144 cleaves. HIGHWAY ENGINEERING PROBLEMS [Ch. 7 



joints, fracturing of the pavement is inevitable. This condition is 

 heightened during excessively wet times and is seasonal as well as cycli- 

 cal over longer periods. A free-draining surficial base course which 

 is not confined laterally by shoulder material that is too dense may 

 provide a solution. It appears obvious that, if the soil moisture 

 brought to the surface by constant traffic vibration cannot be recov- 

 ered by the subgrade, it should be permitted egress in such a manner 

 that the pavement is protected from permanent damage. 



At the present time we do not know enough about what takes place 

 below the road surfacing. Certainly samples taken from the shoulder 

 at the pavement edge do not help in the understanding of changes 

 beneath the insulated and protected part of the highway, away from 

 the edges. A volume of case histories of surfacing failures in rela- 

 tionship to the thickness, sequence, gradation, and particle size of the 

 base courses might be very illuminating. One instance is known in 

 which under a porous surface the base courses graded from coarse to 

 fine downward to the subgrade. The deterioration of the road with 

 respect to bearing capacity of the surficial layers was extreme. Dam- 

 age to the lower layers, however, was negligible; hence resurfacing 

 with essentially waterproof material eventually restored this road 

 to service. 



Space does not permit expansion on the great variety of problems 

 relative to the vast amount of qualitative data available on the sub- 

 ject in the literature. However, under the leadership of such original 

 thinkers in the field of soil mechanics as Terzaghi, Casagrande, Rut- 

 ledge, Taylor, and others, and with the research and quantitative 

 data coming from the Bureau of Public Roads and the highway test- 

 ing laboratories in many states, remarkable progress has been made. 

 Constant awareness of the importance of soils testing in highway 

 construction is essential. 



FUTURE RESEARCH 



The possibilities and scope of research, like the topic of sedimenta- 

 tion in highway construction, are of such magnitude that one can 

 merely "brush" the surface. It goes without saying that continuing 

 research involving soils in relationship to highway construction is 

 necessary. Some basic work concerning slides in soils has been initi- 

 ated, but the need for much more dealing with all types of dry or 

 wet mass movements of soils, soil and rock, and rock alone in varied 

 climatic and topographic environments is indicated. 



Basic research on the bearing values of soils affected by frost action 



