134 cleaves. HIGHWAY ENGINEERING PROBLEMS [Ch. 7 



were developed in nodular clays and in weak clay shales on slopes 

 between 2 to 1 and 3 to 1. In the overlying interbedded shales and 

 sandstones, % to 1 and y 2 to 1 slopes are stable. 



When sedimentary rock strata in a sidehill cut strike parallel to the 

 cut but dip into the hillside away from the road, steep slopes are 

 feasible. Vertical fractures and joints, on the other hand, may make 

 it desirable to modify such slopes. 



Schultz, Cleaves, and Rutledge state (in press) : 



. . . because experience indicates that, once started, there is no way to 

 control deep deformational slides, the modern method of slope design is to 

 determine the stable angle of slope in advance of excavation. It is simple 

 to prevent the development of deformational slides by the expedient of 

 adopting extremely flat slopes. Such practice could, however, involve the 

 excavation of more material than if the slide were actually permitted to de- 

 velop. Consequently it is essential to approach the problem from a quantita- 

 tive point of view. Such a procedure involves close correlation of geology and 

 soil mechanics. 



From the standpoint of soil mechanics the stability of slopes with respect 

 to the possible occurrence of deep deformational slides depends chiefly on the 

 following factors: shearing strength and density of the materials in question 

 in relationship to height and slope of the banks. Shearing strengths are deter- 

 mined in the laboratory, and the proper slope is found by correlating the 

 shearing strengths of the various materials with the depth of the cut and their 

 positions in the banks. It is obvious that weak rocks generally require flatter 

 slopes than stronger materials. 



In regions of hard sedimentary strata such as the Appalachians 

 the following statement of these three authors does not necessarily 

 apply, but in other areas, especially those of less strongly compacted, 

 consolidated, and cemented Mesozoic and Cenozoic strata it may be 

 particularly apropos. 



If the rock is perfectly uniform with respect to shearing strength and density, 

 the higher the banks the flatter the slopes required. If strong rocks are over- 

 lain by weak materials, the stable slope for the former may be found by cor- 

 relating their shearing strengths with the total height of the banks (weaker 

 materials included). 



The slope for the overlying weak materials is determined in a similar manner, 

 except that only this class of materials needs to be considered in determining 

 the effective height of the bank. 



If weak rocks are overlain by stronger rocks, the slope must be the same 

 as for a bank of equal height composed entirely of the weaker materials, but 

 having the same densities as the materials in question. 



The above analysis takes no account of the influence of bedding planes, 

 faults, fissures, and other geological factors. Bedding planes are zones of 



