Not only is the lock of the Industrial Canal one of the largest in the
United States, but its construction solved a soil problem that was
thought impossible. That of the Panama Canal 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 assurance 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 passing 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.
[Illustration: INNER HARBOR--NAVIGATION CANAL
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. Coarser 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 Canal, 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
anticipated. 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 Canal 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 mass 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 canal 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 pass 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 passing 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 Canal and the New Orleans Industrial Canal, 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.
"The concrete work gives the impression of lightness, as well as
strength, as though every yard of concrete was doing its special share
of the work without overstraining, which is, of course, the
characteristic of well-designed reinforced masonry.
"The outer surfaces are particularly smooth and well finished, more so
than in any work I have recently seen.
"The erection of the gates, valves, operating machinery and the
protective dam, has kept up closely with the concrete work, so that no
delays need be apprehended at the close of the construction period.
"The shop and field work in the lock gates is excellent. The rivet
holes match well and the rivet heads appear to be tight and well
formed. The gate leaves seem very straight and true."
The lock was designed by George M. Wells of the George W. Goethals
Company, assisted by R. O. Comer, designing engineer of the Dock Board,
and approved by General Goethals. The methods employed to unwater the
lock were devised by Mr. Wells. J. Devereux O'Reilly, chief engineer of
the Dock Board, to November, 1919, had charge of the details of
installing the unwatering and safety devices. He was succeeded by
General Arsene Perrilliat, who supervised the final unwatering process.
Upon his death in October, 1920, he was succeeded by J. F. Coleman &
Company, in charge of the engineering department, and H. M. Gallagher,
chief engineer, under whom work is being brought to a conclusion.
From first to last, Tiley S. McChesney, assistant secretary and
treasurer of the Dock Board, rendered intelligent and invaluable
service, gathering together and holding the threads of the enterprise,
and attending promptly to the multitude of details connected with the
prosecution of the work.
The lock was formally dedicated May 2, 1921--a ceremony that was the
feature of the Mississippi Valley Association's convention in New
Orleans.
With the dredging of the channel between the river and the lock, a work
that should be finished before January, 1922, ships will be able to
pass from the Mississippi into Lake Pontchartrain. Then New Orleans can
plan its next great development.
[Illustration: CROSS SECTION OF LOCK]
[Illustration: CROSS SECTION OF SIPHON]