142 cleaves. HIGHWAY ENGINEERING PROBLEMS [Ch. 7 



other physical characteristics of the base courses are governed by the 

 expected loadings and topographic and climatic conditions. Empiri- 

 cal rules accepted in the past do not necessarily apply to the modern 

 superhighway when heavy traffic is focused on particular highways. 

 For example: the Lincoln and William Penn highways in Pennsyl- 

 vania used to share the heavy interstate trucking, but the Pennsylva- 

 nia Turnpike has bled off most of this and in addition has drawn 

 heavily on traffic from the National Highway (Route 40) and prob- 

 ably also the Mohawk Trail in New York. This means not only that 

 the heaviest truck loads permissible under Pennsylvania state law 

 use the Turnpike, but also that the frequency of their passage over 

 any point is greater than on ordinary major highways. 



The relaxation interval of the subgrade of the Pennsylvania Turn- 

 pike between loadings is probably less than for any similar route in 

 the country. Inasmuch as this road is a forerunner of others to come, 

 soils testing for such arteries should receive greater attention than 

 ever before. No apt comparison is feasible between the Merritt Park- 

 way in Connecticut, because that highway excludes truck traffic. Nor 

 is a comparison with major airfield flight strips, because on them the 

 traffic is far less although the wheel loadings are greater. It is ap- 

 parent that the recovery time for the subgrade is longer for flight 

 strips. 



Determination of the subgrade soil characteristics is essential from 

 the point of view of drainage and stability. These features are gen- 

 erally related. "Stability means, essentially, resistance to movement 

 under conditions of moisture or load" (Hewes, 1942, p. 165). In ac- 

 cordance with the soil type, topographic position, climatic environ- 

 ment, and artificially or naturally changed conditions, the moisture 

 content and load-bearing capacity may be extremely variable. 



Hittle and Goetz (1946), investigating the factors influencing the 

 load-carrying capacity of base-subgrade combinations, considered 

 such variables as (1) soil type, (2) type of granular-base material, 

 (3) depth of base material, and (4) seasonal moisture. In this study 

 a cyclic-loading technique was developed by which measurements of 

 the base-subgrade combinations were made and the elastic and perma- 

 nent deflection characteristics of both the base and the subgrade 

 ascertained. Their study shows that base stability is determined 

 principally by the grading characteristics and density of the base ma- 

 terials. 



Whereas seasonal moisture changes are of vital importance, so also 

 are long-term climatic changes. A road constructed during a stretch 

 of dry years may suffer serious deterioration when the cvcle moves on 



