248 TUNNELING 



device presented merit, although its design and construction 

 were defective, and he finally undertook the work and carried 

 it to a brilliant success. The tunnel was 1,350 ft. long and 

 7 ft. in diameter, and penetrated compact clay. Work was 

 begun on the first shore shaft on Feb. 12, 1869, and the tunnel 

 was completed the following Christmas, or in something short 

 of eleven months, at a cost of 14,500. 



The shield used was Barlow's idea put into practical shape 

 by Greathead. It consisted of an iron cylinder, or, more 

 properly, a frustum of a cone whose circumferential sides 

 were very slightly inclined to the axis, the idea being that 

 the friction would be less if the front end of the shield were 

 slightly larger than the rear end. The shell of the cone was 

 made of ^ in. plates. The thinned plate cutting-edge of 

 Barlow's shield was replaced by Greathead with a circular 

 ring of cast iron. Greathead also altered the construction of 

 the diaphragm by arranging the angle stiffeners so that they 

 ran horizontally and vertically, and by fastening the diaphragm 

 plates to an interior cast-iron ring connected to the shell plates. 

 This was a decided structural improvement, but it was accom- 

 panied with another modification which was quite as decided 

 a retrogression from Barlow's design. Greathead made the 

 diaphragm opening rectangular and to extend very nearly from 

 the top to the bottom of the shield, thus abandoning the 

 element of safety provided by Barlow in case of an irruption 

 of water. Fortunately the material penetrated by the shield 

 for the Tower tunnel was so compact that no trouble was had 

 from water ; but the dangerous character of the construction 

 was some years afterwards disastrously proven in driving the 

 Yarra River tunnel at Melbourne, Australia. To drive his 

 shield Greathead employed six 2^ in. screw-jacks capable of 

 developing a total force of 60 tons. The tails of the jack bore 

 against the completed lining, which consisted of castriron rings 

 18 ins. wide and i n . thick, each ring being made up of a 

 crown piece and three segments. The different segments 



