The Industrial Canal and Inner Harbor of New Orleans Part 4

The siphon is divided into four compartments. There are two storm chambers, measuring 10 by 13 feet each, one normal weather chamber measuring 4 by 10 feet, and one public utilities duct, measuring 6 by 10 feet. These are inside dimensions. The floor of the siphon is two feet thick; the roof, one foot nine inches. The whole structure is a solid piece of concrete and capable of standing a pressure of more than 2,000 pounds to the square foot. Its total length is 378 feet; the shipway pa.s.sing over it is 105 feet wide and 30 feet deep.

In the public utilities duct are carried the city's water pipes, cables, telephone and telegraph wires, and gas mains.

The storm chambers will handle the rainfall of cloudbursts. In ordinary weather the water will be concentrated through the smaller chamber, in order to produce a strong flow and reduce the settlement of sediment to a minimum.

Eight sluice gates, each 6 by 10 feet, open or close the water chambers. They are operated by hydraulic cylinders of the most approved type.

For sending workmen inside the siphon to make repairs or clearing away an obstruction there are eight manholes. Four measure 6 by 13 feet, two 6 by 6 feet, and two 6 by 4 feet.

As soon as the Florida Walk ca.n.a.l can be deepened and a few link-ups in the drainage system can be made, the entire drainage of New Orleans, in normal weather and during light storms, will, according to announcement by the Sewerage and Water Board, be sent through this outlet. During the occasional cloudbursts it will be necessary to send some of the drainage into the lake, but this will be rapidly flowing water and will sweep offsh.o.r.e. It means a great deal to the suburban development of the city.

A year and a half the siphon was in the making. Preparations for the structure cost more than $250,000--excavation foundation, etc. The concrete alone cost $170,000. Machinery and the work of housing and installing it cost $60,000 more.

Four bascule steel bridges now cross the Industrial Ca.n.a.l. They are the largest in the city. Three of them--at Florida Walk, for the Southern and Public Belt Railways; Gentilly, for the Louisville & Nashville; and on the lake front, for the Southern, weigh 1,600,000 pounds each--superstructure only. The fourth--at the lock--weighs 1,000,000 pounds. They are balanced by 800-ton concrete blocks and concrete adjustment blocks. Their extreme length is 160 feet; the moving leaf has a span of 117 feet.

With a 30-foot right of way for railroad tracks, 11 feet for vehicles and trolley cars and four feet for pedestrians, they are designed to meet traffic conditions of a great and growing city. They will support 50-ton street cars or 15-ton road rollers--New Orleans has nothing as heavy as that now--and trains a great deal heavier than are now coming to the city. No bridge in the South will support as heavy loads.

The tensile strength of the steel of which the bridges are constructed is from 55,000 to 85,000 pounds to the square inch, and they will bear a wind load of 20 pounds to the square inch of exposed surface.

They are operated by two 75-horse power electric motors, 440 volts, 60-cycle, 3-phase current, which is stepped down from 2,200 volts by means of transformers. In addition, there is a 36-horse power gasoline engine, to be used if the electrical equipment is out of order. To open or close the bridges will require a minute and a half.

THE REMARKABLE LOCK.

Not only is the lock of the Industrial Ca.n.a.l one of the largest in the United States, but its construction solved a soil problem that was thought impossible. That of the Panama Ca.n.a.l is simple in comparison.

The design is unique in many respects. The lock is a monument to the power of Man over the forces of Nature, and to the progress of a community that will not say die.

Because of the great variation in the level of the river at low and high water--a matter of twenty feet--it was necessary to make the excavation, for building the lock, about fifty feet deep. In solid soil this would be a simple matter. But this ground has been made by the gradual deposit of Mississippi River silt upon what was originally the sandy bed of the ocean, and through these deposits run strata of water-bearing sand, or quicksand. This flows into a cut and causes the banks to cave and slide into the excavation. Underneath there is a pressure of marsh gas, which, with the pressure of the collapsing banks, squeezes the deeper layers of quicksand upwards, creating boils and blowing up the bottom.

