i/a RAILWAY CONSTRUCTION. 



in quantity and scantling to meet the requirements for wet 

 treacherous clay. 



Figs. 202 and 203 give end view and longitudinal section of 

 timber framework frequently adopted for average tunnel work. 

 The positions of the different pieces will explain themselves and 

 the duty they have to perform. The main struts, or raking 

 pieces, which have to sustain great pressure, may be shored 

 against the finished lengths of masonry or brickwork. The 

 timbering of the sides can be removed as the lining proceeds, but 

 in many cases the round logs and boards near the crown cannot 

 be withdrawn, and have to be left in the work, the space 

 between the top of the arching and under side of the boards 

 being firmly packed with brickwork, masonry, or dry rubble 

 stonework. 



As the tunnel lining is generally carried forward in short 

 lengths, following up the main excavations, the centering for the 

 arching should be of such description that it can be readily 

 transferred or moved forward as the work proceeds. The form 

 of the centering, and the spacing of its upright supports, must 

 admit of sufficient width for one or more lines of rails for the 

 waggons required to remove the excavated debris and convey 

 the building materials used in the lining. 



Picks, bars, and shovels are the tools used in the excavation 

 of the softer material and loose disintegrated rock, but for the 

 hard rock, blasting will be necessary. The tunnel opening being 

 comparatively small, only moderate blasting charges can be used 

 with safety, and these must be placed so as to break up the rock- 

 bed in a suitable manner for working, and without shaking or 

 damaging the already completed excavation. Ordinary hand- 

 drills, or jumpers, may be used for forming the charge holes, 

 a number of them being at work at the same time, and the 

 charges fired very closely one after the other. As the blasting 

 operations necessitate the retiring of the miners to a considerable 

 distance, out of the way of flying fragments, and the remaining 

 away until the foul air has been dispelled, it is advisable to fire 

 off several charges about the same time, and thus minimize as 

 much as possible the stoppage to the drilling and clearing away 

 the loosened material. 



Mechanical drills, worked by compressed air or other motive- 

 power, are now very extensively used where the rock is solid 

 and continuous. They are much more expeditious than the hand 



