36 



BULLETIN" 463, U. S. DEPARTMENT OF AGRICULTURE. 



Table 7. — Mileage of sand-elay roads in several representative States for 1909 



and 191Jf. 



States. 



Sand-clay mileage. 



States. 



Sand-clay mileage. 



1909 



1914 



1909 



1914 





None. 



186 



1,107 



561 

 1,151 

 1,916 





202 



730 



2,254 



758 







4,313 

 3,684 













The construction of sand-clay roads is essentially a matter of em- 

 ploying locally available materials to the best practicable advantage 

 in producing an improved earth road surface. To do this involves 

 an intelligent selection from the local materials and an adaptation of 

 the construction method employed to the material selected. These 

 two features of the work will be taken up separately in the order 

 mentioned. 



SELECTION OF MATERIALS. 



The questions that ordinarily must be given principal considera- 

 tion in the selection of materials for a sand-clay road surface are: 

 (1) Is the soil composing the roadbed such that, if local sand or 

 clay were admixed with it in proper proportion, a durable road 

 surface can be produced? (2) Is topsoil of a suitable character 

 available for use as a surfacing material? (3) Can a sufficient quan- 

 tity of natural sand-clay subsoil for surfacing the road be obtained 

 conveniently? (4) Can the two constituent materials be obtained 

 separately and mixed in place on the road? (5) If a variety of 

 materials are available, what selection or combination would give 

 the best results and prove most economical in the long run? In 

 deciding these questions, there are three ways in which the judgment 

 may be assisted materially. These are, in the order of their impor- 

 tance, by means of service comparisons, field examinations, and labo- 

 ratory tests. The questions and the customary manner of deciding 

 them will be discussed briefly in the following paragraphs. 



THE SOIL OF THE EOADBED. 



In order to determine whether the soil of a given roadbed may 

 be incorporated advantageously in a sand-clay road surface, it is 

 necessary, of course, to consider the quality of the other local ma- 

 terials available for mixing with it, since the fitness of the roadbed 

 soil for use in the surface necessarily must be limited by the quality 

 of the other constituent material. In general, there are two condi- 

 tions where the soil of the roadbed may be used advantageously in 

 constructing the surface. One is where the roadbed is very sandy 

 and plastic clay is more readily available for use in the surface than 



