Fig. 150.—Showing the Joining of the Caissons at the Pont Mirabeau Tunnel under the Seine River.
After the caissons had been sunk to the required place and in continuation of one another, a space of nearly 5 ft. was left between them. The construction of the tunnel within the bank of earth separating the two caissons was as follows: A cofferdam was built around this space. It was formed by two diaphragms closing the ends of the tunnel, and by two longitudinal walls sunk as temporary caissons, one on each side of the tunnel and inclosing their ends. This cofferdam was covered with a metal working-chamber whose lower edges rested on top of the four walls of the cofferdam. The joints were made tight by means of rubber or packed clay. The water in the cofferdam was then pumped out, the earth excavated, and the masonry built in continuation of the two ends of the tunnel sections. The submerged sections of the tunnel which were allowed to remain full of water to render them more stable and to save effort in pumping them, were now made dry; the diaphragms were removed from the ends of the caisson tunnels and the work made continuous. [Fig. 149] shows the cross-section of the caissons.
At the Pont Mirabeau crossing of the Seine, a slightly different method was used, described in “Eng. News,” May 18, 1911. The caissons were sunk to the required line and grade with an intervening longitudinal space of 153⁄4 ins. between two adjoining caissons. At each end of this space, which was filled with the river marl, was sunk against the edges of the caissons a hollow cylinder 20 ins. outside diameter. The interior of these cylinders was excavated and filled with concrete, thus forming a continuous wall on both sides of the two adjoining caissons. The earth from the intervening space was then removed and concrete deposited from bottom opening tremies up to the level of the top of the caisson. After nearly one month the tunnel was entered from the shaft and an opening the shape and size of the tunnel section cut through the diaphragms of the 153⁄4-in. wall and the concrete tunnel lining made continuous between the two sections. [Fig. 150] shows the method of joining the caissons.
The Detroit River Tunnel.
[15]—With some modifications which permitted dispensing with compressed air, the tunnel under the Detroit River was built for the Michigan Central Railroad, connecting Detroit with Windsor, Canada. The tunnel is 6625 ft. long; of this, however, only 2625 ft. are under the river, while the approach on the American side is 2000 ft. long and that on the Canadian side, 4000 ft. The tunnel consists of two parallel circular tubes 23 ft. in diameter, built up of 3⁄8-in. steel plate. They are placed 26 ft. apart, center to center, and are connected by diaphragms at 12-foot intervals.
[15] Condensed from a paper by B. H. Ryder.
Each section of the subaqueous tunnel is approximately 262 ft. long. There are ten of these sections and an eleventh a little over 60 ft. long. These tubes were built at the shipyards of the Great Lakes Engineering Works at St. Clair, about 30 miles from Detroit. After the assembling was completed, the ends of each tube were closed by temporary wooden bulkheads to make them float, and the outside sheathed horizontally with heavy timbers bolted to the diaphragms. This sheathing running lengthwise of the tube made a form or pocket, into which the inclosing jacket of concrete was placed. The sections were then launched and towed down to the tunnel site and sunk separately in a trench on the river bottom that had been previously dredged to receive them. This trench was dug to a width of 50 ft. and depth varying from 25 to 50 ft. by clamshell buckets, swung from a scow, working to a depth below the water level of 60 to 90 ft.
As a foundation for the sections, a grillage was constructed on the surface and sunk in place in the trench by derricks swung from a scow. The grillage was placed underneath each joint between the sections and built up of I-beams imbedded in concrete. This grillage is the width of the trench and about 30 ft. long, with posts projecting downward from the four corners, and these were seated into the river bottom, by means of pile drivers, to the desired grade.
Then the eleven sections of the tunnel were lowered and connected, one at a time. By the aid of air tanks placed on each section the movement was controlled until the final sinking upon the grillage in the trench. This operation called into play the greatest engineering skill and ingenuity. When it is considered that the current velocity at the river bed is about 2 ft. per second and much higher along the surface, some idea can be gained of the problems to be overcome. The movement of the enormous sections must be absolutely under control. Thirty-five-ton blocks of concrete were sunk in the river bottom up and down stream to act as anchors, and through them cables were rigged and connected back to the hoisting engines on the derrick scows. These were prevented from moving by spuds at each corner, securely driven into the river bottom at depths sometimes as great as 90 ft. Controlling cables were also run from the sections to the tremie scow to pull one structure close to the adjoining section previously sunk, and the divers made the necessary connection. [Fig. 151] shows cross-sections and plans of the tunnel as given in “Eng. Record,” March 2, 1907.
