. Railroad construction : theory and practice : a textbook for the use of students in colleges and technical schools . 200 RAILROAD CONSTRUCTION. §170. tain whatever pressure may come on them. Cross-timbersnear the bottom support a flooring on which vehicles for trans-porting material may be run and under which the drainagemay freely escape. As the necessity for timbering becomesgreater, side timbers and even bottom timbers must be added,these timbers supporting poling-boards, and even the breastof the heading must be protected by boards suitably m .^.. ...._.„. ,,.. „..^ Fig. 9
. Railroad construction : theory and practice : a textbook for the use of students in colleges and technical schools . 200 RAILROAD CONSTRUCTION. §170. tain whatever pressure may come on them. Cross-timbersnear the bottom support a flooring on which vehicles for trans-porting material may be run and under which the drainagemay freely escape. As the necessity for timbering becomesgreater, side timbers and even bottom timbers must be added,these timbers supporting poling-boards, and even the breastof the heading must be protected by boards suitably m .^.. ...._.„. ,,.. „..^ Fig. 93.—Timbering for Tunnel Heading. as shown in Fig. 93. The supporting timbers are framed intocollars in such a manner that added pressure only increasestheir rigidity. 170. Enlargement. Enlargement is accomplished by remov-ing the poling-boards, one at a time, excavating a greater or lessamount of material, and immediately supporting the exposedmaterial with poling-boards suitably braced. (See Figs. 93 and94.) This work being systematically done, space is therebyobtained in which the framing for the full cross-section ma}^ begradually introduced. The framing is constructed with a cross- §171. TUNNEI,S. 201 section so large that the masonr}^ lining may be constructedwithin it. 171. Distinctive features of various methods of are six general systems, known as the English, German,Belgian, French, Austrian, and American. They are so named
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