. Railroad construction. Theory and practice. A textbook for the use of students in colleges and technical schools. By Walter Loring Webb . will evidently be in the direction of theline of least resistance. In a strictly homogeneous materialthis will be the shortest line from the center of the explosive tothe surface. The variations in homogeneity on account oflaminations and scams require that each case shall be judo-edaccording to experience. In open-pit blasting it is generallveasy to obtain two and sometimes three exposed faces to therock, making it a simple matter to drill holes so that a
. Railroad construction. Theory and practice. A textbook for the use of students in colleges and technical schools. By Walter Loring Webb . will evidently be in the direction of theline of least resistance. In a strictly homogeneous materialthis will be the shortest line from the center of the explosive tothe surface. The variations in homogeneity on account oflaminations and scams require that each case shall be judo-edaccording to experience. In open-pit blasting it is generallveasy to obtain two and sometimes three exposed faces to therock, making it a simple matter to drill holes so that a blast willdo effective work. When a solid face of rock must be brokeninto, as in a tunnel-heading, thework is necessarily ineffectual andexpensive. A conical or wedge-shaped mass will first be blown outby simultaneous blasts in the holesmarked 1, Fig. %b\ blasts in theholes marked 2 and 3 will then com-plete the cross-section of the head-ing. A great saving in cost mayoften be secured by skilfully takingadvantage of seams, breaks, and irregularities. When the workis economically done there is but little noise or throwing of rock, /. drill holes tunne^l headingFig. 65. 146 BAILROAD CONSTRUCTION. § 120. a covering of old timbers and branches of trees generally sufficingto confine the smaller pieces which would otherwise fly up. 120. Amount of explosive. The amount of explosive requiredvaries as the cube of the line of least resistance. The bestresults are obtained when the line of least resistance is f of thedepth of the hole; also when the powder fills about i of thehole. For average rock the amount of j)owder required is asfollows: Line of least of powder 2 ft. 4 lbs. 6 ft. 8 lbs. Strict compliance with all of the above conditions would re-quire that the diameter of the hole should vary for every this is impracticable, there should evidently be somevariation in the size of the hole, depending on the work to bedone. For example, a V
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