DEAINAGE METHODS FOR COUNTY ROADS. 43 



water about four and one-half times higher than in dry soil.^, and 

 this fact must be considered in designing- cross section and grade 

 across swamps, marshes, or against irrigation or drainage ditches 

 in which water is present for long periods. These conditions affect 

 the design or cross section and the establishing of grade in places 

 where the presence of free water is foreseen or where run-off is ex- 

 pected to be slow and considerable, if the full bearing power of the 

 natural soil is to be developed. If, because of immediate local con- 

 ditions, the design of cross section can not be made to meet the above 

 requirements, then special drainage should be provided or additional 

 bearing value provided through some suitable treatment. 



The way in which the subgracle is prepared also, of course, affects 

 its bearing power to a very considerable extent. In fact, the exercise 

 of proper precautions in the matter of draining and compacting a 

 roadbed sometimes may give the subgrade ample stability for directly 

 supporting the road crust, where otherwise, if these features were 

 neglected, an expensive foundation might be required in order to 

 afford the same degree of stability. 



INFLUENCE OF ROAD CRUST. 



The considerations that should determine the form of support to 

 be provided for any particular type of road crust are (1) whether 

 it is practicable to make the crust sufficient to distribute wheel loads 

 so that the safe bearing power of a well-prepared subgrade will not 

 be exceeded, (2) the relative cost of increasing the depth of the crust 

 so as to take care of the necessary distribution, where this is prac- 

 ticable, as compared to the cost of providing a suitable foundation of 

 cheaper material, (3) the damage that would result to the road sur- 

 face in case of slight settlement of the subgrade. 



All of these considerations depend largely, of course, on the char- 

 acter or type of road crust to be employed. Where the crust is to 

 be of Portland-cement concrete, or some similar type capable of dis- 

 tributing adequately the pressure due to concentrated loads the 

 foundation evidently may be omitted, provided the crust is made 

 sufficiently thick to take care of the distribution of load. In the case 

 of sand-clay, gravel, and macadam, furthermore, slight disturbance 

 of the subgrade might not result in any very serious injury to the 

 surface, because the materials forming the crust which might be dis- 

 turbed would be rebonded soon by the action of traffic. With any of 

 the types mentioned, however, it might be more economical to con- 

 struct a foundation of cheaper material than to provide the addi- 

 tional thickness of crust which otherwise would be required. This is 



^ Movement and distribution of moisture in soils. P. S. Harris and H. W. Turpin. 

 Jour. Agr. Res., July 16, 1917, p. 117. 



