88 I I NNKI.lNci 



non-interference of the shaft with the operations inside the 

 tunnel. Were it not that the side shaft requires the intro- 

 duction of a transverse gallery connecting it with the tunnel, 

 it \\oiild IK- mi the whole superior to the center shaft; but the 

 side gallery ne< essitates turning .tin- cars at right angles, and 

 ci.n>equentlv the use of a very sharp curve or a turntable to 

 reach the shaft bottom, which is a disadvantage that may 

 outweigh its advantages in SOUK- other respects. It is impos- 

 sible to state absolutely which of these methods of locating 

 shafts is the l>est : both present advantages and disadvantages, 

 and the use of one or the other is usually determined more by 

 the local conditions than by any general superiority of either. 



When side shafts are employed they are sometimes made 

 inclined instead of vertical. This form is used when the depth 

 of the shaft is small. By it the hauling is greatly simplified, 

 since the cars loaded at the front with excavated material can 

 be hauled directly out of the shaft and to the dumping-place, 

 surmounting the inclined shaft by means of continuous cables. 

 The short galleries connecting the side shafts with the tunnel 

 proper usually have a smaller section than the tunnel, but are 

 \ ated in exactly the same manner. Another form of side 

 shaft sometimes used is one reaching to the surface when 

 the tunnel runs close to the side of cliff, as is the case with 

 Borne of the Alpine railway tunnels. 



CLASSIFICATION OF TUNNELS. 



Tunnels are classified in various ways, but the most logical 

 method would appear to be a grouping according to the quality 

 of the material through which they are driven ; and this method 

 will IK; adopted here. By this method we have first the fol- 

 lowing general classification: (1) Tunnels in hard rock; (2) 

 tunnels in ordinary loose soil; (3) tunnels in quicksand; 

 ( 4 ) open-cut tunnels ; and (5) submarine tunnels. It is hardly 

 necessary to say that this classification, like all others, is simply 



