192 



EARTHWORK 



topographical conditions make it necessary to use a shorter 

 spiral than the minimum given in the table. 



The least length corresponding to any value of a is found 

 from the formula 



EXAMPLE. Find the least length for the" spiral in the pre- 

 ceding example. 



SOLUTION. The velocity is 40 mi. per hr.; therefore, from 

 the table. a = 2, and L = 6-r2 = 3 sta. = 300 ft. 



EARTHWORK 



FIELD WORK 



Cuts and Fills. In building a railroad, cuts and fills are 

 introduced to equalize the irregularities of the natural soil. 

 Figs. 1 and 2 show a typical fill and cut in ordinary firm earth 

 or gravel. 



Slope Ratio. In cuts in the hardest rock, the average slope 

 is usually made J:l; that is, i horizontal to 1 vertical. As 



-H'-O" 



FIG. 1 



the soil becomes less firm the slope must be flattened until, 

 for a soil of firm earth or gravel, a slope of 1 to 1 may be per- 

 missible, although a slope of 1J:1 is commonly adopted. In 

 very soft soil, the slope ratio is sometimes cut down even 

 as far as 4 horizontal to 1 vertical. The standard practice 



