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



sidered complete until after it lias been subjected to traffic for a con- 

 siderable period. During this period the surface should be kept in 

 shape by repeated dragging, and if necessary additional sand or clay 

 should be supplied at points which show weakness owing to these 

 materials not having been mixed in proper proportions. 



Other features of the construction can best be described separately 

 for each of the four sets of conditions mentioned above. 



Case 1. — To construct a sand-clay surface by admixing clay with 

 sand from the roadbed, first grade the roadbed in the manner already 

 described for earth-road construction, but keep the crown flat on the 

 portion to be surfaced. Then spread clay over that part of the road- 

 way which the surface is to cover, to such depth that, when it is 

 mixed with the sand of the roadbed, a surface about 8 inches thick, 

 after compacting, will be obtained. The loose depth of clay neces- 

 sary to secure a finished surface 8 inches thick usually will vary 

 from 4 inches to 6 inches, according to the amount of sand the clay 

 contains. After the clay has been spread, plow up sand from be- 

 neath in sufficient quantity to make the net proportion of sand to clay 

 about 2 to 1. Ordinarily the plow furrows should extend down into 

 the sand from 4 to 6 inches, depending on the compactness of the 

 roadbed and the amount of sand originally present in the clay. But 

 it is better to have too little than too much sand, because it is easier 

 to add sand from the roadbed than to haul additional clay. Next 

 harrow the surface with a disk harrow. Repeat the operations of 

 plowing and harrowing until the sand and clay are mixed thoroughly 

 and uniformly. Then shape up the surface with a grading machine 

 or road drag and permit traffic upon it. After the first soaking 

 rain, plow and harrow the surface again until the surfacing ma- 

 terial practically becomes mud, after which shape up the surface and 

 keep it in shape by repeated dragging until it has dried out and is 

 thoroughly compacted. Watch the road carefully for several months 

 after it is first constructed, to correct deficiencies of sand or clay and 

 to keep the surface in shape. The cross sections shown in figure 20 

 illustrate the principal steps involved in contracting a sand-clay 

 road surface according to the method just described, as well as the 

 one described in the following paragraph. 



Case %. — To construct a sand-clay road surface by admixing sand 

 with clay from the roadbed, the process is exactly similar to that 

 described for Case 1, except that sand is spread over the graded road- 

 bed and clay is plowed up from beneath to mix with it. The depth 

 to which the sand should be spread for an 8-inch finished surface 

 usually varies from 4 inches to .8 inches according to the amount of 

 sand contained originally in the clay of the roadbed. The depth to 

 which the plow furrows should extend down into the clay usually 

 varies from about 3 inches to about 5 inches, and depends on the sand 



