THE GREAT ALPINE TUNNELS. 



623 



70 80 Metres 



Metres. 3 000 



fG*r or 



THC MOUNT*/* A80V£ 

 T/t£ SiA*f>LON T(/*#£Lt 



height of the mountain above the tunnel is 5,428 feet; the temperature 

 of the roek is greatly influenced by this latter fact. 



The question of the temperature of the rocks passed through in the 

 construction of a tunnel 

 is one of great interest, 

 as it depends upon sev- 

 eral conditions: (1) The 

 character of the rock; 

 (•>) the inclination of the 

 beds, those which attain 

 a vertical or nearly ver- 

 tical position being less 

 able to confine the heat 

 than those which are 

 more or less horizontal; 

 (3) the height of the 

 mountain above the tun- 

 nel, or, in other words, 

 the thickness of the 

 blanket. 



A diagram is shown 

 (see fig. 5) giving the 

 temperature actuall} 7 

 encountered in the St. 

 Gothard and Arlberg 

 tunnels, and from these, 

 aided by the carefully 

 prepared geological sec- 

 tion along the center 

 line of the Simplon Tun- 

 nel, an approximate line 

 (in red) is given of the 

 temperatures which are 

 expected. 



The possibilit}^ of 

 cooling the rocks and 

 the air of the tunnel will 

 be dealt with later on, 

 but there is in addition 

 a permanent lowering 

 of the temperature after 

 the tunnel is complete, 

 particulars of which will be given under the description of the St. 

 Gothard. 



For each 144 feet of superincumbent rock or earth the increase is 

 found to be 1° F. 





■S/MPIO* 

 70S Metret 



Fig. 5. — Curves showing depths corresponding to an increase in 

 temperature of 1° C. for the Mont Cenis, Gothard, and Arlberg 

 tunnels (with curve of probable temperature for the Simplon 

 Tunnel). 



