STORAGE AND DISTRIBUTION. 129 



rupture. In fig. 91 the angle of repose is taken 

 as 30, therefore GD bisecting it gives us the line of 

 rupture, and BCD is the wedge of earth which we 

 have to calculate upon. Calculate the weight of the 

 earth and calculate the weight of the wall, assuming 

 both to be i ft. wide (fig. 92). Make KD one-third 

 the way up from the base, and to scale make KH 

 equal to weight of the wall. Now make the angle 

 KHJ equal to 30 (or the angle of repose), and pro- 

 duce it to meet the perpendicular from K. Now JK 

 to scale is equal to the force acting on the wall in Ibs. 

 Now find the centre of gravity of the wall by any 

 known method. A simple way is by means of the 

 dotted lines, producing the top each side a distance 

 equal to the base, and producing the base likewise 

 a distance equal to the top as shown on the drawing. 

 The centre of gravity will then be at the inter- 

 section of the two diagonals at L. Drop a perpen- 

 dicular LP making MP equal to weight of wall, in this 

 case = 3920 lb., complete the parallelogram. MO is 

 then the all-important line, the line of resistance 

 of the wall, which must pass through the middle 

 third of the base if the wall is to be safe. 



The above graphic construction is of the utmost 

 value and importance to the engineer. It is simple, 

 but must be very accurately drawn to a large scale. 

 If this be done no failure of the wall from natural 

 causes is likely to occur. Its neglect in design may 

 mean complete failure, and if not extravagant use of 

 material. When the ground has a sloping surface, such 

 as occurs in what we termed surcharged revetments, 

 the above graphic construction holds good, because 

 it simply means calculating a larger wedge of earth. 



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