392 ROADS AND PAVEMENTS 



of 1-2-4 may be used for the arch, and a mixture of 1-2 H> 

 for the bottom. The mixing must be very thorough, and the 

 tamping into place carefully done. For sewer work, the mix- 

 ture should be so wet that a spade can be readily thrust down 

 into the mass to work the mixture into homogeneity. 



The thickness of circular concrete sewers built in firm and 

 stable ground and at a depth not exceeding 12 ft. may be taken 

 to be approximately as follows: 



Diameter Thickness of Sewer 



Feet Inches 



3 4 



6 6 



9 8 



12 10 



This thickness must be varied, however, with the character 

 of the soil and the depth of cutting. In wet, running soils, 

 the lower half of the sewer may be from two to four times 

 these thicknesses, with extra thickness at the sides. In 

 trenches 30 ft. deep, the thickness of the arch may be twice 

 the thickness given. 



ROADS AND PAVEMENTS 



HIGHWAYS 



GRADES, CROSS-SECTION, AND CURVES 



In order that a road may be satisfactory for travel, it must 

 be dry and solid, and have easy grades, easy curves, and a 

 smooth surface. These conditions refer to the use of the road, 

 but there are other conditions that are essential to economic 

 construction and maintenance; namely, (1) that the length 

 of the road shall be a minimum; (2) that its surface shall be so 

 placed with reference to the natural surface of the ground that 

 the amount of excavation and embankment shall be a mini- 

 mum; and (3) that it shall be so located as to be free from land- 

 slides, washouts, and snowdrifts. These different conditions 

 often conflict with one another, and there is generally a great 

 deal of difficulty in reconciling them. The question of cost 



