40 BULLETIN I2i, V. S. DEPARTMENT OF" AGEICXJLTUEE. 



sure on the road surface of about 5 tons under each rear wheel. The 

 use of much heavier equipment is in sight, however, and unless some 

 regulations be passed to prohibit it the occurrence of 10-ton or even 

 heavier motor trucks on our local highways adjacent to large cities 

 or between large centers that are only a few miles apart may become 

 common. 



Slight irregularities in the surface, due to faults in construction 

 or developed bj^ the wearing action of traffic, have the effect of 

 producing impact under moving- wheels and this action serves sub- 

 stantially to increase the pressure to be distributed by the road crust 

 and foundation. On account of the impact which they produce 

 swiftly moving motor trucks are sometimes more severe on road 

 foundations than the hea^dest traction engines, and if an improved 

 road is to be subjected to any considerable volume of motor-truck 

 traffic the impact feature should be given especial consideration when 

 the road is designed. 



In asuming the maximum wheel load for any particular road a 

 reasonable allowance should be made for future increase. Since 

 motor trucks have come into use there has been a constant tendency 

 to increase both their rates of speed and the loads they carry, and 

 it is quite probable that this tendency will continue. Furthermore, 

 many roads not now subjected to motor-truck traffic Avill attract such 

 _ traffic after they are improved, and this possibility always should be 

 considered. Increase in the volume of traffic also may be an im- 

 portant factor, because a foundation ample to support an occasional 

 heavy load might fail soon if the same load was often repeated. 



DISTRIBUTION OF PRESSURE. 



The manner in which pressure due to concentrated loads is clis- 

 'tributed by the various materials used in road construction never 

 has been fully investigated, and any assumptions regarding this 

 point that may be made at present would be purely conjectural and 

 of doubtful value. In the past engineers have made the questions 

 of when a foundation should be employed and of how it should be 

 designed wholly matters of judgment, and few, if any, rules have 

 been formulated to aid even in making the judgment consistent for 

 different conditions and materials. This lack of method is due 

 directly to the absence of accurate knowledge concerning the cone of 

 dispersion and pressure distribution under concentrated loads, and 

 until further experimental data are obtained the design of road 

 foundations necessarily must be based on rather uncertain empirical 

 information so far as comparing the efficiency of different materials 

 and methods of construction is concerned. 



