DUST PREVENTION AND ROAD PRESERVATION, 1916. 17 
yards of surface. After the gravel was spread the surface was rolled 
with the 8-ton tandem roller. 
Earth-Oil Asphalt Experiment. 
Location, station 217+00 to station 223+00. 
Total length, 600 feet. 
Total area, 934.8 square yards. 
The experiment was based upon laboratory investigations which in 
May resulted in the construction of a 50-foot section of earth-asphalt 
on the Arlington Farm of the Department of Agriculture. The 
section selected was a part of an earth road, the soil of which was 
largely fine clay. About 2 inches of this material was removed from 
the surface and the section was brought to a uniform grade and cross- 
section and rolled with a 2-ton tandem roller. The excavated mate- 
rial was hauled to a local paving plant, mixed with water and hot 
oil asphalt to form a mixture of proper consistency, which was 
deposited in a layer about 3 inches thick on the earth subgrade above 
described. The mixture in this case was too stiff to be handled 
satisfactorily with rakes and too soft to be rolled. It was shaped 
in the best manner possible with shovel backs and allowed to dry 
out. The surface hardened rapidly, with the formation of numerous 
contraction cracks. These were filled with a thin grout of the same 
mixture used in the surface, and the section was opened to traffic. 
It remained in excellent condition throughout the summer and 
autumn, although the contiguous earth road was almost impassable 
at times in wet weather. At about the time freezing weather set in 
traffic was cut off the section in order to make some drain connec- 
tions. It soon was observed that the soil-asphalt surface began to 
crack, owing probably to the effect of frost action on the underlying 
subgrade, and in a short time complete disintegration extended from 
the sides toward the center. 
The materials used in the longer experiment about to be described 
were a very homogeneous clay soil, asphalt, and water. The prop- 
erties of the soil and asphalt are shown in Tables 21 and 18, respec- 
tively. 
The mixing was done at the same plant used for mixing the bitu- 
minous gravel concrete, but in an entirely different manner. The 
soil was measured first into the mixer just as it came from the bank. 
Water was admixed to form a rather sloppy mud, after which the 
hot asphalt was added and the mixing was continued until a uniform 
gray color was produced. The temperature of the asphalt was about 
250° F., but the temperature of the fresh mixture was only slightly 
higher than that of the air. The proportions of the mixture were 
such that each batch contained approximately 560 pounds of earth, 
100 pounds of asphalt, and 25 to 30 gallons of water. Results of 
analyses of the mixture show an actual percentage of asphalt of 14.68. 
4588°— 18— Bull. 586 3 
