. Railroad construction : theory and practice : a textbook for the use of students in colleges and technical schools . tion. In solidself-sustaining rock, which will not decompose upon exposureto air, it becomes simply a matter of excavating the rock withthe least possible expenditure of time and energy. In softground the heading must be heavily timbered, and as the headingis gradually enlarged the timbering must be gradually extendedand perhaps replaced, according to some regular system, so thatwhen the full cross-section has been ex-cavated it is supported by such timberingas is intended for
. Railroad construction : theory and practice : a textbook for the use of students in colleges and technical schools . tion. In solidself-sustaining rock, which will not decompose upon exposureto air, it becomes simply a matter of excavating the rock withthe least possible expenditure of time and energy. In softground the heading must be heavily timbered, and as the headingis gradually enlarged the timbering must be gradually extendedand perhaps replaced, according to some regular system, so thatwhen the full cross-section has been ex-cavated it is supported by such timberingas is intended for it. The heading issometimes made on the center line nearthe top; with other plans, on the centerline near the bottom; and sometimes tw^osimultaneous headings are run in the twolower corners. Headings near the bot-tom serve the purpose of draining thematerial above it and facilitating theexcavation. The simplest case of head-ing timbering is that shown in Fig. 92,in which cross-timbers are placed at in- yig 92 tervals just under the roof, set in notchescut in the side walls and supporting poling-boards which sus-. 200 RAILROAD CONSTRUCTION. §170. tain whatever pressure may come on them. Cross-timbersnear the bottom support a flooring on which vehicles for trans-porting material may be run and under which the drainagemay freely escape. As the necessity for timbering becomesgreater, side timbers and even bottom timbers must be added,these timbers supporting poling-boards, and even the breastof the heading must be protected by boards suitably braced.
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