260 TUNNELING 



omitted. The shield of the Waterloo tunnel is an example of 

 this construction. In more treacherous materials, however, not 

 only is a diaphragm necessary, but it is also necessary to diminish 

 the size of the openings through it, and to provide means for 

 closing them entirely. Sometimes only one or two openings are 

 left near the bottom of the diaphragm, as in the St. Glair and 

 Mersey tunnel shields ; and sometimes a number of smallei 

 openings are provided, as in the East River and Hudson Rivei 

 tunnel shields. 



In highly treacherous materials subject to sudden and 

 violent irruptions of earth from the excavation face, it some- 

 times is the case that openings, however small, closed in the 

 ordinary manner, are impracticable, and special construction has 

 to be adopted to deal with the difficulty. The shields for the 

 Mersey and for the Blackwall tunnels are examples of such 

 special devices. In the Mersey tunnel a second diaphragm was 

 built behind the first, extending from the bottom of the shield 

 upward to about half its total height. The aperture in the first 

 diaphragm being near the bottom, the space between the second 

 and first diaphragms formed a trap to hold the inflowing material. 

 The Blackwall tunnel shield, as previously indicated, had its 

 front end divided into cells. Ordinarily the face of the excava- 

 tion in front of each cell was left open, but where material was 

 encountered which irrupted into these cells a special means of 

 closing the face was necessary. This consisted of three poling- 

 boards or shutters of iron held one above the other against the 

 face of the excavation. These shutters were supported by 

 means of strong threaded rods passing through nuts fastened 

 to the vertical frames, which permitted each shutter to be ad- 

 vanced against or withdrawn from the face of the excavation 

 independently of the others. Various other constructions have 

 been devised to retain the face of the excavation in highly 

 treacherous soils, but few of them have been subjected to 

 conclusive tests, and they do not therefore justify considera- 

 tion. 



