. Railroad construction. Theory and practice . Fig. 90.—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 implies a complicated con-nection between the linings of the tunnel and shaft, provided 188 RAILROAD CONSTRUCTION. § 168. 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, illust
. Railroad construction. Theory and practice . Fig. 90.—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 implies a complicated con-nection between the linings of the tunnel and shaft, provided 188 RAILROAD CONSTRUCTION. § 168. 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. 90.* Fig. 91 j 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 Fig. 91.—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-wells for the material they passthrough, and sometimes to such an extent that it is a seriousmatter to dispose of the water that collects at the bottom,requiring the construction of large and expensive drains. 168. 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. Thes
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