. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . 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 t
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . 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 X. Fig. 93. —Cross-section, Large Main Shaft. The general consensus of opinion now appears to be that shaftsare worse than useless for ventilation; that the quick passage ofa train through the tunnel is the most effective ventilator; andthat shafts only tend to produce cross-currents and are ineffectiveto clear the air. In consequence, many of these elaboratelylined shafts have been permanently closed, and the more recentpractice is to close up a shaft as soon as the tunnel is always form drainage-w^ells for the material they passthrough, and sometimes to such an extent that it is a seriousmatter to dispose of the w^ater that collects at the bottom,requiring the construction of large and expensive drains. 202. Drains. A tunnel will almost invariably strike veins ofwater which will promptly begin to drain into the tunnel andnot only cause considerable trouble and expense during construc-tion, but necessitate the provision of permanent drains for itsperpetual disposal
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