LOCKS. 



If the wall is surcharged (see Fig. 3), a case which is occasionally met with in prac- 

 tice, the triangle AGD becomes AG'D, G'D being, as before, inclined to the vertical at 

 half the angle of repose of the 

 backing, and the pressure T" be- 

 comes the weight of AG'D. 



Then, if we include the friction 

 on the wall, 



T" = weight of AG'D X 0.643, 



the angle of inclination KLM be- 

 ing as before, about 33 41', or ij 



to i. 



If the slope of the earth does 



not extend high enough to make 



AG'D a complete triangle (see Figs. 



3 and 4), AG'D becomes a four-sided 



figure, as ADG"G'", and its weight 



is reduced accordingly. In this 



case the point of application of 



T" is not at one-third of the height 



of the wall above the base, but 



at that point where a line QL', drawn through the center of gravity of the earth 



mass and made parallel to the slope DG" of maximum pressure, strikes AD. This 



point will lie more than one-third of 

 the height above the base. 



Head- and Tail-bay Walls. The 

 8 functions of the head- and tail- bay 

 walls are, first, to support the con- 

 centrated pressure from the gates, and 

 secondly, to provide width for maneu- 

 vering the operating machinery, such 

 as the capstans for opening and clos- 

 ing the gates. The latter condition 

 usually requires a width of at least 

 10 feet behind the gate recess. 



In providing against the thrust 

 from the gates it may be assumed 

 that the masonry for a certain length, 

 say 15 feet, above the hollow quoins, 

 Thus, let the accompanying Figs. 



acts as a 



D 



FIG. 4. 

 monolith with 



that below them. 



5 and 6 represent the head and tail walls on the river side in plan and section, the 



