TRANSACTIONS OF SECTION G. 753 



About the same time Mr. J. H. Greathead completed the City and South 

 London Railway in London. It consists of two tunnels, made of cast-iron seg- 

 ments, 10 ft. 6 in. in diameter each, and 3 miles long. Compressed air was only 

 necessary for short distances at three points, as it was principally driven through 

 London clay. 



Previous to the construction of the St. Clair Tunnel compressed air had been 

 used in the Hudson Tunnel, which still remains unfinished owing to financial 

 difficulties. ^ The greater portion of its length is constructed of cast-iron rings of 

 19 ft. 8 in. diameter. 



In 1891 Messrs. Pearson and Son's tender for the construction of the Blackwall 

 Tunnel, amounting to 871,000/., was accepted by the Council, and the work was 

 commenced in 1892. Mr. D. Hay and the author were appointed as resident 

 engineers under Mr. A. R. Binnie, and Mr. E. W. Moir took charge of the works 

 for the contractors. 



The Blackwall Tunnel is much larger than any tunnel yet constructed by the 

 methods adopted. The outside diameter of the St. Clair Tunnel, which is the 

 largest one at present, is 21 ft., while that at Blackwall is 27 ft. in external 

 diameter. 



The following are .some of the leading dimensions : — 



Ft. in. 

 Length from entrance to entrance .... 6,200 

 This total distance is divided as follows : — 



Open approaches, flanked by retaining walls . . 1,735 

 Cut-and-cover portion, built of brick and concrete 1,382 



Cast-iron- lined portion 3,083 



The width of roadway is 16 



And the width of each footpath . . . . 3 IJ 



The tunnel is level under the river, and the gradient on the north side is 1 in 

 34, and on the south side 1 in 36. There are four vertical shafts, two on each side 

 of the river, and varying in depth from 75 ft. to 100 ft. below ground level. Each 

 shaft is a wrought-iron caisson of 58 ft. external diameter at the bottom, and 48 ft. 

 internal diameter throughout, and lined with brickwork. Each caisson consists of 

 two wrought-iron skins, 5 ft. apart, braced together, and terminating in a cutting- 

 edge. Two circular holes, which are temporarily plugged while sinking, are left in 

 each caisson to give way for tlie tunnel through the shaft, and provision is made 

 for an airtight floor above the level of the tunnel when necessary. The space 

 between the two skins is filled with concrete. Two caissons have been sunk, and 

 the two others are in course of being sunk. 



The tunnel is constructed of cast-iron rings 2 ft. 6 in. long, and each ring 

 consists of fourteen segments and a key-piece. The thickness of metal is 2 in. 

 and each segment has flanges 12 in. deep, and both longitudinal and circum- 

 ferential joints are planed. 



The shield used for the construction of the tunnel is 19 ft. 6 in. long, and 

 is 27 ft. 8 in. in external diameter. The outer shell consists of four ^-in. 

 steel plates. The shield is divided into a front and back portion by two vertical 

 diaphragms at right angles to its axis. It is thus possible, when necessary, to 

 have a higher air pressure in the working face of the shield than in the completed 

 portion of the tunnel. The space between these two diaphragms forms an air-lock, 

 both diaphragms, of course, being provided with doors, by which access to the 

 working face is obtained. At the back of this air-lock the shield consists only of 

 the outer shell, which always laps over and outside at least one completed ring of 

 the tunnel, and inside of which all the rings are built. The space of 4 in. 

 left outside the rings when the shield is shoved forward is tilled with grout, 

 forced in by air pressure through screwed holes made in each segment for the 

 purpose. Everything is, therefore, quite solid at the back of the cast-iron lining. 

 At the air-lock and in front of it there is an inner shell, connected stiffly to the 

 outer shell by circular girders and in other ways, and both joining together at the 

 cutting-edge. The working face is divided into four horizontal floors and twelve 

 workinfr chambers by vertical and horizontal diaphragms in the Une of the axis of 

 1894. 3 c 



