Bridge Building on a Large Scale.

Kaipara and Waitemata Echo, 23 October 1914, Page 8

Bridge Building on a Large Scale.

NEW STRUCTURE ACROSS THE MISSISSIPPI AT MEMPHIS. FOR USE OF RAILROAD. IMMENSE STONE. PIERS ARE SUNK TO DEPTH.OF. 100' ' . FEET . BELOW. SURFACE. . * OF WATERS ■';■'. The popular' conception of what supports a great bridge like the one being bult by the Rock Island lines across the Mississippi River at Memphis, . ia rather hazy. The person who crosses a bridge on a train does not see the massive stone piers which rise from the water. Even if he did see the piers he would still have little idea as to what it all means. Perhaps he imagines that the piers go down through the water to the bottom of the river, where, by some miraculous means, they find a firm foundation. He little realises that what he sees towering out of the water is only a small fraction of the pier and that its foundation is one hundred feet' below the surface resting on the strong bed stratum which underlies Memphis. • Each of the big stone piers when complete will support thousands of tons of steel and .must be capable of standing a strain equal to its possible load multiplied several times. The silt and sand of- the river bed and the soft alluvium which underlies them would make a poor foundation for a bridge pier. They would have about the resisting power of warm butter when the entire weight of the bridge is placed on them. The bridge builders must delve into the earth until something more permanent is found on which to rest so great a weight. Data supplied by the records of the builders of the "Frisco" bridge and from their own borings show the engineers that at a depth of . 105 feet below the present surface of the water is a hard blue clay," which has been there probably since the world was founded.

The greatest mystery of all to the average person is how the bridge builders are going to reach that stratum, because the piers are to be located in the fiver. It is in fact a tremendous undertaking, but it is made possible by means of the caisson. In this wooden box without a bottom the men work under tremendous air pressure and dig away the sand and earth, which is hoisted to the surface. The water is kej.it from rushing in on them by making the air pressure on the inside balance the uater -pressure from without. The pier is

built on top of the caisson and it is gradually sunk so that all of the stone is laid above the surface of the water. A large engine on a barge lying alongside supplies the air pressure which" keeps the water out. The sand is hoisted through a tube which has a suction like a vacuum cleaner, but when the hard is struck the men will have to hoist the dirt out with buckets.

A long tube with a ladder on the inside allows the men to come to the surface and descend to the bottom of the river. In the tube are two trap doors or valves to keep the air from rushing out when the men enter the tube. The weight of the pier above keeps the. caisson from rising to the surface when the air pressure is put on. The edge of the caisson is shod with steel. The caisson which has been built for the first pier is 90 by 42 feet inside. The space where the men work is painted white so as to reflect the light and it is supplied with electricity. The-tube by which the air for the men to breathe enters is supplied with a valve so that the instant the engines cease pumping for any cause it closes and keeps up the pressure in the caisson so that the water from the outside will not rush in and drown them. The workmen begin with an eight hour day the same as men on the sui-face. After going down for about fifty feet their day is reduced to four hours. At eighty feet they work only one hour at a time then rest for two or three. At ninety feet they begin working in fortyminute shifts with long rests between.