256 TUNNELING 



with its major axis horizontal, was used, and the German en- 

 gineer, Herr Mackensen, has designed an oval shield, with its 

 major axis vertical. A semi-elliptical shield was employed on 

 the Clichy tunnel, and semi-circular shields were used on the 

 Baltimore Belt Line tunnel and the Boston Subway in Amer- 

 ica. Generally, also, tunnel shields are right cylinders ; that is, 

 the front and rear edges are in vertical planes perpendicular to 

 the axis of the cylinder. Occasionally, however, they are 

 oblique cylinders ; that is, the front or rear edges, or both, are 

 in planes oblique to the axis of the cylinder. One of these 

 visor-shaped shields was employed on the Clichy tunnel. 



The Shell. It is absolutely necessary that the exterior sur- 

 face of the shell should be smooth, and for this reason the 

 exterior rivet heads must be countersunk. It is generally 

 admitted, also, that the shell should be perfectly cylindrical, 

 and not conical. The conical form has some advantage in 

 reducing the frictional resistance to the advance of the shield ; 

 but this is generally considered to be more than counterbalanced 

 by the danger of subsidence of the earth, caused by the exces- 

 sive void which it leaves behind the iron tunnel lining. For 

 the same reason the shell plate, which overlaps the forward ring 

 of the lining, should be as thin as practicable, but its thickness 

 should not be reduced so that it will deflect under the earth 

 pressures from above. Generally the shell is made of at least 

 two thicknesses of plating, the plates being arranged so as to 

 break joints, and, thus, to avoid the use of cover joints, to inter- 

 rupt the smooth surface which is so essential, particularly on 

 the exterior. The thickness of the shell required will vary 

 with the diameter of the shield, and the character and strength 

 of the diametrical bracing. Mr. Raynald Legouez suggests as 

 a rule for determining the thickness of the shell, that, to a 

 minimum thickness of 2 mm., should be added 1 mm. for every 

 meter of diameter over 4 meters. Referring to the illustrations, 

 Figs. 128 to 132 inclusive, it will be noted that the St. Clair 

 tunnel shield, 21 ft. in diameter, had a shell of 1-in. steel 



