118 RAILROAD LOCATION AND CONSTRUCTION — MACKENZIE 



Iron Pipes for Culverts. — When iron pipes are used the fol- 

 lowing sizes will he sufficient for the given area, in places where 

 water can be backed up to discharge under a head : 



The dimensions given in the foregoing tables have been 

 extensively used on first-class work in this part of the country. 



Arch culvert stone masonry costs about as follows : 



Retaining -Walls. — For retaining earth, the mean thickness of 

 stone walls are usually made one-third of the height, the top, 

 middle, and bottom thicknesses varying, as 3, 5, and 7. Walls 

 should be calculated to resist overturning, and also sliding at 

 the base, using the following : 



Co-efficient of friction of rubble masonry on wet clay, 0"2 to 0'33 

 " " moist clay, 0"33 



dry clay, O'Sl 



dry earth, O'SO to 066 



sand, 0-66 to 075 



gravel, 0-66 to 0-75 



" " dry wooden platform, 0'60 



wet " 0-75 



