244 TUNNELING 



sisted of two parallel horseshoe tunnels, 13ft. 9 ins. wide and 

 16ft. 4 ins. high and 1200 ft. long, separated from each other 

 by a wall 4 ft. thick, pierced by 64 arched openings of 4 ft. 

 span, the whole being surrounded with massive brickwork built 

 to a rectangular section measuring over all 38 ft. wide and 

 2-2 ft. high. 



The first shield designed by Brunei for the work proved in- 

 adequate to resist the pressures, and it was replaced by another 

 somewhat larger shield of substantially the same design, but of 

 improved construction. This last shield was 22ft. 3 ins. high 

 and 37 ft. 6 ins. wide. It was divided vertically into twelve 

 separate cast-iron frames placed close side by side, and each 

 frame was divided horizontally into three cells -capable of sepa- 

 rate movement, but connected by a peculiar articulated con- 

 struction, which is indicated in a general way by Fig. 122. To 

 close or cover the face of the excavation, poling-boards held in 

 place by numerous small screw-jacks were employed. Each 

 cell or each frame could be advanced independently of the 

 others, the power for this operation being obtained by means 

 of screw-jacks abutting against the completed masonry lining. 

 Briefly described, the mode of procedure was to remove the 

 poling-boards in front of the top cell of one frame, and excavate 

 the material ahead for about 6 ins. This being done, the top 

 cell was advanced 6 ins. by means of the screw-jacks, and the 

 poling-boards were replaced. The middle cell of the frame was 

 then advanced 6 ins. by repeating the same process, and finally 

 the operation was duplicated for the bottom cell. With the 

 advance of the bottom cell one frame had been pushed ahead 

 6 ins., and by a succession of such operations the other eleven 

 frames were advanced a distance of 6 ins., one after the other, 

 until the whole shield occupied a position 6 ins. in advance of 

 that at which work was begun. The next step was to fill the 

 6-in. space behind the shield with a ring of brickwork. 



The illustration, Fig. 122, is the section parallel to the ver- 

 tical plane of the tunnel through the center of one of the 



