. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . at a masonry lining may subsequently be placed insidethe wooden lining and thus postpone a large expense until theroad is better able to pay for the work. In very soft unstablematerial, like quicksand, an arch of cut stone voussoirs may benecessary to withstand the pressure. A good quality of brick isoccasionally used for lining, as they are easily handled and makegood masonry if the pressure is not excessive. Only the bestof cement mortar should be used, econom^^ in this featu
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . at a masonry lining may subsequently be placed insidethe wooden lining and thus postpone a large expense until theroad is better able to pay for the work. In very soft unstablematerial, like quicksand, an arch of cut stone voussoirs may benecessary to withstand the pressure. A good quality of brick isoccasionally used for lining, as they are easily handled and makegood masonry if the pressure is not excessive. Only the bestof cement mortar should be used, econom^^ in this feature beingthe worst of folly. Of course the excavation must include theoutside line of the lining. Any excavation which is made out-side of this line (by the fall of earth or loose rock or by excessiveblasting) must be refilled with stone well packed in. Occasionallyit is necessary to fill these spaces with concrete. Of course it isnot necessary that the lining be uniform throughout the Shafts. Shafts are variously made with square, rectan-gular, elliptical, and circular cross-sections. The rectangular. Fig. 92.—Connection with Shaft, Church Hill Tunnel. cross-section, with the longer axis parallel with the tunnel, ismost usually employed. Generally the shaft is directly over thecenter of the tunnel, but that always impHes a comphcated con-neciion between the Hnings of the tunnel a^d shaft, provided 236 RAILROAD CONSTRUCTION. §202. such linings are necessary. It is easier to sink a shaft near toone side of the tunnel and make an opening through the nearlyvertical side of the tunnel. Such a method was employed in theChurch Hill Tunnel, illustrated in Fig. 92.* Fig. 93 f showsa cross-section for a large main shaft. Many shafts have beenbuilt with the idea of being left open permanently for ventila-tion and have therefore been elaborately lined with masonry.
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