LIMITS OF LENGTH OF OVERTHRUST BLOCKS 



339 



stress exceeds this value a new rupture will start at the surface and be 

 propagated downward. 



For any given angle a which a thrust-plane makes with the horizon we 

 arrive at a limiting value for AB by assuming concrete values for S, w, 

 and f: 



w may be taken as .075 ton per cubic foot; 



S, for the strongest rocks at the surface, may be taken as 15 tons per 

 square inch, or 2,160 tons per square foot. 



The value of / is more debatable. Under laboratory conditions for the 

 determination of engineering data, the maximum value of / for rock on 



.5 



n 

 +» 



o 



•H 

 M 



o 



.3 



9 





32.5 186 224 336 448 560 672 709 



Pounds per square inch 



A wrought iron on wrought iron 



B steel on cast iron 



C wrought iron on oast iron 



Figure 2. — Diagram to show the Increase in the Coefficient of Friction with increasing 



Pressure. (After Rennie.) 



rock is about .65. We may assume .5 as a fair figure for this quantity, 

 as ordinarily determined ; but the value of the coefficient of friction both 

 in the laboratory and in nature varies with varying conditions. Certain 

 experiments reported in engineering handbooks indicate that up to the 

 limit of abrasion the coefficient of friction increases with increasing 

 pressure. 



The graph in the accompanying diagram, figure 2, constructed from 

 results obtained by G. Eennie, 2 shows the rise of the value for the co- 

 efficient of friction with increase of pressure. 



2 John C. Trautwine : The civil engineer's pocket-book, 1900, p. 374&. 



