In highly treacherous materials subject to sudden and violent irruptions of earth from the excavation face, it sometimes 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 excavation 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rods passing through nuts fastened to the vertical frames, which permitted each shutter to be advanced against or withdrawn from the face of the excavation independently of the others.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 consideration.