ON SHIELD TUNNELLING IN LOOSE GKODND. 533 



Tunnel — Ft. In. 



Length, about 800 



Outside diameter 10 



Inside diameter 9 



Low-water level above invert — 



Lancashire end 37 



Cheshire end 31 



High-water spring-tide level above invert- 

 Lancashire end 54 



Cheshire end 48 



The shafts are lined with cast-iron cylinders, the tunnel with cast- 

 iron segments bolted together and planed on the abutting edges. 



The peculiar difficulties of this work arose from the fact that, while 

 the whole of the material (except certain small lenticular masses of clay) 

 through and beneath which the tunnel was driven was perfectly loose 

 and full of water under direct pressure from above, it constantly varied 

 in the size of its component parts from the finest mud to the coarsest 

 shingle, containing comparatively little sand. In homogeneous mud the 

 operations would have been comparatively easy, but the face of the exca- 

 vation was quite commonly composed of three totally dilFerent materials, 

 such as open shingle, sand, and mud, all three under the full and varying 

 pressure of the tidal water above, and all three behaving in a totally 

 different manner ; while in some cases a band of hard clay was met with, 

 impervious in itself, but surrounded by water under pressure, never 

 covering the whole face, but sometimes in the bottom, sometimes in the 

 middle, and sometimes in the top. 



The following considerations will make the great difficulty thus pre- 

 sented more obvious. In sinking a shaft under air pressure we free the 

 ground by excavation within and beneath the circular cutting edge 

 attached to the lovsrest cylinder. That edge being horizontal, we are 

 generally able to maintain the air within the vertical cylinders at such a 

 pressure that the water is kept back at every point of the cutting edge. 

 The cylinders, then, owing to their own or to a superimposed weight, sink 

 farther. They are from time to time lengthened by adding cylinders at 

 the top ; and by freeing the ground below the cutting edge this lining is 

 sunk to the required depth. 



In 1825 Brunei began the use of the first iron tunnelling shield. By 

 this marvellous adaptation of means to ends — of mechanical methods to 

 meet every physical difficulty that great personal experience of tunnelling 

 could suggest — we find an advance in the science of tunnelling which, 

 having regard to the materials then available, was greater than any 

 advance since made. The shield was, like the brickwork forming the 

 twin tunnels, rectangular. It supported the ground in front. It sup- 

 ported the ground overhead, not only by an inverted slipper travelling 

 with the shield, but by an extension backward, consisting of a wrought- 

 iron plate attached to the slipper and resting upon the top of the finished 

 brickwork, over which it was drawn as the shield advanced. For the 

 rest, the shield was divided vertically into twelve vertical frames, each 

 capable of being advanced alternately or in any other order ; and each of 

 these vertical frames supported against the face of the excavation forty- 

 four timber shields, each as long as the frame was wide — viz., 3 feet — and 

 6 inches deep ; and each of these minor shields could be removed or 

 advanced separately. 



