154 AGRICULTURAL ENGINEERING 



than either one alone, and it has been demonstrated fully 

 that this is true. In constructing the sand-clay road, suffi- 

 cient clay is added to the sand, or sand to the clay, as the case 

 may be, to fill the open spaces between the sand particles 

 with clay, causing the mixture to form into a very dense and 

 impervious layer. Tests should be made to determine the 

 amount of clay which must be added to the sand, or the 

 amount of sand which must be added to the clay. The re- 

 quired material is hauled to the roads to be improved and the 

 mixture made by plowing, harrowing, and rolling. If after a 

 time it is noticed that the road surface balls up and sticks to 

 the wheels of the vehicles driven over it, there is not a suffi- 

 cient amount of sand in the mixture. On the other hand, if 

 during the dry weather the surface becomes loose, it would 

 indicate that more clay should be added. Sand-clay roads 

 are very cheap, often their cost does not exceed more than 

 SI 00 to $200 per mile and seldom exceeds $400 per mile. 

 The sand-clay road is simply a step toward the gravel road. 



Gravel Roads. Gravel consists of particles of stone 

 which have been rounded by the action of water and ice. 

 and which are deposited in banks. Gravel of the right kind 

 is a material from which a very satisfactory road may be 

 constructed. It is not suited, however, to heavy traffic. 

 It is suited to average country conditions, and in many 

 localities where gravel can be had conveniently it is the most 

 desirable material to use. 



Durability of Gravel. Gravel that is satisfactory for the 

 surfacing of roads should be durable and not so soft as to be 

 ground into dust by much traffic, neither should it be so 

 brittle as to be easily shattered or broken. As a general 

 rule, most gravel may be depended upon to be fairly durable, 

 for if it were not so it would not exist as gravel after having 



