THE GTCEAT ALPINE TUNNELS. 



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 be driven down hill, and consequently the 

 water collects at the working face A; the bottom can not therefore be 

 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 bore hole is now sunk from B to /-», whilst 

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

 advanced. 



By the adoption of the Greathead shield much of this difficulty can 

 be avoided; but one subaqueous tunnel through water-bearing strata, 

 at considerable depth, is sufficient for a lifetime. 



As an illustration of the danger to which men are exposed in such 

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





WaCer L esel (for brain aye) / en 600 {v> 



1 



Fig. 1. — Diagrammat 



(Not to Scate) 

 ction to illustrate method of constructing tunnel below river bed. 



standing every precaution being taken, all the men engaged in driving 

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

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

 Mersey — driving by Greathead shield under pressure — the mortality 

 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 

 easily intelligible, the following particulars may be given: 



Length of tunnel in miles 



North <>r east portal above sea level feet.. 



South or west portal above sea level do 



Highest level 



Maximum grade in tunnel per 1,000 



Maximum height of mountain above tunnel feet.. 



Possible maximum tempera lure of rock deg. Fahr.. 



St.Goth- 

 ard. 



9.3 

 3, 039 



.;, t:>7 



3,788 



5. 82 



5, 598 



85 



Mont 

 Cenis. 



Arlberg. 



7.9S 

 3,766 

 1,164 



4,12-18 

 30 



:.. 128 

 85 



6.36 

 4,296 



3, 998 



4,300 



15 



2, 362 

 65 



Simplon. 



12. 26 



2, 2o4 

 2, 080 

 2,314 



7 



7,005 



104 