New Orleans has had plenty of experiences with these flowing sands in its shallow sewerage excavations. How, then, expect to make an excavation fifty feet deep? asked the doubting Thomases. It couldn't be done. The quicksands would flow in too fast. The dredges would drain the surrounding subsoil, but that wouldn't get beyond a certain depth.

Furthermore, what a.s.surance was there that the soil that far down would supply sufficient friction to hold the piles necessary to sustain the enormous weight of the lock and the ships pa.s.sing through it?

Undaunted by these croakings, the engineers, from test borings, calculated the sliding and flowing character of the soil, and estimated the various pressures that would have to be counteracted, balanced this with the holding power of pine and steel and concrete, evolved a plan, and began an excavation of a hole 350 feet wide by 1,500 feet long, gradually sloping the cut (1 to 4 ratio) to a center where the lock, 1,020 by 150 feet, outside dimensions, was to be built.

[Ill.u.s.tration: INNER HARBOR--NAVIGATION Ca.n.a.l Lock and Vicinity]

The gentle slope of the cut was to prevent slides.

It had been ascertained that the first stratum of quicksand began twenty-eight feet below the ground surface (-3 Cairo datum) and was three feet thick; the second stratum, forty-eight feet below the surface (-23 Cairo datum) and ten feet thick. Coa.r.s.er sand extended eleven feet below this, from -33 Cairo datum. The second stratum of flowing sand began just below where the lock floor had to be laid. The third layer was 80 feet below the surface (-55 Cairo datum); the tips of the piling would just miss it.

Excavation began in November, 1918. While the dredges were at work a wooden sheet piling cofferdam was driven completely around the lock, and about 125 feet from the edge of the bank, to cut off the first quicksand stratum. About 150 feet further in, when the excavation was well advanced, a second ring of sheet piling was driven, to cut off the second stratum, which carried a static pressure of 55 feet and was just a foot or so below where the floor of the lock would be. It was not thought necessary to cut off the third stratum.

The excavation was made in the wet. When it was finished the dredges moved back into the Ca.n.a.l, the entrance closed, and the work of unwatering the lock site began. This was in April, 1919.

There had never been such a deep cut made in this section.

Consequently, the character of the soil, while it could be estimated, could not be known absolutely. And the exact pressure of the gas could not be known.

The sands proved to be more liquid and the gas pressure stronger than antic.i.p.ated. Quicksands ran through the sheet piling as through a sieve. The walls of the excavation began to slough and cave. The gas pressure became alarming when the weight of earth and water was taken off; sand boils began to develop at the bottom; the floor of the cut was blowing up.

The fate of the Industrial Ca.n.a.l hung in the scale.

To meet the situation the engineers pumped a great volume of water into the excavation. Its weight counterbalanced the earth pressure of the side and the gas pressure of the bottom.

Then another ring of sheet piling was driven inside the other two. This one was of steel, and the walls were braced apart by wooden beams ten inches square and fifteen feet apart in both directions. This is one of the largest cofferdams of steel ever driven. As an added precaution against the danger of a blowout by the third stratum of quicksand, which had a static head of 75 feet, 130 ten-inch artesian wells were driven inside the steel cofferdam. Fifty-six similar wells were driven between the steel and the wooden cofferdams to dry out the second stratum of quicksand, as much as possible, and lessen its flowing character.

In November, 1919, the work of unwatering the lock site again began.

Only one foot every other day was taken off. Engineers watched every timber. It was not until January 4, 1920, that the unwatering was complete. The plan had worked. Only in one place had there been any movement--a section of the wooden sheet piling about 300 feet long bulged forward a maximum distance of three inches, when the bracing caught and stopped it.

Then began the work of driving the 24,000 piles on which the lock was to be floated. They are 60 feet long and their tips are 100 feet below the surface of the ground.

