ALTERNATIVE TO OUR HARBOUR BRIDGE?

Sun (Auckland), Volume II, Issue 388, 23 June 1928, Page 26

ALTERNATIVE TO OUR HARBOUR BRIDGE?

Giant American Scheme To Build Underwater Highway .. . Accommodation For 12,000 Motorcars Daily . . . Tunnel Sections Built in Open Air And Then Sunk to Required Position . . .

UILDING a tunnel in the I j open air and then plished in California. This method of construction was carried out with the first subway tunnel under the Harlem River in New York, many years ago. but the California tunnel is a much larger affair, being no less than three-

quarters of a mile long (says 4 Popular Science Monthly,” New York). ‘•Early this summer Californians will celebrate the opening of the most remarkable underwater highway, in many respect, that engineers have ever attempted. It is the 4,500,000 dollars (£900,000) Oakland-Alameda Estuary Tube, running nearly threefourths of a mile tinder old San Antonio Creek, an arm of San Francisco Bay, and replacing the Webster Street Bridge, which now links the two cities. ••The spectacular and daring achievement marks a distinct advance in tunnel building; first, because it la the roomiest tube of its kind in

I the world, and second because of its ! novel construction. Instead of being i driven beneath the bed of the wateri way, as was the great Holland tunnel under the Hudson River, it is constructed in 12 separate precast sections of reinforced concrete, built in a dry dock, floated ten miles down San Francisco Bay to the spot, and lowered to a trench in the bed of the estuary. There, 42ft below the surface, the mammoth cylinders were

joined in a continuous, water-tight line. “The completed tube measures 3,545 ft between portals, 2,400 ft being under water, and its outside diameter is 37ft, seven and a-half feet wider than the Holland tunnel. It will be capable of accommodating more than 12,000 motor-cars daily, and also will contain double lines of street-car tracks. In design and scheme of ventilation it resembles the Hudson tunnel. On each side of a 23ft roadway are sidewalks protected by guardrails. Powerful 16ft fans in the portal buildings at each end of the tube constantly force in fresh air and draw out stale air.

"The sections, weighing 4,500 tons apiece, were cast in enormous forms in a 750 ft dry dock at Hunter’s Point, San Francisco. Into each went 250 tons of reinforcing steel, 2,500 cubic yards of concrete, and 25,000 square feet of three-ply membrane waterproofing. “As each ponderous segment was completed, it was made water-tight by bulkheads and towed by tugs to

a position above where it was to rest. Then, with 2,000 tons of sand and water ballast inside, it was slowly lowered by winches, being kept in accurate position by guiding masts at each end. “The final and most difficult problem was that of joining the segments 42ft under the water. It was solved by the ingenious scheme of casting square collars near the ends of each segment. Where the segments join, these collars are six feet apart. Sheets of steel are locked in place across these collars, forming a steel compartment around each joint. The compartment then is filled with concrete, sealing the joint perfectly.”