534 REPORT— 1892. 



Next came Mr. Peter Barlow, who more nearly imitated, in horizontal 

 driving, the sinking of a cast-iron cylinder. He acknowledged his 

 indebtedness to Brunei, the great originator of shield tunnelling. He 

 substituted cast iron for brickwork, made his tunnel circular, and sur- 

 rounded his circular shield by a cylinder which overlapped the tunnel, 

 not merely at the top, but all round. For the screws used by Brunei to 

 propel his great frames and minor shields Mr. Barlow substituted 

 hydraulic rams, and he pushed the shield forward in stages to a sufficient 

 extent to enable him to add ring by ring to the forward end of the tunnel 

 lining. 



This device was explained by Mr. Barlow in a pamphlet published in 

 1867, ' On the Relief of London Street Trafl&c, with a Description of the 

 Tower Subway, now shortly to be executed.' According to his method, 

 he carried out with complete success the Tower Subway, eight feet in 

 diameter, beneath the Thames. Since that time Mr. Greathead has 

 introduced many ingenious improvements and additions. 



In applying this method directly loose materials under water pressure 

 are met with the much greater difficulty of driving horizontally than of 

 smking vertically is experienced. If artificial air pressure is employed — 

 and in many cases it has not yet been found practicable to dispense with 

 it — new difficulties arise. 



The conditions of equilibrium between the air pressure, within the 

 tunnel and the water pressure without the tunnel are obviously diffei'ent 

 at different levels of the face. Such equilibrium as permits the work to 

 be continued is difficult to attain, and when attained is highly unstable. 

 To meet this difficulty Mr. Greathead has devised many ingenious 

 arrangements, in one of which he excavates by means of hydraulic jets in 

 front of the closed shield or diaphragm (still surrounded by the annular 

 shield overlapping the finished work), and removes the fluid ground by a 

 pipe passing through the shield to the tunnel. 



But the great variety of material met with in the Mersey Aqueduct 

 Tunnel, including large shingle and in many cases heavy oak timber in 

 good condition, lying prostrate across or parallel with the axis of the 

 tunnel, rendered the use of any such methods impracticable. 



In order to understand rightly the nature of the problem, regard must 

 be had to the following considerations : — 



Imagine a face with loose material at the end of the finished portion 

 of the tunnel subject to the pressure, let us say, of 30 feet of water to the 

 top, and of 40 feet of water to the bottom of the tunnel, the tunnel being 

 charged with air to balance, let us suppose, the 40 feet. One of two 

 things immediately takes place. Either the air pressure, overbalancing 

 the water pressure where at the higher part of the face it has only a 

 head of 30 feet, rushes out in such quantity as to form an upward funnel 

 through the strata, in which case water rushes in below where the pres- 

 sure in front of the shield is 40 feet ; or the air simply displaces the water 

 in the interstices of the porous ground. This latter action is also very 

 rapid. The wet surface of the face seems instantly to dry up ; in fact, so 

 complete and rapid is the drying that in a few seconds.it is difficult to 

 imagine that the face has been wet within the last twenty-four hours. But 

 with this change the whole condition of equilibrium is altered. The head 

 of water at the face is partly displaced by a head of air — a great bubble — 

 in the interstices of the ground, having the form of a balloon, continuously 

 supplied with air, from the end of the tunnel, which it dissipates from its 



