206 TUNNELING 



which was made 5 ft. in diameter, and lined with brickwork, 

 whilst the old drainage heading was enlarged to 9 ft. in diam- 

 eter, and lined with brickwork, so as to aid in the permanent 

 ventilation of the tunnel. The lowering of the level, moreover, 

 converted the bottom tunnel headings into top headings, so 

 that along more than a mile of the tunnel the semicircular arch, 

 26 ft. in diameter, was built first, and then, after lowering the 

 headings, the invert was laid and the side walls were built up. 

 Bottom headings were driven along the remainder of the tunnel, 

 and the work was expedited by means of break-ups. Ventila- 

 tion was effected in the works by a fan 18 inches in diameter 

 and 7 ft. wide, fixed at the top of the new deep shaft ; the rock 

 was bored by drills worked by compressed air ; the blasting was 

 eventually effected exclusively by tonite, owing to its being 

 freer from deleterious fumes than any other explosive ; and the 

 workings were lighted by Swan and Brush electric lamps. The 

 tunnel is lined throughout with vitrified brickwork, between 

 2] ft. to 3 ft. thick, set in cement, and has an invert 1^ ft. to 

 3 ft. in thickness ; the lining was commenced towards the end 

 of 1880, the headings under the river were joined in Septem- 

 ber, 1881, and the last length of the tunnel, across the line of 

 the great spring, was completed in April, 1885. 



Water came in from the river through a hole in a pool of 

 the estuary, close to the Gloucestershire shore, in April, 1881, 

 during the lining of a portion of the tunnel, but fortunately 

 before the headings were joined. This influx was stopped by 

 allowing the water to rise in the tunnel to tide-level, to prevent 

 the enlargement of the hole, which was then filled up at low 

 water with clay, weighted on the top with clay in bags. The 

 great spring broke out again in October, 1883, and flooded the 

 works a second time ; but within four weeks the water had 

 been pumped out and the spring again imprisoned. During 

 this period an exceptionally high tide, raised still higher by 

 a southwesterly gale, inundated the low-lying land on the Mon- 

 mouthshire side of the estuary, and, flowing down one of the 



