THE GREAT ALPINE TTTNNELS. 



621 



When the heading is driven .sufficiently forward to justify the 

 commencement of the main tunnel, a fresh difficulty presents itself. 

 This main tunnel has to ])c driven down hill, and consequently the 

 water collects at the working face A; the bottom can not therefore l)e 

 removed until a bore-hole is put down from A to a. When this is 

 done the remaining- excavation can be taken out, and a further length 

 of tunnel driven to B. A l)ore hole is now sunk from B to />, whilst 

 that from A to a can be plugged up; and thus the tunnel is gradually 

 advanced. 



B}' the adoption of the (iri-eathead shi(dd nuich of this difficulty can 

 be avoided; but one su))a(iueous tunnel through water-bearing strata, 

 at considerable depth, is sufficient for a lifetime. 



As an illustration of th(^ danger to which men are exposed in such 

 work, it is stated, with nuich regret, that in a certain tunnel, notwith- 



f 



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 ^^^^^1^~ 



U^dr: 



7'(yA^/\/£:^ 



^^^^^^^. 



W 



Wa(e' Ic'-ci {-'jr Dratnage) / i.n 600 Jn> 



(Not to ScaCc) 

 Fli;. \. — Diagrammatic section to illustrate method of cDnstructint; tiiniiel helow river lied. 



standing every precaution l)cing taken, all the men engaged in driving 

 the drainage heading by means of a tunneling machine have died; 

 and in the case of the iirst Vyrnwy tunnel crossing of the River 

 Mersey — driving by Greathead shield under pressure — the mortalit}^ 

 was great. 



Having explained in very general terms some of the difficulties of 

 tunnel construction, we will proceed to the case of the great tunnels 

 through the Alps, and for the purpose of rendering the subject more 

 easih^ intelligible, the following particulars may be given: 



Length of tunnel in miles 



North or east portal above sea level feet. . 



South or west portal above sea level. do 



Highest level 



Maximum grade in tunnel per 1,(H)0 



Maximum height of mountain abiive tunnel feet.. 



Possible maximum temperature of rock deg. Fahr. . 



St. Goth- 

 ard. 



9.3 

 3,639 

 3, 757 



3,788 



5.82 



5,598 



85 



Ment 

 Cenis. 



7.98 

 3, 766 



1,1 (U 

 4,LM8 



30 

 5, 428 



85 



Arlberg. 



6.36 

 4, 296 

 3.998 

 4,300 



15 

 2, 362 



65 



Simplon. 



12.26 

 2, 254 

 2,080 

 2,311 

 7 

 7,005 

 104 



