218 



TUNNELING 



drilling through the test-pipe. The rock beyond the soft 

 seam was a soft white limestone. 



With the prospect of resuming work the question now 

 arose as to the best method of proceeding; and, as a great 

 deal depended upon the success of driving through the present 

 headings, it was strongly recommended that the safest and 

 surest method, that of shield tunneling, be adopted in both 

 headings, although necessarily entailing a large expenditure in 

 plant, and delay in time for installation. This plan met with 



i'w.say*' 



longitudinal Section. 



FIG. 117. Section and Elevation of Shield, East River Gas Tunnel. 



the company's approval, and a shield and hydraulic plant were 

 designed. As the nature of the material to be penetrated be- 

 yond the test-pipes was unknown, this shield was so made that 

 in passing from rock to soft material, or back again to rock, it 

 could be erected or taken apart again with a minimum of time 

 and labor, so that it might almost be called a portable shield 

 (Fig. 117). As in both the tunnel headings there was but one 

 air-lock, and as it was inadvisable to remove the air pressure 

 from the headings, the different parts of the shield had to be of 

 such size as could be passed through the air-lock doors. This 



