONSTRUCTION NOTES 281 



After the foundation footings are in place the form work for the remain- 

 der of the wall above the footings is erected and the horizontal steel is put 

 in the proper place. In all reinforced concrete walls it is almost without 

 exception necessary to construct forms for both faces of the wall. When 

 these forms are in place and carefully wired to prevent spreading, the 

 thoroughly mixed concrete is placed in the forms. It is very necessary to 

 make certain that the concrete on the inner side of the wall toward the 

 bank is thoroughly and evenly distributed around all of the reinforcing 

 steel. If the retaining wall is to be veneered with stone or brick (Plate 116, 

 figure 3) then the wall ties of galvanized iron should be placed in the forms 

 in such a way that, when the concrete has been poured and the forms 

 removed, one end of these wires or iron straps is thoroughly embedded in 

 the concrete and the other end projects from the face of the concrete wall, 

 later to be embedded in the joints of the stone or brick veneer. These 

 wall ties should be staggered at intervals of 1 8 inches to two and one-half 

 feet horizontally and vertically. 



(b) Gravity walls of concrete or stone masonry. Gravity walls may be 

 divided into two classes for purposes of this discussion: (a) Concrete walls 

 and stone or brick masonry walls (Plate 115, figures I, 5 and 6), and (b) 

 Dry walls (Plate 115, figures 2, 3 and 4). The stability of gravity walls 

 depends upon the weight of the wall and the position of the center of gravity 

 of the wall. In order to resist the lateral pressure under average soil con- 

 ditions the base of the wall should approximate in width not less than .35 

 to .40 of the total height of the wall. This width may be reduced if the 

 face of the wall has a definite batter (see Plate 115, figures 2 and 4). It is 

 necessary to make the foundation of a greater width in proportion to the 

 height of the wall when the wall retains a water-bearing soil such as clay 

 or fluid mud, as compared with a well-drained gravel or average earth. 

 Under some conditions the width of the base may approach 70 per cent of 

 the height of the wall. The efficient height for a gravity wall of concrete is 

 approximately six to seven feet. The availability of local materials and the 

 cost of purchasing such materials may create a condition where it is much 

 more economical to build a reinforced concrete wall (Plate 116, figure 2) 

 than a gravity wall (Plate 115, figures 5 and 6). 



The following is a tabulation which may be of interest in determining 

 the width of the base to be constructed for retaining walls of different soil 

 conditions: 



Equivalent fluid pressure Width of base divided 

 Kind of soil in pounds per square foot by height of wall 



Well-drained gravel or average earth, 20 to 33 0.35 to 0.40 



Wet sand, 50 0.58 



Water-bearing soil, 62.5 0.65 



Fluid mud, 80 0.73 



