DUST PREVENTION AND ROAD PRESERVATION, 1916. 9 
used instead of tar for both the penetration and the seal coat, and 
clean screened gravel and pea gravel were substituted for the trap 
rock. The mechanical analyses of the gravels are shown in Table 5. 
Experiment No. 7. — Gravel Construction, with Surface Treatment op Residual 
Asphaltic Petroleum. 
Location, station 82+61 to station 85+61. 
Total length, 300 feet. 
Total area, 533.3 square yards. 
Iii this experiment the surfacing was constructed of a 4-inch layer 
of pit-run gravel, upon which was placed a 1-inch layer of clean, 
washed gravel. The entire course was harrowed with a spike-tooth 
harrow, sprinkled thoroughly and rolled, after which the entire experi- 
ment was opened to traffic for one month. The surface, which then 
had become thoroughly compacted, was roughened with picks and 
harrowed loose to an average depth of 1 inch. The bituminous treat- 
ment followed immediately after the harrowing, and consisted of 
(1) an application of 0.68 gallon of hot residual asphaltic petroleum 
per square yard, covered with pea gravel, and rolled ; and (2) an ap- 
plication of 0.55 gallon of the same oil per square yard, covered with 
sand, and rolled. After the surface had been finished for a day or 
two there was a tendency toward bleeding, and more sand was applied. 
Experiment No. 8 was planned originally as a part of experiment 
No. 7, and no distinction was made between them in the records and 
cost data. Therefore they are considered together in other parts of 
this report. 
Experiment No. 8. — (Originally a Part op Experiment No. 7.) — Gravel Con- 
struction with Surface Treatment of Residual Asphaltic Petroleum. 
Location, station 85+61 to station 87+61. 
Total length, 200 feet. 
Total area, 355.5 square yards. 
This experiment was similar in all respects to experiment No. 7, 
except as follows: The surface treatment consisted of a single appli- 
cation of hot residual asphaltic petroleum, at the rate of 0.693 gal- 
lon per square yard, with a covering of pea gravel. A thin coat of 
dry sand-clay, of very homogeneous quality was applied over the 
surface treatment without any additional oil, and rolling completed 
the experiment. 
summary of cost data for entire project. 
The cost data for the experiments described above are presented 
in Tables 9 to 16. 
4588°— 18— Bull. 586 2 
