102 



HIGHWAY CONSTRUCTION 



9 inches long, and 6 inches deep; the round blocks are commonly 6 

 inches in diameter and 6 inches long. 



The kinds of wood most commonly used are cedar, cypress, juni- 

 per, yellow pine, and mesquite; and recently jarrah from Australia, 

 and pyinyado from India, have been used. 



The wood is used in its natural condition, or impregnated with 

 creosote or other chemical preservative. 



The blocks of wood are laid either on the natural soil, on a bed 

 of sand and gravel, on a layer of broken stone, on a layer of concrete, 



Fig. 61. Gravel Heater. 



Fig. 62. Melting Furnace. 



or, sometimes, on a double layer of plank. The joints are filled either 

 with sand, paving-pitch, or Portland-cement grout. 



Advantages. The advantages of wood pavement may -be stated 

 as follows: 



(1) It affords good foothold for horses. 



(2) It offers less resistance to traction than stone, and slightly 

 more than asphalt. 



(3) It suits all classes of traffic. 



(4) It may be used on grades up to five per cent. 

 It is moderately durable. 



It yields no mud when laid upon an impervious foundation." 

 It yields but little dust. 



(5) 

 (6) 

 (7) 



