42 BULLETIN 463, U. S. DEPARTMENT OF AGRICULTURE. 
siclered complete until after it has 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. Xext 
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 contructing a sand-clay 
road surface according to the method just described, as well as the 
one described in the following paragraph. 
Case 2. — 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 
