204 TUNNELING 



waukee tunnel is sufficient ; to illustrate those of the third class, 

 the Van Buren Street tunnel in Chicago is selected ; and to 

 illustrate those of the fourth class, the East River gas tunnel 

 and the Milwaukee water supply tunnels are excellent examples. 



THE SEVERN TUNNEL. 



The Severn tunnel, which carries the Great Western Rail- 

 way, of England, beneath the estuary of a large river, is 4 miles 

 642 yards long. It penetrates strata of conglomerate, limestone, 

 carboniferous beds, marl, gravel, and sand, at a minimum depth 

 of 44f ft. below the deepest portion of the estuary bed. The 

 following description of the work is abstracted from a paper by 

 Mr. L. F. Yernon-Harcourt. * 



Tfye first work was the sinking of a shaft, 15 ft. in diameter, 

 lined with brickwork, on the Monmouthshire bank of the Severn, 

 200 ft. deep. After the Monmouthshire shaft had been sunk, a 

 heading 7 ft. square was driven under the river, rising with a 

 gradient of 1 in 500 from the shaft on the Monmouthshire shore, 

 so as to drain the lowest part of the tunnel. Near to the first, a 

 second shaft was sunk and tubbed with iron, in which the 

 pumps were placed for removing the water from the tunnel 

 works, connection being made by a cross-heading with the 

 heading from the first shaft. There was also a shaft on the 

 Gloucestershire shore ; and two shafts inland from the first on 

 the Monmouthshire side, to expedite the construction of the 

 tunnel. Headings were then driven in both directions along the 

 line of the tunnel, from the four shafts ; and the drainage head- 

 ing from the old shaft was continued, in the line of the tunnel, 

 under the deep channel of the estuary, and up the ascending 

 gradient towards the Gloucestershire shore, till, in October, 

 1879, it had reached to within about 130 yards of the end of 

 the descending heading from the Gloucestershire shaft. During 

 this period, though the work had progressed slowly, no large 



* Proceedings Inst. C.E., vol. cxxi. 



