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. [Illustration: ~HALF CROSS SECTION Y-Y~ ~HALF CROSS SECTION Z-Z~ ~HALF HORIZONTAL SECTION X-X~ ~HALF TOP VIEW~ FIG. 151.--Cross-Sections and Plans of the Detroit River Tunnel.] Steel masts had been previously attached to each end of the sections to enable the engineers on shore to determine the alignment and locate the exact position during the sinking.