EARTH, SAND-CLAY, AND GRAVEL ROADS. 
43 
content of the clay and the compactness of the roadbed. It should 
be borne in mind that, in this case, it is better to have a surplus of 
sand than a surplus of clay in the surface, because the former is more 
easily corrected. 
Case 3. — Figure 21 illustrates the two principal steps involved in 
constructing a sand-clay surface with topsoil or other natural sand- 
clay mixture. The roadbed should be graded in the usual way, ex- 
cept that the portion to be surfaced should be trenched out with the 
grading machine to receive the surfacing material. The surfacing 
material should be spread to such a depth that it will be at least 8 
— s — 
Flat or slightly crownec H 
Cross section ready to receive the surfacing material 
■ r See text for this depth 
Cross section showing material spread on the road 
ready for mixing 
Cross section of completed road 
TYPICAL CROSS SECTIONS SHOWING METHOD OF CONSTRUCTING 
SAND-CLAY ROAD BY MIXING SAND OR CLAY WITH THE ROAD BED MATERIAL. 
"W'should be at least 12 feet for single track road, and "S'generally not less than 
5 feet, for double track "W* should be not less than 14 feet. and"S % 'not less than 3feet. 
Uncrown, varies from A" to the foot for level grade, to I" to the foot for a grade 
of5%. 
Fig. 20. 
inches thick at the center when compacted. After this is done per- 
mit traffic upon the road and keep the surface in shape with a road 
drag while it is being compacted. Where surfacing material may be 
obtained cheaply trenching is frequently omitted altogether, and the 
artificial surface is extended out over the shoulders. 
Case 4- — Where it is necessary to bring the sand and clay on the 
road separately and mix them in place, the method employed should 
be a combination of the methods already described. That is, a 
trench should be formed with the grading machine to receive the 
bottom layer of material, which may be either the sand or the clay, 
and the top layer should then be spread and the mixing done as 
described in Case 1 or Case 2. The depths of the respective layers 
should be such as to secure a completed surface at least 8 inches 
thick, and the proportions should be fixed in the manner already 
described in discussing the other cases. Where the roadbed material 
