12 



TUNNELING 



Wire 



sated according to the method of least squares. The probable 

 error in the fixed direction was calculated to be 0.8" of arc at 

 Goeschenen and 0.7" of arc at Airolo. From this it was 

 assumed that the probable deviation from the true center would 

 be about two inches at the middle of the tunnel, but when the 

 headings finally met this deviation was found to reach eleven 

 inches. 



Comparatively few tunnels are driven by working from the 

 entrances alone, the excavation being usually prosecuted at 

 several points at once by means of shafts. In these cases, in 



oi-der to direct the excavation cor- 

 rectly, it is necessary to fix the 

 center line on the bottom of the 

 shaft. This is accomplished in 

 two ways, one being employed 

 when the shaft is located directly 

 over the center line, and the other 

 when the shaft is located to one 

 side of the center line. 



When the shaft is located on 

 the center line two small pillars 

 are placed on opposite edges of 

 the shaft and collimating with the 

 center line as shown by Fig. 3. 

 On these two pillars the points 



corresponding to the center line are correctly marked, and con- 

 nected by a wire stretched between them. To this wire two 

 plumb bobs are fastened as far apart as possible. These plumb 

 bobs mark two points on the center line at the bottom of the 

 shaft, and from them the line is extended into the headings as 

 the work advances. Compass readings are employed to check 

 the transit lines ranged on the plumb bobs. Where there are 

 rocks containing iron ore a miner's transit should be employed 

 for making the compass reading. 



When the shaft is placed at one side of the tunnel the 



FIG. 3. Method of Transferring the 

 Center Line down Center Shafts. 



