. Railroad construction : theory and practice : a textbook for the use of students in colleges and technical schools . n line Although this is probably the safest method,even these plugs are not always undisturbed, as the material,unless very hard, will often settle slightly as the excavationproceeds. When a tunnel is perfectly straight and not too long,alignment-points may be given as frequently as desired from permanent stations located outsidethe tunnel where they are not liableto disturbance. This has been ac-complished by running the align-ment through the upper part of thecross-section,
. Railroad construction : theory and practice : a textbook for the use of students in colleges and technical schools . n line Although this is probably the safest method,even these plugs are not always undisturbed, as the material,unless very hard, will often settle slightly as the excavationproceeds. When a tunnel is perfectly straight and not too long,alignment-points may be given as frequently as desired from permanent stations located outsidethe tunnel where they are not liableto disturbance. This has been ac-complished by running the align-ment through the upper part of thecross-section, at one side of the cen-ter, where it is out of the way ofthe piles of masonry material,debris, etc., w^hich are so apt tochoke up the lower part of thecross-section. The position of thisline relative to the cross-sectionbeing fixed, the alignment of anyrequired point of the cross-sectionis readily found by means of a lightframe or template with a fixed tar-get located where this line would intersect the frame whenproperly placed. A level-bubble on the frame will assist insetting the in its proper § 163. TUNNELS. 193 In all tunnel surveying the cross-wires- must be illuminatedby a lantern, and the object sighted at must also be powerful dark-lantern with the opening covered with groundglass has been found useful. This may be used to illuminate aplumb-bob string or a very fine rod, or to place behind a brassplate having a narrow slit in it, the axis of the slit and platebeing coincident with the plumb-bob string by which it ishung. On account of the interference to the surveying caused bythe work of construction and also by the smoke and dust in theair resulting from the blasting, it is generally necessary to makethe surveys at times when construction is temporarily sus-pended. 163. Accuracy of tunnel surveying. Apart from the verynatural desire to do surveying which shall check well, there isan important financial side to accurate tunnel surve3dn
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