. Railroad construction. Theory and practice . 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 introduce
. Railroad construction. Theory and practice . 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. Fig. 94. from the origin of the methods, although their use is not con-fined to the countries named. Fig. 95 shows by numbers (1 to 5)the order of the excavation within the cross-sections. The Eng-lish, Austrian, and American s}stems are alike in excavating theentire cross-section before beginning the construction of themasonry lining. The German method leaves a solid core (5)until practically the whole of the lining is complete. This hasthe disadvantage of extremely cramped quarters for work, poorventilation, etc. The Belgian and French methods agree inexcavating the upper part of the section, building the arch atotice, and supporting it temporarily until the side walls arebuilt. The Belgian method then takes out the core (3), removesvery short sections of the sides (4) immediately underpinningthe arch with short sections of the side walls and thus graduallyconstructing the whole side wall. The French method digs outthe sides (3), supporting the arch temporarily with timbers andthen
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