LOCKS AND DAMS
An elevator system for ships is being installed at Panama at a cost of $58,000,000. These elevators, known as locks, will raise ships to and lower them from the great artificial, inland lake which is 85 feet above sea-level.
In a sea-level canal, such as Suez, ships steam through a dug-out channel from one ocean to another. But, at Panama, the plan adopted involves the lifting of ships over the Isthmus and the locks are the means whereby they are lifted. For this physical operation there are six locks on the Atlantic side and six on the Pacific side, at each end of the Gatun Lake.
A ship arriving at Colon from New York, on its way to San Francisco, enters the sea-level channel Limon Bay and steams for seven miles through the canal, which is 500 feet wide and 41 feet deep, to Gatun. Here its way is barred by a massive pile of masonry with impressive steel gates and towering 85 feet above the ship is the surface of the Gatun Lake. To the West of the ship runs the man-made mountain, the Gatun dam, which hold the lake in bounds. The problem is to lift the ship to this lake.
As if by magic, the gates swing open and an electric locomotive, which has run out on a guide wall and fastened to the ship, tows it into the first lock. The gates swing together and the ship is imprisoned in a chamber 1,000 feet long and 110 feet wide and built of concrete. In a moment the water in this chamber begins to rise, being supplied through holes in the bottom, and the ship rises with the water.
Fifteen minutes after enter the lock, the ship has risen with the water for 27 1/2 feet. If the full capacity for filling the lock should be used the ship would rise that height in eight minutes. Another set of gates swing open in front of the ship, and locomotives tow it into the second lock, a concrete chamber of the same dimensions. The gates having closed behind, this chamber begins filling with water until the ship is raised again for 27 1/2 feet. A third set of gates open and the ship is towed into the final lock where the operation is repeated with a raise of 30 feet, or a total lift for the three locks of 85 feet. When the gates in front swing open the ship steams out into the Gatun Lake. The time spent in climbing 85 feet was an hour and a half.
For sixteen miles through this lake the ship steams in a channel 1,000 feet wide; for four miles in a channel 800 feet wide, and for three miles in a channel 500 feet wide, or twenty-three miles in all. Then it enters the famous Culebra cut, which is 300 feet wide through the continental mountain divide, and nine miles long. At the end of the cut is the Pedro Miguel lock, thirty-two miles from Gatun.
After entering this lock, which essentially is the same as the ones on the Atlantic side, the ship goes through the reverse of the process at Gatun. The water in the concrete chamber begins falling, taking the ship down with it. When it has fallen 30 feet the gates in front open and the ship goes out into another artificial lake, a mile and a half long, at the end of which are the Miraflores locks. These two locks lower the ship 27 1/2 feet each, or a total for the three locks of 85 feet, which was the height the ship was raised on the other side. The ship then steams through a sea-level channel for seven miles to the Pacific, having made the while journey from deep water in the Atlantic to deep water in the Pacific, fifty miles, in ten hours.
Thus it will be seen that the Atlantic and Pacific oceans are still separated by thirty-two miles of land at Panama, on which is a fresh-water lake 85 feet above sea-level. The locks simply are so many stair-steps up to and down from this lake. At both ends the locks are built in pairs, or twins, so that ships going in opposite directions may pass through them simultaneously. A wall 60 feet thick separates the locks, and if one set would become disabled, the adjoining set still would be available for passage. The time required for a ship to mount the three locks on one side and descend the three locks on the other side is three hours.
On the Atlantic side, the locks at Gatun are connected and constitute one solid piece of masonry. On the Pacific side the lock at Pedro Miguel is separated from two locks at Miraflores by a small lake a mile and a half long. This lake, like the great Gatun Lake, is formed by damming rivers. A dam at the Pedro Miguel lock, which is the first lock encountered going toward the pacific, holds the waters of Gatun Lake from spilling down the Pacific slope.
Chief Engineer Stevens began the excavations in the Gatun and Pedro Miguel lock sites in 1906, shortly after the decision was made for a lock-type canal, but most of the excavation and all the concrete laying has been done under Col. Goethals. It was necessary to remove about 5,000,000 cubic yards of rock and earth from the site of the three locks at Gatun to prepare a foundation for the tremendously heavy structure. Careful borings had been made to ascertain if a suitable foundation could be found there.
