. Railroad construction. Theory and practice . ^ not be found wdse to de-crease, for the sake of economy, the dimensions which practicehas shown to be sufficient for the work. The economy thatwould be possible would be too insignificant to justify any the usual dimensions, given in §§ 139 and 140, shouldbe employed. CHAPTER V. 160. Surface surveys. As tunnels are always dug from eachend and frequently from one or more intermediate shafts, it isnecessary that an accurate surface survey should be madebetween the two ends. As the natural surface in a localitywher
. Railroad construction. Theory and practice . ^ not be found wdse to de-crease, for the sake of economy, the dimensions which practicehas shown to be sufficient for the work. The economy thatwould be possible would be too insignificant to justify any the usual dimensions, given in §§ 139 and 140, shouldbe employed. CHAPTER V. 160. Surface surveys. As tunnels are always dug from eachend and frequently from one or more intermediate shafts, it isnecessary that an accurate surface survey should be madebetween the two ends. As the natural surface in a localitywhere a tunnel is necessary is almost invariably very steep andrough, it requires the employment of unusually refined methodsof work to avoid inaccuracies. It is usual to run a line on thesurface that will be at every point vertically over the center lineof the tunnel. Tunnels are generally made straight unlesscurves are absolutely necessary, as curves add greatly to thecost. Fig. 85 represents roughly a longitudinal section of the. Fig. --€000 ^ -tOOO -r 600G r—-50C^—i 85.—Sketch of Section of the Hoosac Tunistel. Hoosac Tunnel. Permanent stations were located at A, B, (7,D, E, and F, and stone houses were built at A, B, C, and were located with ordinary field transits at first, and thenall the points were placed as nearly as possible in one verticalplane by repeated trials and minute corrections, using a verylarge specially constructed transit. The stations D and F werenecessary because E and A were invisible from C and B. Thealignment at A and E having been determined with great accuracy, the true alignment was easily carried into the tunnel. 179 180 RAILROAD CONSTRUCTION. § 161. The relative elevations of A and E were determined withgreat accuracy. Steep slopes render necessary many settingsof the level per unit of horizontal distance and require that thework be unusually accurate to obtain even fair accuracy perunit of distance. The levels are usually re-run
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