The form and arrangement of the timbering, as well as the number, sizes, and positions of the pieces, must be determined by the material of the excavation and the contour line of the finished arching or lining. The framework, which would be sufficient to support ordinary soft material, must be largely augmented both
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the sides can be removed as the lining proceeds, but in many cases the round logs and boards near the crown cannot be withdrawn, and have to be left in the work, the space between the top of the arching and under side of the boards being firmly packed with brickwork, masonry, or dry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 débris 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