On August 24, 1909, the first concrete was laid in the Gatun lock site. Rock of a desirable kind for use in making the concrete, as well as sand, could not be found in the Canal Zone, and experiments along the coast showed that at Porto Bello, twenty miles East of Colon, good rock could be quarried, and sand was discovered in suitable quantities and quality at Nombre de Dios, forty miles East of Colon. These two places are the oldest on the Isthmus, Columbus having been there in 1502.
Rock crushing began at Porto Bello on March 2, 1909. If all the rock and sand removed from Porto Bello and Nombre de Dios was placed in barges separated by the usual distances in a tow, they would reach from Colon to New Orleans, or 1,500 miles. This material was towed to Colon and thence through the old French canal to Gatun. Here it was unloaded by machinery and stored conveniently for the concrete mixing plant.
All the machinery and equipment for building the locks was designed on a scale commensurate with the unprecedented size of the structures. Eight giant mixers were fed with rock, sand, and cement by cars operated by electricity, the finished product coming from each of the mixers at the rate of 64 cubic feet for each complete operation.
To get the concrete into place, four cableways, suspended across the lock site on towers 85 feet high, were installed. Electrically operated cars brought the concrete to these towers where great buckets were filled. These buckets then were run up to the cables, and out on the cables to a given point, where they were lowered and the concrete dumped into the proper position.
After the floors of the locks had been laid, the walls were built in the usual manner of erecting steel forms, which were removed when the concrete had hardened. At Gatun the walls of the locks were built in sections 36 feet long, and joined together, on the idea that such construction would have less tendency to settle and crack than if it was built in one solid, continuous wall. This may be appreciated when it is understood that at Gatun the locks form a concrete wall about 3,500 feet long, or considerably more than half a mile. The usable part of each lock is 1,000 feet long and there are three in flight. The twin locks have an outside wall 52 feet wide, an inside measurement 100 feet wide, a separating wall 60 feet wide, another inside measurement of 110 feet, and a final outside wall 52 feet wide, or a total width for both locks lying side by side, from outer wall to outer wall, of 384 feet.
In each of the outside walls and in the center wall tunnels 18 feet in diameter were constructed for use in filling and emptying the locks with water during the processes of raising and lowering ships. Smaller tunnels run out from these main longitudinal tunnels, under the floors of the locks with openings through which the water is turned into or withdrawn from the lock chambers by gravitation. Valves operated by electricity regulate the flow of the water. The water for operating the locks starts from the Gatun Lake and flows through the tunnels downgrade, through the three locks, until it finally is used in the lowest lock when it is spilled into the sea-level channel.
The first concrete for the Pacific side locks was laid at Pedro Miguel on September 1, 1909, seven days after the beginning of operations at Gatun. It was in February, 1910, that concrete work was started in the two locks at Miraflores, which, in 1912, were the most backward feature of canal construction. For all twelve locks, 4,302,563 cubic yards of concrete is required. three years after beginning the concrete work, or in August, 1912, the locks were more than 90 per cent completed, the one at Pedro Miguel being the nearest done with 98 per cent of the estimated concrete in place. The three locks at Gatun then had about 95 per cent in place and the two at Miraflores about 80 per cent.
For the three locks at Gatun, 2,000,000 cubic yards of concrete was required; for the lone lock at Pedro Miguel, 889,827 cubic yards; and for the two locks at Miraflores, 1,412,736 cubic yards. A contract was awarded the Atlas Portland Cement Co. for 4,500,000 barrels of cement, with the privilege of increasing this order by 15 per cent, and in 1912 another 1,000,000 barrels were bought to complete the canal. The stability of the locks depends upon the quality of cement used, hence the Government inspectors have watched this factor jealously.
Rock for the Pacific locks has been obtained at a quarry opened in Ancon hill, at the Pacific entrance of the canal. The sand has been brought from Chame, about 23 miles up the coast from Panama. The Pacific division has been at much less expense in obtaining materials than the Atlantic division, accounting for the difference in the cost of construction in the two divisions. The Pacific division was at one disadvantage in that the three locks were not together, as on the Atlantic side, necessitating practically two separate jobs. The amount of excavation at Pedro Miguel to secure a foundation was 770,000 cubic yards and at Miraflores, 2,247,600 cubic yards, a total for the three locks of 3,017,600 yards, which is nearly 2,000,000 yards less than had to be excavated in the site of the three Atlantic locks.
All twelve locks were half done as regards the concrete work, about May 1, 1911. The best month's record for laying concrete was made in April, 1912, in the Pacific division, when 97,735 cubic yards were laid. The concrete is all of reinforced construction, and an unusual feature has been the placing of rocks weighing many tons throughout the walls. The lock walls at Pedro Miguel were not built in sections as at Gatun, but as one solid piece of masonry more than 1,000 feet long. At Miraflores the two locks were built in sections, as at Gatun.
