128 cleaves. HIGHWAY ENGINEERING PROBLEMS [Ch. 7 



The applications of seismology and electrical resistivity methods 

 for determining the depth to bedrock, and the position of the water 

 table, when used in conjunction with check core borings, speed up 

 preliminary subsurface surveys and reduce the guesswork in estimat- 

 ing common and rock excavation. 



Through the utilization of these and other methods, and upon the 

 basis of geological understanding, the geologist can determine the posi- 

 tion of bedrock, the ground-water table, subsurface divides, possible 

 perched ground water, and can fairly appraise the nature of the under- 

 lying materials. 



Because the modern major highway is planned for the purpose of 

 satisfying the principal traffic needs of an area, and because of the 

 advent of modern excavation and earth-moving equipment, it is now 

 mandatory to construct roads with gentler grades and straighter align- 

 ment than ever before. In general, regional alignment is fixed for the 

 modern high-speed highway, and only very minor local adjustments 

 may be made. This means that, locally, sections must be built through 

 terrain, often with undesirable physical characteristics such as swampy 

 ground, mine subsidence areas, heavy cuts, and poor aggregates. Nev- 

 ertheless, within the economic limits controlling the enterprise the 

 project must be built as laid out. 



The problems in the Appalachians are different from those on the 

 West Coast, in the tropics or in the Arctic. It would be impossible to 

 treat adequately the significant sedimentation characteristics of soils 

 alone for such varied areas, yet an attempt will be made to outline 

 some of the applications of sedimentation as they affect construction 

 on and in the soil cover and sedimentary rock. Brief reference will 

 also be made to slides and areas of subsidience in the hope that some 

 of the lessons learned in recent years may be helpful to others. 



CONSTRUCTION ON SOILS 



In highway or airfield construction, soil is the overburden to the en- 

 gineer, and he is less concerned with its origin and classification than 

 he is with the equipment he can use to move it most efficiently and 

 cheaply. The soils studies for airfields have, in general, been much 

 more thorough than those for highways, but the time has come when 

 equally careful studies of the soils must be made for roads. This ap- 

 plies specifically to the subgrade and the placing of base courses be- 

 neath the surfacing of modern superhighways. It follows naturally 

 that the soils engineer will do a better job of interpreting his tests 



