EARTH, SAND-CLAY, AND GRAVEL ROADS. 37 



a natural mixed sand-clay. The other is where the roadbed consists 

 of a good quality of plastic clay and sand is available for the surface. 

 In either case the appearance of the roadbed material may some- 

 times be very misleading under certain weather conditions, and be- 

 fore deciding whether it is suitable for use in the surface its behavior 

 should be observed for all weather conditions to which the surface 

 will be subjected. 



For example, an earth road which appears very sandy in dry 

 weather in reality may be composed of soil that will be converted 

 into mud by even a very moderate rain. This is particularly true 

 of soils that contain a high percentage of either very fine sand or 

 silt. Where the roadbed is composed of such soil, no part of it should 

 be incorporated in a sand-clay surface. The sand contained in a 

 sand-clay surface is supposed to supply stability to the surface in wet 

 seasons, and the roadbed material, to be adapted for this purpose, 

 must be composed of sandy soil that will increase in stability with an 

 increase in moisture content, and will not become muddy under any 

 circumstances. 



Qn the other hand, a roadbed that in wet seasons appears to be 

 composed of clay suitable for use in a sand-clay surface may crumble 

 into fine dust on drying out under even very light traffic. While a 

 road surface constructed by mixing sand with soil of this kind might 

 be a considerable improvement over the old earth road in both wet 

 and dry weather, it would be very much inferior to a surface con- 

 structed by mixing together a good quality of npnslaking clay and 

 sand. 



In the Piedmont and upper Coastal Plain regions of the South 

 Atlantic States it happens often that the soil over limited areas of 

 land is of such quality that a road surface made of it will possess all 

 the desirable characteristics of a well-constructed sand-clay surface. 

 A rather large mileage of roads has already been surfaced with soil 

 of this kind and much of it is giving excellent satisfaction. Such 

 roads are known locally as " topsoil roads." 



The best topsoil for road surfacing usually is found in fields which 

 have been under cultivation for a numbar of years. The probable 

 reason for this is not only that cultivation produces a more intimate 

 mixture of the soil constituents, but that the repeated aeration tends 

 to improve the stability of the soil by causing the oxidation of certain 

 component minerals. Also, where the original soil contains a too 

 high percentage of clay, cultivation may have improved its quality by 

 increasing the rate at which clay is leached out. Since cultivation 

 ordinarily extends to a depth of only a few inches below the ground 

 surface, the layer of soil suited for use in road surfacing is, in most 



