TRANSACTIONS OP SECTION J. 



491 



5. A Grapliical MetJiod for the Design of Beservoir-ivallS: 



By R. W. Chapman, M.A., B.C.E. 



Plates XV. and XVI. 



The problem of the design of a masonry reservoir- wall, in which 

 there shall be no waste of material, may be shortly stated 

 thus : We have to so vary the thickness of the wall from top to 

 bottom that, if we consider any horizontal section, that portion 

 of the wall above that must have sufficient stability, due to its 

 own weight, to resist the overturning influence of the hydro- 

 static pressure. It is usually considered advisable to neglect 

 in calculation the strength which the wall receives from the 

 adhesion of the cement, and consequently, as it is a question 

 of stability only, no factor of safety is required. Moreover, the 

 pressure must be so distributed over the base of the wall that 

 there is always some pressure at the face, and consequently 

 the centre of pressure on the base must never be at a greater 

 distance from the face of the wall than two-thirds of the base. 

 Such are the conditions under which reservoir-walls are 

 designed by most writers on the subject. 



I pro]30se in this paper to give an account of a method by 

 which the proper width of the wall may be directly determined 

 at intervals by a simple graphical construction. I shall sup- 

 pose that the face of the wall 

 exposed to the hydrostatic pres- 

 sure is vertical, as the slight in- 

 clination usual at the lower por- 

 tion of the face is not sufficient 

 to change the direction of the 

 resultant fluid pressure much from 

 the horizontal, and by neglecting 

 the slight deviation we make our 

 wall really a little stronger than 

 theory requires. 



As in the figure, which repre- 

 sents a section of the wall, suppose 

 that the top width is a, and b, c, 

 d, e, f, &c., denote the widths at 

 intervals of h feet, proceeding from 

 the top to the bottom. These 

 measurements are all supposed 

 to be made in feet. 



The top width of the wall, a, has to be decided upon at the 

 start. For still water there is no pressure on the wall right at 

 the top, but impact of wind and wave necessitate of course 



