. Railroad construction. Theory and practice . 190 RAILROAD CONSTRTJCTIOK. §170. tain whatever pressure may come on them. Cross-timbersnear the bottom support a flooring on ^^hieh 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 Fig. 93.—Timbering for Tunnel Heading. as shown in Fig. 93. The supporting timbers are framed intocollars in
. Railroad construction. Theory and practice . 190 RAILROAD CONSTRTJCTIOK. §170. tain whatever pressure may come on them. Cross-timbersnear the bottom support a flooring on ^^hieh 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 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 pohng-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 may begradually introduced. The framing is constructed with a cross- § 171. TUNNELS. 191 section so large that the masonry lining may be constructedwithin it. 171. Distinctive features of various methods of construction. There are six general systems, known as the English, German,Belgian, French, Austrian, and American. They are so named
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