20 TUNNELING 



hand, are the standard tools now, as in the past, for excavating 

 soft-ground tunnels. 



Rock-Excavating Machines. At one period during the work 

 of constructing the Hoosac tunnel considerable attention was 

 devoted to the development of a rock excavating, boring, or 

 tunneling machine. This device was designed to cut a groove 

 around the circumference of the tunnel thirteen inches wide 

 and twenty-four feet in diameter by means of revolving cutters. 

 It proved a failure, as did one of smaller size, eight feet diame- 

 ter, tried subsequently. During and before the Hoosac tunnel 

 work a number of boring-machines of similar character were 

 experimented with at the Mont Cenis tunnel and elsewhere in 

 Europe ; but, like the American devices, they were finally 

 abandoned as impracticable. 



Hand Drills. Briefly described, a drill is a bar of steel 

 having a chisel-shaped end or cutting-edge. The simplest form 

 of hand drill is -worked by one man, who holds the drill in one 

 hand, and drives it with a hammer wielded by his other hand. 

 A more efficient method of hand-drill work is, however, where 

 one man holds the drill, and another swings the hammer or 

 sledge. Another form of hand drill, called a churn drill, con- 

 sists of a long, heavy bar of steel, which is alternately raised 

 and dropped by the workman, thus cutting a hole by repeated 

 impacts. 



In drilling by hand the workman holding the drill gives it a 

 partial turn on its axis at every stroke in order to prevent 

 wedging and to offer a fresh surface to the cutting-edge. For 

 the same reason the chips and dust which accumulate in the 

 drill-hole are frequently removed. The instruments used for 

 this purpose are called scrapers or dippers, and are usually very 

 simple in construction. A common form is a strong wire hav- 

 ing its end bent at right angles, and flattened so as to make a 

 sort of scoop by which the drillings may be scraped or hoisted 

 out of the hole. It is generally advantageous to pour water 

 into the drill-hole while drilling to keep the drill from heating. 



