134. Blasting under water has been practised to some extent, and with great success by Messrs. Maillefert and Raasloff, both in New York harbor and in the St. Lawrence River. The method is merely to explode bodies of powder upon the surface of the rock, the water itself being a sufficient source of reaction to the blast.
TUNNELLING.
135. Tunnels are driven through hills to avoid very deep cutting. When in rock of a solid nature, the roof supports itself; but when in earth or in loose rock, an artificial arched lining becomes necessary. Figs. 58 and 59 show sections in both rock and earth; the invert b b is placed in a bed of concrete. In excavating earth, a temporary roof is made use of while the work is in progress, which is afterwards replaced by an arch of brick or stone. The back of the arch must be closely wedged, grouted, and the earth well rammed in.
Fig. 58.
Fig. 59.
The great disadvantages attending the construction of tunnels are want of air, light, room, and drainage. To facilitate the latter requirement, a very light grade may be introduced; this may easily be done, as they generally occur on summits, or on the approach to summits; 1
1000 or five feet per mile is sufficient.
In working a tunnel which is upon a grade, one end naturally drains itself if the approach is taken out; the other drains the wrong way, to meet which obstacle we must resort to pumps which follow the work, keeping always in the lowest place, or by sinking a well at the shaft through which the water is raised to the surface.
The ventilation of tunnels is effected by drawing off the bad air when a fresh supply must enter.