OFFICE OF PUBLIC ROADS. 



819 



oil; and 1 macadam road at Clarksville, Tenn., on which only the 

 preparatory work was done. ]\fore details concerninfr this road will 

 he given in the next annual report. In addition, the office supervised 

 the concrete work for 3 bridges with spans of 80 feet, 16 feet, and 

 10 feet, respectively. One other object-lesson road was not finished 

 and two more are not yet reported. The total cost of the work, ex- 

 clusive of the salaries and expenses of engineers furnished by this 

 office, was, as has been given above, $91,877. 



The following table is given to show the number of square yards 

 of each type of road constructed during the fiscal years 1905-1912, 

 inclusive : 



Object-lesson roads constructed during 1905-1912, inclusive. 



BITUMINOUS MACADAM ROADS. 



Sii-VEB Springs, Md. — A bituminous macadam drive 2,776 feet in length from 

 Silver Springs st:ition westward to the Blair estate wns built betrN'eeu June 17, 

 1911, and September 5, 1911. The work was interrupted 6 days by unfavorable 

 weather. The country is rolling. The natural soil is loam from station to 

 station 8, and clay and gravel from station 8 to station 27+76. The earth 

 surface was ]onsene<l with spikes and picks. The maximum grade of 8i per 

 cent was rodured to 54 per cent. Five 6-inch drains, having a total length of 

 210 feet, and one 4-inch drain 18 feet long were laid. A 10-ton roller was used 

 on the work. The maximum haul for excavation was 200 feet, and the average 

 haul from car to road. 2..'00 feet. Broken stone was shipped by rail 49J miles, 

 and water for sprinkling was hauled about ],0O0 feet. 



Limestone of good binding and fair wearing qualities was used for both 

 foundation and surfacing material. Subgrade to the extent of 4,656 square 

 yards was prepared for surfacing. The road was surfaced 12* feet wide for 

 a distance of 2,776 feet. The total width of the finished roadway was IS feet. 

 The bottom course of compacted! stone was 4 inches and the top course from 2 

 to 2* inches, making the total depth from 6 to 64 inches. Tlie sizes of stone in 

 tlie itottoni course v:iry from 1 iiu-li to ?, inches; in the .second course from O.-'i 

 Inch to li inches'; in the third course from 4 to } inch; and in the fourth 

 course from dnst to 4 inch. 



The subgrade and bottom course were built as in the case of water-bound 

 macadam roads. After the No. 2 stone had l)een placed and rolled, 14 gnllons 

 of bitnniinous m:iteri;i] were applie<l in a thin sheet, by steam pressnre, from 

 a tank wagon, which was fitteil with a flexible liose and nozzle, thus giving 

 a very uniform application. 0:i this surface screenings from 4 to 3 iiuii in 

 size were Sjiread in suflicient quantity to prevent the bituminous material from 

 eticking to the wheels of the roller. Another application of 0.6 gallon of 



* Considerable dust unavoldnbly occurred In this course. 

 70481°— AQB 1912 54 



