SII;MAI:INK TINNKUM; 261 



Rear-End Construction. Hy tlie rear end of the shield is 

 that ixu-tion at the rear of the diaphragm. It may be 

 dividrd into two parts, called respectively the body and the 

 tail of the shield. The chief purpose of the body of the shield 

 is to furnish a place for the location of the jacks, pumps, 

 motors, etc., employed in manipulating the shield. It also 

 serves a purpose in distributing the weight of the shield over 

 a large area. To facilitate the passage of the shield around 

 curves, or in changing from one grade to another, it is desirable 

 to make the body of the shield as short as possible. In the 

 Mersey, Clichy, and \Vatt-rloo tunnel shields, and, in fact, 

 in most others which have been employed, the shell plates of 

 the body have been reinforced by a heavy cast-iron ring, within 

 and to which are attached the jacks and other apparatus. The 

 latest opinion, however, seems to point to the use of brackets 

 and In-ams for strengthening the shell for the purpose named, 

 rather than to this heavy cast-iron construction. In the 

 Hudson River, St. Clair River, and East River tunnel shields, 

 A\ ith their long and strongly braced front-end construction to 

 cany the jacks, the body of the shield, so to speak, is omitted, 

 and the rear-end construction consists simply of the tail plat- 

 ing. In the Blackwall shield, the body of the shield shell 

 provides tin- space necessary for the double diaphragms and 

 the cells which they inclose. In a general way, it may be 

 said that the present tendency of engineers is to favor as 

 short and as light a body construction as can IHJ secured. 



The tail of the shield serves to support the earth while the 

 lining is being erected; and for this reason it overlaps the 

 forward ring of the lining, as shown clearly by most of 

 the shields illustrated. To fulfill this iturjM.se, the tail-plates 

 should lc perfectly smooth inside and outside, so as to slide 

 easily between the outside of the lining plates and the earth, 

 and should also be as thin as practicable, in order not to leave 

 a large void behind the lining to be filled in. In soils which 

 are fairly stable, the tail construction is often visor-shaped : 



