srilM.\i:i\K il NNKLINti 259 



Referring first to the horizontal divisions, it may be noted 

 that they serve different purposes in different instances. In the 

 Cliehy tunnel shield the horizontal divisions formed simply 

 working platforms ; in the Waterloo tunnel shield they were 

 designed to ahut closely against the working face by means of 

 special jacks, and so to divide it into three separate divisions; in 

 the St. Clair tunnel they served as working platforms, and also 

 had cutting-edges for penetrating the material ahead ; and in 

 the Blackball tunnel shield they served as working platforms, 

 and had < Mining-edges as in the St Clair tunnel shield, and in 

 addition tin 1 middle division was so devised that the two lower 

 chambers of the shield could be kept under a higher pressure of 

 air than the two upper chambers. Passing now to the vertical 

 divisions, they serve to brace the shell of the shield against ver- 

 tical pressures, and also to divide the horizontal < lianiliers into 

 cells; but unlike the horizontal plates they are not provided 

 with cutting-edges. The St. Clair, Hudson River, and Black- 

 wall tunnel shields illustrate the use of the vertical bracing for 

 the double purpose of vertical bracing and of dividing the hori- 

 zontal cl aml>ers into cells. The Waterloo tunnel shield is 

 an example of vertical bracing employed solely as bracing. 

 The vertical division of the Hast River tunnel shield was 

 employed in order to allow the shield to l>e dissembled in 

 quadrants. 



The Diaphragm. The purpose of the shield diaphragm is to 

 clo.se the rear end of the shield and the tunnel In-hind from an 

 inrush of water and earth from the face of the excavation. It 

 alsosenes the secondary purpose of stiffening the shell diamet- 

 rically. Structurally the diaphragm separates the front-end con- 

 struction previously described from the rear-end construction, 

 which will IHJ descril>ed farther on : and it is usually composed 

 of iron or steel plating reinforced by beams or girders, and 

 pierced with one or several openings by which access is had 

 to tho working face. In stable material, where caving or an 

 inrush of water and earth is not likely, the diaphragm is 



