. Shield and compressed air tunneling . asing. Rope haulage in this part of the tunnelis conveniently arranged by installing an electrically or air drivenhoisting engine near the bulkhead, which hauls the cars singlyor in trains by a cable. This system is most satisfactory whenthe tunnel is on a down gradient, so that the cars will move tothe shield by force of gravity, held in check by the cable. Other-wise, another hoisting engine must be placed at or near the shieldand it must be moved forward as the shield advances. 198 Sill ELD TUNNELING 69. Electric Haulage.—In the case of electric haula
. Shield and compressed air tunneling . asing. Rope haulage in this part of the tunnelis conveniently arranged by installing an electrically or air drivenhoisting engine near the bulkhead, which hauls the cars singlyor in trains by a cable. This system is most satisfactory whenthe tunnel is on a down gradient, so that the cars will move tothe shield by force of gravity, held in check by the cable. Other-wise, another hoisting engine must be placed at or near the shieldand it must be moved forward as the shield advances. 198 Sill ELD TUNNELING 69. Electric Haulage.—In the case of electric haulage a bareconductor is placed through the tunnel high enough to clear theheads of men walking in the tunnel, and small locomotives areused to haul the cars. This gives a compact, self-contained unitand has many advantages, including the possibility of handlingthe trains through the locks by means of the locomotive. Thedrawback to the system is the bare conductor, although meanscan be provided to decrease this difficulty (see Fig. 88).. Fig. 88. -Electric locomotive with overhead trolley wire in tunnel. (Courtesyof Hudson and Manhattan Railroad.) 70. Other Methods of Haulage.—Compressed air and storagebattery locomotives would do the work, but their weight requiresa heavy floor and track system. Nevertheless, constant im-provements are being made with these machines and it may beworth inquiring into them for a given project. I. THE EXCAVATION EQUIPMENT 71. Mechanical Excavators.—In Chap. IX several mechanicalexcavating machines which have been used in connection withsoft ground shield driven tunnels are described. These machinesare most suitable in ground of a uniform character and onlywhen it is self sustaining. 72. Rock Drills.—For tunneling through rock the present ten-dency is to use the small type of hand drills instead of the heavy ILANT AND KQllIMKST «)1»
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