. Railroad construction. Theory and practice . ptible of general analysis except as questions ofrailway economics and must be treated individually. 175. Cost of tunneling. The cost of any construction whichinvolves such uncertainties as tunneling is very variable. Itdepends on the material encountered, the amount and kind oftimbering required, on the size of the cross-section, on the priceof labor, and especially on the reconstruction that may be neces-sary on account of mishaps. Headings generally cost $4 to $5 per cubic yard for excava-tion, while the remainder of the cross-section in the sa
. Railroad construction. Theory and practice . ptible of general analysis except as questions ofrailway economics and must be treated individually. 175. Cost of tunneling. The cost of any construction whichinvolves such uncertainties as tunneling is very variable. Itdepends on the material encountered, the amount and kind oftimbering required, on the size of the cross-section, on the priceof labor, and especially on the reconstruction that may be neces-sary on account of mishaps. Headings generally cost $4 to $5 per cubic yard for excava-tion, while the remainder of the cross-section in the same tunnelmay cost about half as much. The average cost of a largenumber of tunnels in this country may be seen from the follow-ing table: * { C^ost per cubic yard Cost perlineal foot. Excavation. Masonry. Material. Single. Single. Double. Single. Double. Double. Hard rock Loose rock Soft . $ $ $ $ $ $ * Figures derived from Drinkers (To face page 195.) LONGITDDINAI, SECTION OP PoKTAI. 196 RAILROAD CONSTRUCTION. § 175. A considerable variation from these figures may be found inindividual cases, due sometimes to unusual skill (or the lack ofit) in prosecuting the work, but the figures will generally besufficiently accurate for preliminary estimates or for the com-parison of two proposed routes. CHAPTER AND MINOR BRIDGES. 176. Definition and object. Although a variable percentageof the rain falling on any section of country soaks into theground and does not immediately reappear, yet a very largepercentage flows over the surface, always seeking and followingthe lowest channels. The roadbed of a railroad is constantlyintersecting these channels, which frequently are normally order to pre^ent injury to railroad embankments by the im-pounding of such rainfall, it is necessary to construct waterwa3sthrough the embankment through which such rainflow mayfreel
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