Compressed Air Work And Diving 1909

104 COMPRESSED AIR WORK. It will be necessary, when working round the cutting edge, to carry the holes well behind it, and it must also be remembered, especially when low grade explosives are being used, that a length of 6 in. of the hole drilleri and the rock surrounding it will usually remain intact after the charge has been firecl. In tunnel work the biasting must be starteel with a small heading, and if possible this should be kept in advance of the main tunnel. There are two ways of starting’ such a heading; one is to drill four or more holes meeting at a point, so that when the biast is made the rock comes away leaving a cone-shaped hole. The other way is to drill two vertical lines of holes inelining towards each other, so that the rock is brought away in a V-shaped weclge. In either case the subsequent holes will be in a direction nearly parallel to the direction of the tunnel. Here again, although it is best to have some system, the skiil of the driller in pointing his drill is of great value. The author knew one drill foreman who pointeel his drills according to no known law, and never- theless did great execution, in which unfortunately the shield sometimes had its share. Great care must be taken when working round the cutting edge of the caisson to see that it is not damaged by using too heavy charges. It is also important that the holes should be so placed that the shattered rock is not hurled in a direc- tion where it will do damage. With a shield of rather light design the author has found it necessary to use planks placed in front of the divisions forming the com- partments of the shield, in order to prevent them from getting bent or fractured. I )rills for caisson work are usually mountecl 011 tripods, and the sizes vary from 2 in. diameter of cylinder and 5 in. stroke to 5 in. diameter of cylinder and 8 in. stroke.