The Diaphragm.
—The purpose of the shield diaphragm is to close the rear end of the shield and the tunnel behind from an inrush of water and earth from the face of the excavation. It also serves the secondary purpose of stiffening the shell diametrically. Structurally the diaphragm separates the front-end construction previously described from the rear-end construction, which will be described farther on; and it is usually composed of iron or steel plating reinforced by beams or girders, and pierced with one or several openings by which access is had to the working face. In stable material, where caving or an inrush of water and earth is not likely, the diaphragm is 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. Clair and Mersey tunnel shields; and sometimes a number of smaller openings are provided, as in the East River and Hudson River tunnel shields.
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rods passing through nuts fastened to the vertical frames, which permitted each shutter to be advanced against or withdrawn from the face of the excavation independently of the others.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.
Rear-end Construction.
—By the rear end of the shield is meant that portion at the rear of the diaphragm. It may be divided into two parts, called respectively the body and the tail of the shield. The chief purpose of the body of the shield is to furnish a place for the location of the jacks, pumps, motors, etc., employed in manipulating the shield. It also serves a purpose in distributing the weight of the shield over a large area. To facilitate the passage of the shield around curves, or in changing from one grade to another, it is desirable to make the body of the shield as short as possible. In the Mersey, Clichy, and Waterloo tunnel shields, and, in fact, in most others which have been employed, the shell plates of the body have been reinforced by a heavy cast-iron ring, within and to which are attached the jacks and other apparatus. The latest opinion, however, seems to point to the use of brackets and beams for strengthening the shell for the purpose named, rather than to this heavy cast-iron construction. In the Hudson River, St. Clair River, and East River tunnel shields, with their long and strongly braced front-end construction to carry the jacks, the body of the shield, so to speak, is omitted and the rear-end construction consists simply of the tail plating. In the Blackwall shield, the body of the shield shell provides the space necessary for the double diaphragms and the cells which they inclose. In a general way, it may be said that the present tendency of engineers is to favor as short and as light a body construction as can be secured.
The tail of the shield serves to support the earth while the lining is being erected; and for this reason it overlaps the forward ring of the lining, as shown clearly by most of the shields illustrated. To fulfill this purpose, the tail-plates should be perfectly smooth inside and outside, so as to slide easily between the outside of the lining plates and the earth, and should also be as thin as practicable, in order not to leave a large void behind the lining to be filled in. In soils which are fairly stable, the tail construction is often visor-shaped; that is, the tail-plates overlap the lining only for, say, the roof from the springing lines up, as in one of the shields for the Clichy tunnel. In unstable materials the tail-plating extends entirely around the shield and excavation. The length of the tail-plating is usually sufficient to overlap two rings of the lining, but in one of the Clichy tunnel shields it will be noticed that it extended over three rings of lining. This seemingly considerable space for thin steel plates is made possible by the fact that the extreme rear end of the tail always rests upon the last completed ring of lining.
In closing these remarks concerning the rear-end construction, the accompanying table, prepared by Mr. Raynald Légouez, will be of interest, as a general summary of principal dimensions of most of the important tunnel shields which have been built. The figures in this table have been converted from metric to English measure, and some slight variation from the exact dimensions necessarily exists. The different columns of the table show the diameter, total length, and the length of each of the three principal parts into which tunnel shields are ordinarily divided in construction as previously described:
| Name of Shield. | Length in Feet. | ||||
|---|---|---|---|---|---|
| Diameter. | Tail. | Body. | Front. | Total. | |
| Concorde Siphon | 6.75 | 2.51 | 2.55 | 1.16 | 6.67 |
| Clichy Siphon | 8.39 | 2.51 | 2.55 | 1.16 | 6.16 |
| Mersey | 9.97 | 5.61 | 2.98 | 2.98 | 11.58 |
| East River | 10.99 | 3.51 | 0.32 | 3.67 | 7.51 |
| City and South London | 10.99 | 2.65 | 2.82 | 1.01 | 6.49 |
| Glasgow District | 12.07 | 2.65 | 2.82 | 1.01 | 6.49 |
| Waterloo and City | 12.99 | 2.75 | 2.98 | 1.24 | 6.98 |
| Glasgow Harbor | 17.25 | 2.75 | 2.98 | 1.08 | 8.49 |
| Hudson River | 19.91 | 4.82 | 2.98 | 5.67 | 10.49 |
| St. Clair River | 21.52 | 4.00 | 2.98 | 11.25 | 15.25 |
| Clichy Tunnel | 23.7-19.8 | 4.00 | 2.98 | 6.88 | 17.22 |
| Clichy Tunnel | 23.8-19.4 | 7.44 | 11.90 | 4.46 | 23.65 |
| Blackwall | 27.00 | 6.98 | 5.90 | 6.59 | 19.48 |
| Waterloo Station | 24.86 | 3.34 | 5.51 | 1.14 | 10.00 |
A shield of 60 or 100 tons weight can hardly be directed along the line of the proposed tunnel and also through curves and grades, especially when driven through loose or muddy soils. The tunnels of the New York and Hudson River Railroad under the Hudson, and the tunnel of the New York Rapid Transit Railway under the East River, show marked evidence of how troublesome this work is. To avoid these and other inconveniences encountered in every shield, the Author has designed a new shield which was briefly described at [page 251].
