I9I0.] DAVIES— TUNNEL CONSTRUCTION. 173 



and obviously, therefore, it is carrying on its boring operations 

 under air pressure equal to the water head. 



Brunei's shield comprised a cutting edge and front shell to form 

 the tunnel and hold the sides and roof; the front, or breast, being 

 held by shutters adjustable to the work as excavated. His shield 

 was to be advanced by screw jacks. The modern shield is a great 

 piece of machinery, but in principle is but little different from that 

 of Brunei. The shield is particularly adapted to the construction 

 of a circular tunnel ; though roofing shields, simply to support the 

 roof while lining an arch, are not at all uncommon. 



Any other form than the circle would be very difficult to use, 

 owing to the trick a shield has of rotating as it advances. The 

 shield structure consists of a drumshell having an internal diameter 

 slightly larger than the external diameter of the finished lining. 

 The front end is shod with very heavy steel castings to form a 

 sharp V-shaped cutting edge protecting and stiffening the shell. 

 The shell is built up of rolled steel plates in two- or three-ply 

 thickness, with all rivets on outside and inside countersunk 

 smooth and no projecting butt straps at joints. This construction 

 gives a smooth external surface to slide through the soil and a 

 smooth flush internal surface in the tail section. The central sec- 

 tion of this drum shell is strengthened by a massive girder construc- 

 tion, which forms the main structure of the shield to withstand the 

 pressures and strains. The rear of this girder construction is plated 

 solid and mud-tight, except for doors in each pocket section which 

 can be opened at the control of the workmen, either to admit mate- 

 rials or allow passage for men. This solid plate of the shield is 

 known as the diaphragm. The depth of the girder construction is 

 regulated largely by the length of the hydraulic jacks, which pro- 

 ject through the diaphragm and are heeled as near the cutting edge 

 as possible. To provide for the jacks there is constructed within 

 the outer drum shell an annular space, stiffened between each jack 

 by heavy girders connecting the outer shell to the internal jack space 

 shell. The portion of the shield projecting in the rear of the dia- 

 phragm, consisting of only a portion of the drum shell (finished 

 flush inside and outside) is known as the "tail." This only serves 

 the function of providing a safe place in the shelter of which to 



