IT XX ELS THKorci! HAUD ItUCK 85 



neers usually employ [>ercussion drills, while European 

 engineers also use rotary drills extensively. A comparison 

 between these- t\vo types of drills was made in excavating the 

 A a rl berg tunnel in Austria, where the Brandt hydraulic 

 rotary drill was used at one end, and the Ferroux percussion 

 drill was used at the other end. The rock was a mica-schist. 

 The average monthly progress was 412 ft, with a maximum 

 of 646 ft., with the rotary drills, and an average of 454 ft. with 

 the percussion drill. 



Excavation. Since considerable time is required to get the 

 power plant established, the excavation of rock tunnels is often 

 begun by hand, but hand work is usually continued for no 

 longer a period than is necessary to get the power plant in 

 operation. Generally speaking, the greatest difficulty is 

 encountered in excavating the advanced drift or heading. 

 Based on the mode of blasting employed, there are two methods 

 of driving the advanced gallery, known as the circular cut 

 and the center cut methods. In the first method a set of holes 

 is first drilled near the center of the front in such a manner that 

 they inclose a cone of rock ; the holes, starting at the perimeter 

 of the base of the cone, converge toward a junction at its 

 apex. Seldom more than four to six holes are comprise.! in 

 this first set Around these first holes are driven a ring 

 of holes which inclose a cylinder of rock, and if necessary 

 succeeding rings of holes are driven outside of the first rin_ir. 

 These holes are blasted in the order in which they are driven, 

 the first set taking out a cone of rock, the second set enlarging 

 this cone to a cylinder, and the other sets enlarging this 

 cylinder, Thes<> holes are seldom driven deeper than 4 or 5 

 ft In the center-cut method, which is the one commonly 

 employed i:i America, the holes are arranged in vertical rows, 

 and are driven from lf> ft to 20 ft deep. Figs. 60 to 68 

 inclusive show the arrangement of the holes nnd the method 

 of blasting them. The two center rows of holes converge 

 toward each other so as to take out a wedge of rock, but the 



