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June, 1913 form total displacement. As _ previously stated, the tubes under normal condi- tions are supported throughout their whole length, . but submerged viaducts resting on piers should be designed to carry their own weight in case of acci- dental flooding. In such a _ condition, water and moving load would not occur together, and the only buoyancy would be that from the submerged metal, con- crete and other materials, amounting to the difference between their weight in air and water. The weight of the submerged tube should be such as to nearly equal the weight of the water' displaced; If slightly less than the weight of water, there will be an upward pull on the anchors or piers when empty, and a corresponding downward thrust under moving load, and if so arranged that this upward and downward pressure are equal, the forces under normal con- ditions will be a minimum, and the cross section of the tube may be made nearly or quite uniform throughout. It should, however, be proportioned to support its own weight if flooded. Flotation By way: of illustration, a ttbe is assumed which is 18 ft. diameter, made of '4-in, steel plates. Ji fully sub- merged it would displace 255 cu. ft. of water per lineal foot, weighing about 8 tons. But since its circumference is 5614 ft, its weight with 1%-in. metal would not exceed 1 ton per lineal foot, - and if other materials in, the tube weighed another ton per foot, or a total of 2 tons altogether, the tube would be only one-quarter submerged. The buoy- ancy of a tube 24 ft. in outside diame- ter, including concrete walls 3 ft. thick, would be about 15 tons per lineal foot, and by varying the wall thickness, the weight can be adjusted and made slight- ly less than its buoyancy, that it may- not sink. Tubes may have double metal cylin- ders, of either wrought iron, cast iron or wrought steel, placed one inside the other, with a filling of concrete between them. When of. structural steel, the inner and outer tubes may be % and 14 inch thick respectively, those actually built under the river at Detroit being each 3 in. thick. If cast iron is used it would be much thicker, perhaps 1 to 2 in. The thickness of concrete walls should be made to suit the required buoyancy, and will probably be from 114 to 3 ft. A fine light effect may be secured inside the tubes by lining the whole interior with enameled brick, as proposed for a tunnel at Sydney. The metal may be protected by cov- ering it with cement or concrete, or by surrounding it with a case of sheet copper, a precaution which is hardly THE MARINE REVIEW necessary. Materials that are submerged in sea water soon become coated with barnacles and other growth, which in themselves are a protection, and in any case, if the outer tube should be wholly destroyed by rust, the solid wall of con- crete would still remain. Tube sections may be of any con- venient length up to 500 ft., those at Detroit being 260 ft. long, though 400-ft. lengths have been proposed for other places, -- Anchors When water depth does not exceed about 100 feet, a submerged viaduct or tube on piers, is preferable to a floating tube held in place only by floats and anchors. Piers are effective anchors and they also hold the tube in line, but in water which is too deep for piers, the tubes must be anchored down with chains or ropes. 'For this purpose, hollow cast iron screw piles 2 to 3 feet in diameter, with 12 to 18-in. blades, may be screwed into the channel bot- tom, the turning shaft being held in position by anchored scows, and turned by steam tugs. The pulling resistance of ad-ft. pile 20.11, in the ground is quite uncertain, but has been given by one authority as 2,000 to 3,000 tons, Rotation, Ventilation and Safety Pre- When the width of tube is much greater than its depth, as, for example, when two or more tracks lie side by side, rotation is avoided and the struc- ture will float in its proper or horizon- tal position. But the rotation of single track tubes must be prevented by proper connection to the floats and anchors. Piers Power and ventilation in tunnels and tubes, are closely related, and any mo- tors producing gas, steam or smoke should be avoided. Electric traction is good, but compressed air, if properly applied, may be better. Each train should in itself be a piston, fitting tightly into the tube to keep the air a motion. The accidental iirush of water from a break could then be ar- ranged to force the trains out to safety. Two or more tubes side by should, for safety, be separated by long- itudinal walls, so the flooding of one would not destroy the whole. Each track should also have watertight doors at intervals of about 500 feet, to close automatically in case of flood. Struc- tures are seldom built to resist earth- quakes, cyclones of other accidental con- yulsions of nature which may never come, and it is doubtful in this case if provision need be made for supporting a sunken ship, if one should fall across the tube. As _ previously stated, the chief pur- pose of 'piers is to serve as anchors, and side | . 203 to hold the tubes properly in position. and in line. The load on them will usually be very small, and chiefly from their own weight, but they must never- theless be well founded to prevent un- dermining. Iron cylinders filled with concrete are probably the best in most cases, for the metal casing may be car- ried above the water during construc- tion, and the upper part removed be- fore placing the tubes. Floating piers are practicable in water' of great depth, and as far as possibil- ities of construction are concerned, a bridge might be built across the Atlantic supported on floating piers. They have occasionally been used for revolving bridges, as for. example, under two swing bridges at Dublin, and the later: one at Norwich, England. Some inter-. esting designs for floating piers were prepared in the recent competition for a new bridge over the Hooghly river at Calcutta. One of these showed piers floating on the surface and varying in. elevation with the tide, while another had floats anchored to the bottom, hold- ing the piers and bridge at a fixed ele--- vation regardless of the water height. They were proportioned for very heavy loads, -carrying half a- S00- fixed span at one side and a 150-ft. swing span at the other, the roadway in both cases being very wide. Construction Tubes. must usually be built on shore with closed ends, and then towed out between scows and sunk into place. Water must be excluded from the tubes during construction, and the dams for this purpose, with air-lock doors, should be 3 to 5 feet back from the ends, leav- ing working space for the divers while bolting the sections together. When the tube sections are lowered on the piers, -- a hood or roof may be applied over the connection, and the space filled up with concrete. Ball and socket joints are convenient for circular forms, and since expansion at a depth of 30 to 40 ft. below water does not exceed 1 inch in 1,000 ft. little provision for expansion is needed. : Cost and Time for Construction The estimated cost of a double track submerged viaduct, 3,500 ft. long be- tween portals, under a channel 1,500 ft. wide, was $925,000, with two years time for completion, while a _ high level bridge, 4,900 ft. long, would have cost $3,600,000. For the same location a double track tunnel 1,680 ft. long through solid rock, would cost. $1,050,- 000 with end spirals, and $950,000 with end lifts. A somewhat similar sub- merged viaduct four miles long, over a strait 214 miles wide, for a single line 2