. Railroad construction. Theory and practice. A textbook for the use of students in colleges and technical schools. By Walter Loring Webb . 93 —TiMBEEixa FOK Tunnel Heading. supporting timbers are framed into collars in such a manner thatadded pressure only increases their rigidity. 170. Enlargement. Enlargement is accomplished by remov-ing the poling-boards, one at a time, excavating a greater or less PLATE VI. -■ 7^^ ;~^^ ■///// j^^>-4 •:. •^^-.^./ / Tunnel timeeiung—German System (a). :■:::-^:;:>,r.,Xir\ •^v//^ii,

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