The gates for the locks were contracted for, in 1910, to cost $5,374,474.82. Their construction and erection are by the McClintic-Marshall Construction Company, of Pittsburgh, under the inspection of the Commission. This concern, in 1912, had more than 1,000 men at work and were rushing the construction to meet the dates agreed upon for their completion. Under the contract this company had until January 1, 1914, to finish the work, but estimated that this time could be beaten by six months. The date for finishing the gates at Pedro Miguel lock was May 1, 1912, but the contractor was behind on this program; at Gatun the gates were to be erected by February 1, 1913; and at Miraflores by June 1, 1913. Work was rushed on the gates at the lake end of the Gatun locks, in the summer of 1912, to hold out the rising water. On July 1, 1912, out of a total of 58,000 tons of steel required in all the gates, 19,631 tons had been erected, or 34 per cent, leaving to be erected before September, 1913, when the first ship is scheduled to go through, 38,369 tons.
There are 46 gates in all twelve locks, with two leaves to the gate, or 92 leaves. The gates are from 47 to 79 feet high, are 7 feet thick, and weigh from 300 to 600 tons each leaf. They are constructed with interior cells, which at the bottom will be air chambers to assist in their manipulation, and at the top, water chambers, to increase their weight as the water rises in the locks. the sheathing is with steel plates riveted on heavy girders. These gates will be opened and shut, to permit the entrance or egress of ships, by electrical apparatus.
As 95 per cent of the vessels in the world are less than 600 feet long, it would be a great waste of water and time to use the full 1,000-foot lock in each passage. So intermediate gates are being constructed which will permit the use of only 400 or 600 feet as the particular vessel may require. There are recesses in the lock walls which allow the gates to be opened and still leave a clear width of 110 feet. At the entrance of the locks, a chain, with links three inches in diameter, stretches from one side to the other to stop vessels which might not obey the signals. In case the first gates should be rammed and broken, a second set of gates especially provided for emergencies have been constructed behind the first set. If both sets of gates should be demolished, the water would rush through with a fearful velocity, but provision has been made against this contingency by having in readiness emergency dams, which would be swung out over the lock and forced down through the in-rushing water. This dam, built of steel, is open at the bottom and steel plates are then shoved down it, gradually closing the openings until the flow is stopped. A floating caisson would then be placed in position and sunk, completely shutting out water from the lock, the emergency dam would be raised, and repairs begun.
It is to prevent such accidents that the plan of towing vessels through the locks with electric locomotives was adopted, as then no misunderstanding of signals from the captain to the engineer of a ship could result. The tracks for these locomotives are on each side wall of the locks, and two will fasten to the rear and two to the front of a ship to effect a passage. If all twelve locks were joined end to end they would make a canyon nearly three miles long, 10 feet wide and 80 feet deep.
THE GATUN DAM
The natural topography of the isthmus at Panama permitted the Chagres River to escape into the Caribbean Sea through a break in the mountains at Gatun. Engineers logically considered that this was the point at which a dam should be thrown across the Chagres River. Two valleys were formed at Gatun by a hill which rose in the center to an elevation of 110 feet, and the dam that was designed runs from the Gatun locks to this hill and from this hill to the mountains, a total distance of 7,500 feet, or a mile and a half.
As the Chagres River every year discharges enough water to fill the lake, some means of disposing of the surplus water had to be provided. The plan adopted called for a spillway to be constructed in this hill, about third-way in the dam site. This spillway is of concrete, requiring 225,000 cubic yards to complete. On July 1, 1912, it was more than 90 per cent completed.
The floor of the spillway is 10 feet above sea-level, and 300 feet wide through the hill, which involved excavation through rock for a depth of 100 feet at the highest point of the hill. A concrete dam was built on this floor to a height of 69 feet above sea-level and in shape like a semicircle. On top of the concrete dam, piers were built with an arrangement for steel gates. These steel gates will be electrically operated and regulate the flow of water out of the lake. As much as 140,000 cubic feet of water per second may escape through the spillway when the gates are open.
There will not be a complete loss of this water, as on the east side of the spillway a power plant of the hydro-electric type will be operated. A drop of 75 feet by the water will operate turbine engines which in turn will operate the electric machinery that will generate all the power and illuminating current needed from one end of the canal to the other. But an additional power plant will be maintained at Miraflores for emergencies. The power to operate the lock gates will come from the spillway plant.
