TUNNELS THROUGH SOFT GROUND 141 



heading is not driven very far in advance, the earth from the 

 front is usually conveyed to the rear in wheelbarrows, and 

 dumped into the care standing on the tracks below. In firm 

 soils, where the heading is driven too far in advance to make 

 this method of conveyance inadequate, tracks are also laid on 

 the floor of the heading, and an inclined plane is built connect- 

 ing it with the tracks on the next level below. In place of 

 these inclined planes, and also in place of those between the floor 

 of the tunnel and the level above, some form of hoisting device 

 is sometimes employed to lift the cars from one level to the 

 other. There are some advantages to this method in point of 

 economy, but the hoisting-machines are not easily worked in 

 the darkness, and accidents are likely to occur. 



In the advanced top heading and in the upper part of the 

 section narrow-gauge tracks are necessarily employed, and these 

 may be continued along the floor of the finished section, or the 

 permanent broad-gauge railway tracks may be laid as fast as 

 the full section is completed. In the former case the perma- 

 nent tracks are not laid until the entire tunnel is practically 

 completed ; and in the latter case, unless a third mil is laid, the 

 loads have to be transshipped from the broad- to the narrow- 

 gauge tracks or vice versa. It is the more general practice to 

 use a third rail rather than to transship every load. 



Modifications. Considering the extent to which the Belgian 

 method of tunneling has been employed, it is not surprising 

 that many modifications of the standard mode of procedure 

 have been developed. 'Hie modification which differs most 

 from the standard form is, perhaps, that adopted in excavating 

 the Roosel>eck tunnel in Germany. This method preserves the 

 principal characteristic of the Helgian method, which is the 

 construction of the upper part of the section first; but instead 

 of building the side walls from the bottom upward, they are 

 built in small sections from the top downward. The excavation 

 begins by driving the center top heading No. 1, Fig. 67, whose 

 floor is at the level of the springing lines of the roof arch, and 



