LARGEST BRIDGE.

Star (Christchurch), Volume LXIV, Issue 936, 5 December 1933, Page 5

LARGEST BRIDGE.

To Span San Francisco Harbour. GIGANTIC TASK DESCRIBED. (Special to the “Star.”) SAN FRANCISCO, November 8. Building skyscrapers from the top down—sinking structures higher than any building in California into San Francisco’s bay—is a feat that is being accomplished in the building of the world’s greatest bridge, which will span the harbour of San Francisco. Commuters crossing the bay daily to their work are glimpsing this world's ! record-breaking engineering feat being i performed on the underwater constriction of the San Francisco-Oakland bay bridge. “How is it done?” the commuters ask. On the bay they now see three huge structures like rectangular ships or barges. About the middle of November the world’s largest caisson will be towed midway between San Francisco and Verba Euena Island for construction of the centre anchorage. llow. then. are these skyscraper foundations to be built to unprecedented depths under the bay waters. Step by step, here are the operations being followed to construct the centre anchorage—a gargantuan block of concrete the height of a forty-eight-storev building that will rise from the bedrock of the bay. Caisson 197 ft Long. A caisson, which may be described as a large and strong watertight box or casing, has been constructed. It is 197 feet long and 92 feet wide. It is built like a great rectangular egg crate of steel and timber. It is divided into cells like the little cardboard boxes in which eggs arc packed. There are fifty-five of these cells, to be used as dredging wells, on the centre anchorage caisson. On the bottom edges of the caisson there is a steel cutting

knife—a thick wall of steel tapered to a cutting edge. Already launched, this caisson is now being built up to a height of fifty to seventy-five feet, after which it will shortly be towed to the centre of the west bay. Then begin operations that are unique in the history of engineering. On past underwaver foundation building “ sand hogs ” have gone down into airtight caissons or cofferdams and excavated for the piers. But “ sand-hog-ging " is eliminated from the deep water construction on the bay bridge. One of the piers must be sunk to a depth of 235 feet below the water line. The air pressure at this depth would be quick death to “ sand hogs.” Once the caisson is anchored by 100-ton anchors in the bay, the process of building the skyscrapers downward to bedrock is started. The sides and the cylinders of the caisson are built up higher and higher by degrees. Then concrete is poured into the walls between the cells of the gigantic eggcrate. As the structure gets heavier under the weight of hundreds of tons of concrete it sinks gradually down into the water. The cylinders are capped with big domes that keep air inside the caisson, thereby causing it to float, instead of plunging precipitously to the bottom of the bay. Cutting Into the Bottom. The centre anchorage caisson will be sunk until it hits the mud line ninety feet below the water line. Then its sides will be built still higher; more tons of concrete will be poured into the forms above water. The weight will force the cutting knives on the bottom into the mud, sand and clay. Dredging machines operated entirely from the surface will then replace the usual “ sand hogs ” at doing the excavation job. Heavy pressure, 2001 b jet hoses will be lowered into each of the fifty-five cylinders to loosen the sand and clay. Suction pumps will bring up mud in semi-liquid form. Clamshell dredging buckets operated from construction docks will also be sent down into each dredging well, bringing up some 60001 b of clay at each dip. Finally, the caisson will force its enormous weight down through 90 feet

of clay to bedrock. Then the only human beings who will go down to the bottom of the pier will do their work. William Reed, chief deep sea diver on the project, will go down some 180 feet below the water line to where the caisson rests on rock. Others divers may assist. The diver, working inside the cylinders, will test the bedrock, plant dynamite where it requires to be blasted down and by telephone to the surface, report his inspection findings. When the bedrock is cleaned out, all the cylinders and cells of the caisson will be filled with concrete to form the centre anchorage. There will be 590,000,000 pounds of concrete poured into this one foundation, which will rise 300 feet above the water and go down nearly 200 feet below the surface. In addition, 16,200,000 pounds of steel and 1,198,000 board feet of timber will be used in this one structure. Like Sinking Hotels. The quantity of concrete and steel in the centre anchorage would be sufficient to rebuild all the major downtown office buildings of San Francisco, according to bridge officials. The foundation construction is likened to the sinking of seven St Francis hotels and a Palace Hotel into the bay, since the piers are of that comparative size, and anyone who has visited San Francisco knows the huge size of the two leading hotels mentioned. The bay bridge method of building the piers with compressed air flotation caissons has never been used before. The engineering world is watching the feat. The system is known as the Moran method, named for Daniel E. Moran, of New York, bridge consulting engineer. The flotation idea might be likened to binding together a lot of barrels, most of them closed at each end, so that they float. Then visualise pouring concrete around the barrels, taking the tops off the barrels a few at a time and eventually filling them in entirely with concrete. There is another layman’s conception of the amazing engineering process by which the hazards of 4 ‘ sand hogging ” have been eliminated from the construction of this huge bridge.