POETLAND CEMENT CONCRETE ROADS. 



13 



pavement, the speed of the vehicle, the character of the tires, and 

 the percentage of the total load which is carried above the springs 

 of the vehicle. Under very unfavorable conditions it may be as high 

 as five times the amount of the static load. 



The pavement itself depends upon the subgrade for support, and 

 this support is extremely nonuniform in character. The supporting 

 power of a subgrade depends upon the type of soil, its capillarity, the 



TRUCK PASSING TRUCK 

 Fig. 1. — Width of road required for safe passage of vetiicles. 



proximity of ground water, the condition of surface drainage, the 

 amount of sustained rainfall, and the extent of freezing and thawing. 



All of these factors are extremely variable, and in combination are 

 almost indeterminate, so that it is almost impossible to reduce the 

 determination of pavement thickness to a simple mathematical com- 

 putation. The behavior of concrete pavements of known thickness 

 under known soil conditions and known conditions of traffic is the 

 most satisfactory index of the thickness of pavement required. 



It has been more or less customary in the past to use a fiat subgrade 

 for concrete pavements, and obtain the necessary crown in the pave- 



