. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . The cooling effect of the suddenexpansion of compressed air only reduces the otherwise objection-ably high temperature sometimes found in tunnels. Since pureair is being continually pumped in, the foul air is thereby forcedout. 207. Excavation for the portals. Under normal conditionsthere is always a greater or less amount of open cut precedingand following a tunnel. Since all tunnel methods depend (tosome slight degree at least) on the capacity of the exposed ma-terial to act
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . The cooling effect of the suddenexpansion of compressed air only reduces the otherwise objection-ably high temperature sometimes found in tunnels. Since pureair is being continually pumped in, the foul air is thereby forcedout. 207. Excavation for the portals. Under normal conditionsthere is always a greater or less amount of open cut precedingand following a tunnel. Since all tunnel methods depend (tosome slight degree at least) on the capacity of the exposed ma-terial to act as an arch, tjiere is implied a considerable thicknessof material above the tunnel. This thickness is reduced tonearly zero over the tunnel portals and therefore requires specialtreatment, particularly when the material is very soft. Fig. 98 * * Rziha, Lehrbuch der Gesammten Tiinnelbaukunst. 242 KAILROAD CONSTRUCTION, §208, illustrates one method of breaking into the ground at a loose stones are piled on the framing to give stability to theframing by their weight and also to retain the earth on the. 1 Fig. 98.—Timbering for Tunnel Portal. ,slope above. Another method is to sink a temporary shaft tothe tunnel near the portal; immediately enlarge to the full sizeand build the masonry lining; then work back to the method is more costly, but is preferable in very treacherousground, it being less liable to cause •landslides of the surfacematerial, 2o8. Tunnels vs. open cuts. In cases in which an open cutrather than a tunnel is a possibility the ultimate considerationis generally that of first cost combined with other financial con- IE
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