32 HIGHWAY CONSTRUCTION 



speedy than if the drainage of the outer half of the road has to pass 

 over the inner half. The inner half of such a road is usually sub- 

 jected to more traffic than the outer half. If formed of a straight 

 incline, this side will be worn hollow and retain water. The inclined 

 flat section never can be properly repaired to withstand the traffic. 

 Consequently it never can be kept in good order, no matter how 

 constantly it may be mended. It is always below par and when 

 heavy rain falls it is seriously damaged. 



DRAINAGE. 



In the construction of roads, drainage is of the first importance. 

 The ability of earth to sustain a load depends in a large measure upon 

 the amount of moisture retained by it. Most earths form a good 

 firm foundation so long as they are kept dry, but when wet they lose 

 their sustaining power, becoming soft and incoherent. 



The drainage of roadways is of two kinds, viz., surface and sub- 

 surface. The first provides for the speedy removal of all water 

 falling on the surface of the road ; the second provides for the removal 

 of the underground water found in the body of the road, a thorough 

 removal of which is of the utmost importance and essential to the 

 life of the road. A road covering placed on a wet undrained bottom 

 will be destroyed by both water and frost, and will always be trouble- 

 some and expensive to maintain; perfect subsoil drainage is a neces- 

 sity and will be found economical in the end even if in securing it 

 considerable expense is required. 



The methods employed for securing the subsoil drainage must 

 be varied according to the character of the natural soil, each kind of 

 soil requiring different treatment. 



The natural soil may be divided into the following classes: 

 silicious, argillaceous, and calcareous; rock, swamps, and morasses. 



The silicious and calcareous soils, the sandy loams and rock, 

 present no great difficulty in securing a dry and solid foundation. 

 Ordinarily they are not retentive of water and therefore require no 

 underdrains; ditches on each side of the road will generally be found- 

 sufficient. 



The argillaceous soils and softer marls require more care; they 

 retain water and are difficult to compact, except at the surface j 

 and they are very unstable under the action of water and frost. 