In March, 1920, the work of laying the concrete began. The work was done in 15-foot sections, for only a few of the braces could be moved at one time. When it was finished in April, 1921, the lock was in one piece, a solid ma.s.s of steel and stone, 1,020 feet long, 150 feet wide, and 68 feet high, weighing, with its gates and machinery, 225,000 tons, and filled with water, 350,000 tons.

The concrete floor of the lock is 9 to 12 feet thick, the walls 13 feet wide at the bottom, decreasing to a two foot width at the top. Six thousand tons of reinforcing steel were used in the construction, and 125,000 barrels of cement. There are 90,000 cubic yards of concrete in the structure. Two and a half million feet of lumber were used in building the forms.

Usable dimensions of the lock are 640 feet long, 75 feet wide, and 30 feet (at minimum low water of the river) deep.

The top of the lock is 20 feet above the natural ground surface and 6 feet above the highest stage of the Mississippi River on record. To the top the ground will be sloped on a 150-foot series of terraces. This will brace the walls against the pressure of water within the monolith.

It will be developed to a beautiful park. Heavy anchor-columns of concrete will hold the walls against the pressure of these artificial hills when the lock is empty.

Traffic crosses the ca.n.a.l here by a steel bascule bridge 65 feet wide, with two railroad and two street car tracks, two vehicle roadways, and two ways for pedestrians. Concrete viaducts lead to the bridge.

Gas and water mains, sewer pipes and telephone, telegraph and electric wires pa.s.s under the lock in conduits cast in the living concrete.

Water is admitted into and drained from the lock by culverts cast in the base. These are 8 by 10 feet, narrowing at the opening to 8 by 8 feet, and closed by 8 sluice gates, each operated by a 52-horsepower electric motor. It will be possible to fill or empty the lock in ten minutes.

There are five sets of gates to the lock. They are built of steel plates and rolled shapes, four and a half feet thick and weighing 200 tons each. And there is an emergency dam weighing 720 tons, which in case of necessity can be used as a gate.

Four pairs of the gates are of 55-foot size; one of 42-foot. Each gate is operated by a 52-horsepower electric motor. When open, the gates fit flush into the walls of the locks.

In the emergency dam is the refinement of precaution--designed as it was to save the city from overflow in the remote event of the lock gates failing to work during high water, and to insure the uninterrupted operation of the lock in normal times, if the gates should be sprung by a ship, or otherwise put out of commission.

This dam consists of eight girders or sections, 80 feet long, 3 feet wide and 6 feet high. They weigh 90 tons each. They are kept on a platform near the river end of the lock. Nearby is the crane with a 300-horsepower motor, that picks up these girders and drops them into the slots in the walls of the lock. To set this emergency dam is the work of an hour.

A ship pa.s.sing through the lock will not proceed under her own power.

There are six capstans, two at each end of the lock and two at the middle, each operated by a 52-horsepower electric motor, and capable of developing a pull of 35,000 pounds, which will work the vessels through.

The lock complete, counting the bridge and approaches, cost $7,500,000.

One and a half million of this is for machinery, and $56,000 for the approaches.

Henry Goldmark, the New York engineer who designed the gates of the Panama Ca.n.a.l and the New Orleans Industrial Ca.n.a.l, in a letter of March 24, 1921, to the engineering department of the Dock Board, comments as follows on the remarkable lock:

"I was much impressed by the uniformly high grade of construction of the lock, the systematic and energetic way in which the work was being carried on, and especially by the admirable spirit of team work, shown by the employees of the Dock Board, of different grades, as well as the contractors, superintendents and foremen.

"The desire to get the best possible results in all the details, at the least cost, was manifest throughout.

"The unique method used for carrying on the very difficult and risky work of excavation has attracted much professional attention in all parts of the country. Its successful completion is very creditable to all concerned, in the inception and carrying out of the method used.