132 cleaves. HIGHWAY ENGINEERING PROBLEMS [Ch. 7 



The ability to handle rock strata without the use of explosives de- 

 pends on the physical characteristics and structural attitude of the 

 layers. In the Appalachian region most sedimentary rock must be 

 drilled and blasted. Exceptions are flat-lying coals and some shales 

 and siltstones. These may often be loosened with a rooter and sub- 

 sequently removed by carryall scrapers or elevating graders. In the 

 shales and siltstones, the fissility of the rock and its horizontal posi- 

 tion determine whether or not explosives are necessary. Many clay 

 shales which must be blasted when initially encountered become de- 

 hydrated so rapidly on exposure that they must be treated as soils 

 shortly after uncovering or removal from their original sites. Many 

 Cenozoic strata are so weakly cemented that they too may be exca- 

 vated with conventional road-building equipment. 



Aside from drainage, slides, and subsidence problems, the chief ques- 

 tions relative to sedimentary rocks involve the design of cut slopes. 

 Until relatively recent years and the advent of modern excavation 

 and earth-moving equipment, highways utilizing deep cuts in order 

 to minimize grades were not practical. However, with the availability 

 of such equipment and because of the tremendous demand made on 

 roads by modern truck transportation, the excavation of deep cuts 

 has become a necessity. 



The design of cuts depends mainly on the physical characteristics 

 and structural attitude of the soil or rock to be excavated. Relative 

 to soils, Terzaghi (1929) states that 



. . . the stability of all our clay fills and clay cuts depends essentially on 

 cohesion. Due to this fundamental fact, the factor of safety of slopes with a 

 given inclination rapidly decreases beyond the critical height at which the soil 

 can stand with a vertical face. Hence a stable fill of a certain height and con- 

 sisting of a certain clay soil is no indication of stability in a fill of twice that 

 height, with the same slope and consisting of the same material. In computing 

 the factor of safety of a cut or fill, the curvature of the sliding surface must be 

 taken into account, else the results of the computation may be very misleading. 



There does not appear to be, and may never be, sufficient empirical 

 data to omit the advantages to be gained by soil mechanics testing 

 of soils in proposed deep cuts. Fundamental data obtained relative 

 to the stability of the slope in one soil cut may not apply to another 

 adjacent to it because of the vagaries of normal sedimentation proc- 

 esses. Such vagaries are more often the rule than the exception. 



In sedimentary rock cuts, even with horizontal strata, no general 

 rule applies. Nevertheless, in massive-bedded, strongly cemented, 

 horizontal sandstones, limestones, and dolomites, of generally homog- 



