50 HIGHWAY CONSTRUCTION 



wall or fence raised upon it, to protect pedestrians against accident. 

 (See Fig. 24.) 



For the formula for calculating the dimensions of retaining walls 

 see instruction paper on Masonry Construction. 



Roadways on Rock Slopes. On rock slopes when the in- 

 clination of the natural surface is not greater than one perpendicular 

 to two base, the road may be constructed partly in excavation and 

 partly in embankment in the usual manner, or by cutting the face 

 of the slope into horizontal steps with vertical faces, and building- 

 up the embankment in the form of a solid stone wall in horizontal 

 courses, laid either dry or in mortar. Care is required in proportion- 

 ing the steps, as all attempts to lessen the quantity of excavation by 

 increasing the number and diminishing the width of the steps require 

 additional precautions against settlement in the built-up portion 

 of the roadway. 



When the rock slope has a greater inclination than 1 : 2 the 

 whole of the roadway should be in excavation. 



In some localities roads have been constructed along .the face 

 of nearly perpendicular cliffs on timber frameworks consisting of 

 horizontal beams, firmly fixed at one end by being let into holes 

 drilled in the rock, the other end being supported by an inclined 

 strut resting against the rock in a shoulder cut to receive it. There 

 are also examples of similar platforms suspended instead of being 

 supported. 



Earth Roads. The term "earth road" is applied to roads 

 where the surface consists of the native soil; this class of road is the 

 most common and cheapest in first cost. At certain seasons of the 

 year earth roads when properly cared for are second to none, but 

 during the spring and wet seasons they are very deficient in the im- 

 portant requisite of hardness, and are almost impassable. 



For the construction of new earth roads, all the principles pre- 

 viously discussed relating to alignment, grades, drainage, width, etc., 

 should be carefully followed. The crown or transverse contour 

 should be greater than in stone roads. Twelve inches at the center 

 in 25 feet will be sufficient. 



Drainage is especially important, because the material of the 

 road is more susceptible to the action of water, and more easily 



