ROADS AND PAVEMENTS 399 



foundation will not be required and the macadam system will 

 be found the cheaper and better adapted to the conditions. 



The varieties of rock most suitable for road metal are trap, 

 syenite, granite, chert, limestone, mica-schist, and quartz. 

 These are named in the order of their relative values. Sand- 

 stone, clayey slate, and rock of indurated clayey material are 

 not suitable for this purpose. Sandstone has practically no 

 binding properties; the fragments do not bind together to form 

 a solid mass, but remain simply an accumulation of separate 

 fragments, which soon become ground and crushed into sand 

 by the traffic. Clayey stones have poor binding qualities, and 

 when saturated with water become very soft and are easily 

 crushed into mud. The broken stone is applied in layers of 

 from 3 to 5 in. The first layer is spread uniformly over the 

 road, sprinkled with water, and rolled with a suitable roller* 

 Upon this a second layer, and sometimes even a third layer, 

 depending on the depth required, is treated in the same manner. 

 When the last course has been properly completed, a layer of 

 stone dust, which is usually called the binder, is spread to a 

 depth of J to f in., after which the road is again sprinkled 

 with water and rolled until consolidation is complete. 



A common rule requires that the stone shall be broken small 

 enough to pass through a 23-in. ring. It is also a not uncommon 

 practice to use somewhat larger pieces in the bottom courses 

 of the roadway than at the top, the stones at the bottom being 

 from 2 to 3 in. in greatest dimension and those at the surface 

 not more than 2 in. This is probably a good practice, though it 

 may be doubtful whether it is sufficiently advantageous to 

 warrant the additional expense of separating the sizes. 



The thickness of the covering of broken stone should not be 

 less than 4 in. and a thickness greater than 12 in. is seldom 

 required. Macadam considered 10 in. of well-compacted 

 broken stone on a solid, well-drained earth foundation sufficient 

 for a roadway sustaining the heaviest traffic. A thickness of 

 from 8 to 10 in. is generally considered sufficient. 



Bituminous Macadam Roads. The introduction of exten- 

 sive automobile traffic upon our highways has made the 

 maintenance of macadam roads very difficult. The heavy 

 wheels disturb the binding material, and the rapid air-currents 

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