DRAINAGE METHODS FOR COUNTY ROADS. 43 
water about four and one-half times higher than in dry soil. 1 , 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 subgrade 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 
1 Movement and distribution of moisture in soils. F. S. Harris and H. W. Turpin. 
Jour. Agr. Res., July 16, 1917, p. 117. 
