ROADS AND BRIDGES 



337 



Figure 224. Cross sectional view of a semi-sta- 



tage in this method. In no case should the upper 

 surface of the larger rocks be near the surface. They 

 should be covered with several inches of finely crushed 

 stone. Particles of stone lying on top of a hard rock 

 and struck with the tire of a wagon are subjected to a 

 sudden blow, as the hammer strikes a walnut laid on an 

 anvil. For this reason 

 rock at this point may 

 become crushed and 

 result in forming a rut. 

 Rock crushers. In 

 Figures 222 to 225, in- 

 clusive, are shown rock 

 crushers. Where the 

 rock is to be used in 

 large quantities, a 



1 . < , i 1 i tionary rock-crushing plant. Wagons are driven 



Crushing plant SnOUlCl under the four compartments, and the crushed 

 , 1-1 j j stone runs into the wagon box. 



be so devised and de- 

 veloped that comparatively little manual work is neces- 

 sary. The rock can be loosened from the ledge by 

 means of the drill and blast and sometimes further 

 broken by hand, and then placed in wheelbarrows 



run on cables, or on 

 dump cars, and thus 

 carried to the crusher 

 by power machinery. 

 Some hand work is 

 necessary to feed the 

 crusher. The crusher, 

 propelled by a power- 

 ful engine, breaks the 

 stone between hard 

 steel plates. The 

 broken particles of 



S,one-c t ush l n g p,an,,ope,,, I on. st(me faj , .^ ^ ^ 



vator and are carried to a revolving screen. The finest 