The Gatun dam is so stupendous that it almost seems to be a continuation of the hills that enclose the lake. It in fact does complete the natural mountain chains that form the barriers of the Chagres River. It is 105 feet high, or 20 feet above the ordinary level of the lake at 85 feet elevation. The plan of construction has been to build parallel mounds, for the mile and a half, 1,200 feet apart. Between these mounds, built of rock and earth, a core for the dam has been constructed by pumping mud and sand from the bed of the Chagres River. About 20 per cent of the material pumped is solid matter, and when it has deposited the water is pumped off. This operation has been repeated until an impervious heard has been made in the dam. Even if water from the lake penetrated the outside walls of rock and earth, it would find this core water-tight. The dam is nearly half a mile thick at the base, 398 feet thick where the water surface strikes it at 85 feet, and is 100 feet wide at the top. The outer coverings of rock and earth on the dam close over the hydraulic core at the crest. For about 500 feet the dam will be subjected to the full pressure of 85 feet of water, at other points to a less severe pressure.
Engineers consider the dam excessively safe and the layman has no difficulty in appreciating its strength. This feature was subjected to a storm of criticism throughout the early days of the canal because some engineers believed the earth would not support so heavy a structure, but the present Chief Engineer never has doubted its stability. About half of the material required, 21,994,111 cubic yards, has been brought from the Culebra cut. On July 1, 1912, the dam was more than 90 per cent completed, leaving less than 10 per cent to be done before the passage of the first ship.
On the Pacific side, the first dam encountered is at Pedro Miguel and serves to hold the waters of Gatun Lake at its southern end. It is 1,400 feet long and forty feet wide at the top. The maximum height of the water against this dam will be 40 feet. The plan of construction is much the same as at Gatun, but only about 1,000,000 cubic yards will be required.
After a ship is lowered 30 feet by the Pedro Miguel lock, it finds itself in an artificial lake a mile and a half long. This lake is formed by two dams, the one to the west being 2,300 feet long, and 40 feet wide at the top, holding a maximum head of water of 40 feet. It is constructed with a hydraulic core like the Gatun dam. On the east a concrete dam 500 feet long, and provided with a spillway, as at Gatun, and capable of discharging 7,500 cubic feet of water per second, will hold the small lake in control. The Cocoli River is the principal feeder of this lake.
Records kept by the French, and by the Americans since 1904, show conclusively that enough water always will be available to keep the Gatun Lake and the tiny Miraflores Lake adequately supplied with water. No trouble at all can develop during the eight months of rainy season, and in the dry season of four months enough water will have been stored in the lake by means of the regulating works in the Gatun dam spillway to allow for all losses through evaporation, seepage, power consumption, and loss through the locks. During the wet season the lake will be raised from elevation 85 for two feet, to elevation 87, over an area of 164 square miles. This water could be used until the lake falls to about 82 feet, or five feet over the 164 square miles. In an average dry season this would permit 58 complete transits of the canal every 24 hours, if the full 1,000-foot capacity of the locks is used, or more than the period would allow if vessels followed at intervals of one hour.
The Gatun Lake is backed up among the hills by the dam until it reaches a width of more than twenty miles at the widest point, and a length between Gatun and Pedro Miguel of thirty-two miles. It will be broken by many small islands, and stretches of high lands, and is narrowest in the Culebra cut where for nine miles the width is 300 feet. From Gatun to the entrance of the cut, a distance of twenty-three miles, lighthouses are stationed at commanding points to guide ships at night. The channel throughout is at an average depth of 45 feet. In order to raise the relocated Panama Railroad above the level of the lake it was necessary to make fills to the extent of 16,425,292 cubic yards.
The Navy Department has selected a site near San Pablo, about twenty miles inland from the Atlantic, and on the East side of Gatun Lake, for a high power wireless station. It is to be at an elevation of 110 feet above the level of the lake and capable of sending a message for 3,000 miles, to Washington, D.C., or to a similar station on the California coast. Smaller stations will be maintained at colon and Balboa in the Canal zone, and at Porto Bello. The Republic of Panama and private companies will not operate stations in competition with the American government.
If the great Gatun dam should break, the water in the lake might sweep devastatingly over the city of Colon, seven miles away, or pass through the old bed of the Chagres River harmlessly into the Caribbean Sea. While the pressure on the dam will be terrific, no such catastrophe is considered probable.