After that, the last traces of the rough rock and silt are removed, the iron rings of the tunnel made fast together, the air pressure released, the cutting-shields, that formed so essential a feature of the construction, removed. Then there remains only the work of installing conduits and wiring and laying the tracks before the tunnel is ready for the traffic of the railroad.
The Michigan Central has recently finished a tunnel under the busy Detroit River, at Detroit, which eliminates the use of a car-ferry at that point. The tunnel was built in a manner entirely new to engineers. The river at Detroit is about three-quarters of a mile wide, and its bed is of soft blue clay, making it difficult to bore a tunnel safely and economically. To meet this obstacle a new fashion of tunnel-building was created.
The tunnel itself consists of two tubes, each made from steel ⅜ of an inch in thickness and reinforced every twelve feet by outer “fins.” The channel was dredged and a foundation bed of concrete laid. The sections of the tunnel, each 250 feet long, were then put in position one at a time. The section-ends were closed at a shore plant with water-tight wooden bulkheads. They were then lashed to four floating cylinders of compressed air and towed out to position. After that it was merely a matter of detail to drop the sections into place, pour in more concrete and make the new section fast. The wooden bulkheads next the completed tube were then removed and the structure was ready for the track-layers. The sub-aqueous portion of the new Detroit Tunnel is 2,600 feet long; it joins on the Detroit side with a land tunnel 2,100 feet long, and on the Canadian side with a land tunnel of 3,192 feet.
It takes more than a river, carrying through its narrow throat the vast and growing traffic of the Great Lakes—a traffic that is comparable with that of the Atlantic itself—to halt the progress of the railroad.
CHAPTER V
BRIDGES
Bridges of Timber, then Stone, then Steel—The Starucca Viaduct—The First Iron Bridge in the U. S.—Steel Bridges—Engineering Triumphs—Different Types of Railroad Bridge—The Deck Span and the Truss Span—Suspension Bridges—Cantilever Bridges—Reaching the Solid Rock with Caissons—The Work of “Sand-hogs”—The Cantilever over the Pend Oreille River—Variety of Problems in Bridge-building—Points in Favor of the Stone Bridge—Bridges over the Keys of Florida.