. The elements of railroad engineering . is light, portable, and uniform in side walls are all of well-scabbled rubble of good-sizedstones, even beds, and laid in courses with cement impost courses 5 and T should be of well-cut stone,twelve inches in thickness and of full width of wall. Thearch is either brick or rubble. The caps and roof struts 958 RAILROAD CONSTRUCTION. interfere somewhat with arching. Holes are left in themasonry where these timbers interfere until a section of thearch is complete, when they are removed and the gaps filledwith masonry, the joints being
. The elements of railroad engineering . is light, portable, and uniform in side walls are all of well-scabbled rubble of good-sizedstones, even beds, and laid in courses with cement impost courses 5 and T should be of well-cut stone,twelve inches in thickness and of full width of wall. Thearch is either brick or rubble. The caps and roof struts 958 RAILROAD CONSTRUCTION. interfere somewhat with arching. Holes are left in themasonry where these timbers interfere until a section of thearch is complete, when they are removed and the gaps filledwith masonry, the joints being thoroughly grouted. Allother timbers are left in place. The spaces A, /., etc., , between the arch and roof timbers, are usually filledwith concrete. When the material through which the tunnel passes isvery soft, with slight coherence, all the energy and skillof engineer and workmen are required to make is considered the better practice to drive the heading atthe bottom of the tunnel instead of the top, as by the time. Fig. 452. the heading is driven the ground composing the remainderof the section will have become thoroughly drained, and maybe taken out with much greater safety and less expensethan with a top heading. The mode of driving a headingthrough such material is illustrated in Fig. 452, in whichA represents a cross-section and B a longitudinal section ofthe heading, with complete system of timbering. A full section of timl)ers is called a set, of which the up-right timber C is called the leg; the horizontal timber D thesill, and E the cap or collar. The short boards F, F, whichextend from collar to collar, and are in direct contact withthe sustained material, are called/^V/z/^^/;^v?;7/.s\ They aresharpened to a cutting edge, and are driven into the face ofthe heading Avith sledges, a wedge-shaped block G being RAILROAD CONSTRUCTION. 959 placed above them to keep them at a proper angle. Theplanks H which protect the sides of the heading are termedlagg
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectrailroadengineering
