DUST PREVENTION AND ROAD PRESERVATION, 1914, - q 
EXPERIMENTS AT WEST PALM BEACH, FLA. 
OIL, TAR, OIL-ASPHALT, CORALLINE ROCK. 
These experiments, located on the Miami-Quebec highway, about 
2 miles south of West Palm Beach, were constructed in April and 
May, 1914, by Palm Beach County, under the direction of the Office 
of Public Roads. The purpose was to develop a more economical 
and durable road surface than is obtainable by the use of coralline 
rock under ordinary methods of construction. 
There are in this county about 150 miles of hard-surface roads. 
The material used exclusively for this surfacing is a soft, odlitic 
limestone known as coralline rock, or locally as simply ‘‘rock.” 
It is obtained from two pits in Dade County, which borders Palm 
Beach County on the south. One of these pits, located at Fort 
Lauderdale, furnishes a coralline rock which is softer than the average 
and contains a considerable proportion of sand. For ordinary con- 
struction, it has given apparently satisfactory results. The rock 
quarried from the other pit, located at Ojus, is blasted and excavated 
from under water. It is comparatively hard and contains a relatively 
‘small amount of fine material. It was therefore chosen as the more 
suitable of the two for bituminous construction, and used in the 
experiments. 
The experience gained from the construction of the experiments 
at Miami! and from their inspection after long usage by traffic 
showed conclusively that the best method of treatment involved 
the use of rock from which the finer particles had been removed to 
provide for uniform penetration of the bitumen. The experiments 
at West Palm Beach were therefore all constructed on this basie 
‘principle. The general method of construction preparatory to the 
application of the bitumen was, in brief, as follows: 
The preparation of a thoroughly compacted subgrade parallel 
to the finished surface and composed of the rock originally in the 
ee upon this, a course of screened rock with a uniform loose 
depth of 3 inches, lightly Cee to allow a maximum penetration 
of bituminous material. 
The sereened rock used was produced at great disadvantage and 
consequently high cost. Furthermore, in Addison to screening out 
the material below one-half inch in size, it would have been desirable, 
if practicable, to screen out all above 24 inches. These larger sizes 
had to be picked out by hand by the rakeman on the screen and by 
the spreaders on the road. In any large-scale future work, a small 
crushing plant, elevator, revolving screens, and bins should be 
installed, preferably at the quarry. All rock was spread by shoveling 
back from piles dumped a little in advance of the work. 
1 Bulletin of the U. 8S. Department of Agriculture No. 195. 
93332°—Bull. 257—15——2 
