The Saint Clair River tunnel is a single track railroad tunnel built in 1890-91 by the Grand Trunk Railroad. The total length of the tunnel is 6,000 feet, made up of 1,162 feet on the United States side of the river, 844 shield, usually the upper part. The shield for the use of the East Boston tunnel recently constructed was segmental and was used for the roof of the tunnel only, as is also shown in the illustrations. The front portion of the cylindrical shell, consisting of the front portion of the shield as a whole, may be vizor-shaped if material is not too soft. In such a case the upper part of the fresh excavation is protected by the vizor exten sion of the shield. Where the material is stiff enough to hold itself up for a short time the front extension of the shell is not needed.
feet on the Canada side of the river, and 2,310 feet under the Saint Clair River. It is built through the clay underlying the river and has a clear inside diameter of 19 feet 10 inches, the cast-iron shell being two inches thick and having an outside diameter of 21 feet. The cast-iron shell is lined with six inches of brick and cement. The shield was 15 feet 3 inches long and required from 450 to 2,000 tons to move it during construction. The maximum daily progress through the clay was 15.3 feet. The average monthly progress was 230.4 feet from the American side and 219 feet from the Canadian side.
The transverse section of a shield may he circular, elliptical, or any other shape; it may even be rectangular, as in the case of Brunel's Thames tunnel. The shield for the Clichy sewer tunnel in Paris was elliptical in outline. Again, if the material is not too soft, the shield may be segmental only, that is, com prising but a part of the outline of the full It is thus seen that the purpose of the shield is to prevent the inrush of water and soft material while excavation is being made, the diaphragm of the shield acting as a bulk head and the openings in it being so devised as to be quickly closed if necessary. The extension of the shield in front of the dia phragm is designed to prevent the falling or flowing in of the exposed fact of the new excavation. The extension of the shell back from the diaphragm is designed to afford opportunity for putting in place the finished tunnel lining, whether of cast iron or of steel plates lined with masonry. Where the mate rial is saturated with water it is usually neces sary to use compressed air in connection with the shield. The intensity of this air pressure is determined by the depth of the tunnel be low the surface of the water above it. A greater intensity of pressure is needed for the material at the bottom of the tunnel than at the top, but as the pressure required for the bottom must in general be used, there will be danger of ublow-outso at the top, requiring great caution in soft material. With a diameter
of shield of 22 feet the pressure at the bottom of the excavation may exceed the water pres sure at the top by between 9 and 10 pounds per square inch. Such inequalities tend to produce either a "blow-out" at the top of the heading or an inrush of water and soft material at the bottom. The following table was compiled by Mons. R. Le Gouez and shows the dimen sions of the cylindrical shells of some of the principal shields used in tunneling through soft material up to the present time.
The friction on the outside of the shell of the shield against the surrounding material may be over 1,000 pounds per square foot or much less than that. The amount of nomi nal hydraulic jack power required may be roughly taken at 4,000 to 6,000 pounds to every square foot of frictional surface on the outside of the shell of the shield.
Some of the principal later tunnels con structed by the aid .of shields, in addition to the early tunnels of Brunel and others, are the Sar nia tunnel under the Saint Clair River, near De troit, Mich. (already described) ; the East Bos ton tunnel, a part of the rapid transit system of the city of Boston; the two parallel Hudson Manhattan (McAdoo) single track electric railway tunnels opened to the public in 1908, under the North River between Hoboken and New York, and the tunnels under the North and East rivers that were constructed by the Pennsylvania Railroad Company. The accom panying tables of double-track and single-track tunnels relate to some of the principal tunnel structures of the world and give the quality of material penetrated, with the cost per linear foot, as brought together by Charles Prelim, in his
— In the recent extension of the Pennsyl vania Railroad into New York City and Long Island, involving engineering enterprises, the cost has been estimated at $100,000,000. The carrying out of the gigantic project necessitated the construction of numerous tunnels, both land and sub-aqueous. These consist of : (1) The Bergen Hill, N. J., tunnels. (2) The North River tunnels starting from Weehawken, N. J., running beneath the Hudson River to New York City and connecting with the Penn sylvania Railroad station at 33d street, near 11th avenue. (3) Tunnels under 33d and 32d streets from the railroad station to First avenue. (4) East River tunnels from Manhattan shaft of New York City side of the river, running under East River to Long. Island shaft and Long Island City.