426 



Mr. Francis Fox 



[May 25, 



in the galvanometer, the conclusion could be safely drawn that the 

 wire was hanging freely and truly. 



In driving the necessary adit or heading for drainage purposes 

 beneath a sub-aqueous tunnel, a rising gradient from the shaft 

 bottom of 1 in 500 is allowed, to enable the water at the " face " to 

 flow away from the workmen to the pumps in the " sump " or shaft 

 bottom (see Fig. 4). 



TFtecdolita 



■Wire 



Water Level (For Drainage) I m 500 „ . . V? 





( "Hot to Scale. I 



Fig. 4. — Diagrammatic Section to illustrate Method of Constructing 

 Tunnel below River Bed. 



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 cannot 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 b, 

 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 sub-aqueous 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- 

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

 the drainage heading by means of a tunnelling 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. 



