60 BULLETIN *724, U. S. DEPARTMENT OF AGRICULTUBE, 



fairly well by particles of stone crushing under the roller and in such cases the 

 addition of sand or screenings may not be necessary. The character of the 

 road crust also should be considered in deciding this ix)int. 



6. In order to lock the particles of stone together satisfactorily it frequently 

 is necessary to sprinkle a moderate amount of water over each layer of the 

 foundation while the rolling is in progress. This is especially true of limestone 

 where screenings are added after the stone is spread. In all cases where 

 water is used it is merely to flush the fine chips and dust into the interstices 

 and by no means to produce a grouted surface, and it should be applied gently 

 and in such quantities as not to saturate the subgrade and should be done only 

 on sections of the foundations where rolling is in progress. 



COST OF CKUSHED-STONE FOUNDATIONS. 



The cost of constructing a crushed-stone foundation, after the 

 stone has been delivered on the work, usually can be estimated with a 

 fair degree of accuracy when the conditions affecting the work are 

 understood. If the stone is hauled out in suitable dump wagons and 

 the price of labor is about 20 cents per hour, the spreading can be 

 done at an average total cost of from 6 to 8 cents per cubic yard. 

 Where the stone is dumped on boards and spread by hand this cost 

 ordinarily will be just about doubled. 



The cost of rolling varies with the quality of the stone used. If 

 a hard limestone or trap is used, about 100 cubic yards can be com- 

 pacted per 10-hour day with a 10-ton roller, while with sandstone or 

 soft limestone the quantity compacted per 10-hour day may be as 

 much as 150 or 175 cubic yards. Assuming the loose depth of the 

 foundation at 9 inches and the cost per day of a roller, including 

 labor, fuel, and depreciation, at $12, the total cost of rolling would 

 average from 2 to 3 cents per square yard. The cost of spreading 

 and rolling combined would average from 3^ to 5 cents per square 

 yard. 



The cost of quarrying and crushing varies with the character of 

 the stone, the layout of the quarry, and the equipment employed. 

 Some of the largest producers of crushed limestone market their 

 product at about 50 cents per ton, or 65 cents per cubic yard, while 

 in other cases, where the quariy face is shallow and the plant is small, 

 the actual cost of quarrying and crushing sometimes is more than 

 $1 per cubic yard. The following record is of a limestone quarry in 

 lioudon County, Tenn., operated under the supervision of the Bureau 

 of Public Roads in connection with a single road improvement 

 project, ma}^ be of assistance in estimating costs for similar work 

 in other localities. The quarry had been faced up, but required 

 heavy stripping, and the presence of numerous clay seams made 

 drilling and l)lasting expensive. The stone is a dark-blue, tough 

 limestone, very similar in appearance to Knoxville dolomite. The 

 depth of quarry face was from 12 to 18 feet. Blasting was done 



