SIUMARINE TUNNELING 205 



quantity of water had been met with in any of the headings, in 

 spite of their already extending under almost the whole width 

 of the estuary. On October 18, 1889, however, a great spring 

 was tapped by the heading which was being driven landwards 

 from the old shaft, about 40 ft. above the level of the drainage 

 heading ; and the water poured out from this land spring in 

 such quantity that, flowing along the heading, falling down the 

 old shaft, and thus finding its way into the drainage heading 

 and the long heading of the tunnel under the estuary in con- 

 nection with it, it flooded the whole of the workings in com- 

 munication with the old shaft, which it also filled within twenty- 

 four hours from the piercing of the spring. 



To obtain increased security against any influx of water 

 from the deep channel of the estuary into the tunnel, the 

 proposed level portion of the tunnel, rather more than a 

 furlong long under this part, was lowered 15 ft. by increas- 

 ing the descending gradient on the Monmouthshire side from 

 1 in 100 to 1 in 90, and lowering the proposed rail level on 

 the Gloucestershire side 15 ft. throughout the ascent, so as not 

 to increase the gradient of 1 in 100 against the load. A 

 new shaft, 1.8 ft. in diameter, was sunk slightly nearer the 

 estuary on the Monmouthshire shore than the old one; two 

 shafts also were sunk on the land side of the great spring for 

 pumping purposes ; and additional pumping machinery \\ as 

 erected. The flow from the spring into the old shaft was 

 arrested by a shield of oak fixed across the heading; and 

 at last, after numerous failures and breakdowns of the pumps, 

 the headings were cleared of water, after a diver, supplied witli 

 it knapsack of compressed oxygen, had closed a door in the 

 long heading under the estuary ; and the works were resumed 

 nearly fourteen months after the flooding <>< -eurred. The great 

 spring was then shut off from the workings by a wall across 

 tin- heading leading to the old shaft ; and, owing to the lower- 

 ing of the level of the tunnel, a new drainage heading had to 

 be driven from the bottom of the new shaft at a lower level, 



