Building the Dams
The construction of the Panama Canal required the
construction of three dams, two on the Pacific side and one on the Atlantic side.
The dam at Gatun is so huge in its proportions, and was for years the subject of so much
controversy, that it eclipses the two dams on the Pacific side. The vastness of its
proportions can scarcely be conceived. It is really nothing more or less than a hill
thrown across the Chagres valley, so as to impound the waters of the Chagres River,
converting them into a lake with an area of 164 square miles, and with a shore line of
1.016 miles. The Chagres River drains a watershed of 1,320 square miles. The
average rainfall on this watershed approximates 100 inches annually. It is the
mission of the Gatun Dam to impound enough of this water in the lake to carry the commerce
of the world across the continental divide, and to provide lockage water for putting the
ships through the locks at the two sides of the isthmus. The dam impounds
183,000,000,000 cubic feet of water, or practically as much as the Chagres River brings
down in an average year.
The valley of the Chagres was framed by the hand of Nature in such a way as to fit admirably into the plans of the canal engineers for a lock canal across the isthmus, with the Atlantic locks at Gatun. In the upper and middle reaches of the valley, after the river strikes the line of the canal, the hills which border it are far flung. As you approach Gatun, coming down the valley, they swing in toward the canal on both sides, and at that point are only a mile and a half apart. Midway across this valley there was a small hill with a foundation of rock. The problem was simply one of building a dam between the hills at the two sides of the valley, utilizing the small hill in the middle of the valley as a site for the spillway, through which the surplus waters in the lake should be passed over the dam and down into the sea. The dam which was built is a mile and a half long, half a mile wide at the base, 300 feet wide at the water line, and 100 feet wide at its crest. It is 105 feet high, making the crest twenty feet higher than the normal surface of the water in the lake. The dam contains 22,000,000 cubic yards of material. Its total weight approximates 30,000,000 tons. It covers 288 acres of ground, and contains enough earth and rock to build a fence eighteen inches thick and three feet high around the earth at the equator; or a wall around the State of Ohio ten feet high by twelve feet wide.
In the construction of the dam two parallel ridges of stone 1,200 feet apart were first thrown across the Chagres valley. The height of these ridges ranged from thirty feet on the down-stream side to sixty feet on the up-stream side. Between these ridges, which were technically known as "toes," there was built up a dam of earth. The central portion of this earth dam is known as the hydraulic core. Huge dredges were put to work breaking loose the soft natural cement of the valley below the dam and pumping it into the great pond maintained on the surface of the dam as it rose upward. The material carried in suspension in the water was allowed to settle, and then the water was drawn off and sent back for another load of liquid silt. In this way there was built across the valley, in the heart of the dam, a wall of natural cement 850 feet wide at the base and as high as the dam itself.
After the dam was built to its desired height, the entire up-stream side was armored with stone. Wherever the wave action is strong, huge boulders weighing many tons are planted on the face of the dam, to break the force of the waters. Although the dam impounds the largest artificial lake in the world, its bulk is so great that sixty-three pounds of material would have to be pushed aside for every pound of pressure than can be brought against the dam.
The spillway by which the surplus waters brought down by the Chagres into Gatun lake are passed to the sea is a most interesting structure. It consists of a large semi-circular masonry dam built on the small hill in the middle of the valley, with the arc jutting up into the lake. On the crest of this dam were set thirteen piers and two abutments, making fourteen openings, each of which is forty-five feet wide and twenty feet high. Hugh gates close these openings, moving up and down on roller bearings. They weigh forty-two tons each, and are operated by electricity. The water is permitted to pass through these openings, and the torrents thus crated are made to converge at the bottom of the spillway, so that their fury is neutralized as they rush together. In addition to this, there are huge steel-faced baffle piers erected near the bottom of the spillway apron, which further arrest the maddened waters as they make their sixty-foot plunge from the surface of the lake to the surface of the river channel below. the spillway is so constructed that whenever the water flows deeper than six feet it adheres to the apron of the spillway instead of rushing over and making a perpendicular descent.
The Chagres River is one of the moodiest streams in the world. Now it flows along as a peaceful, lazy little tropical river not over two feet deep, moving sleepily on its journey to the sea. A day later it may become a wild, raging torrent, forty feet deep, madly hastening on toward the sea. Once there was nothing to oppose its moods, and it swept out to sea like a tidal wave. But today the mouth of the Chagres, properly speaking, is no longer at the Caribbean; it is some thirty miles inland, at Gamboa. What once was the valley of the Chagres between Gamboa and Gatun is now Gatun Lake,,, and the Chagres flows into it at Gamboa. instead of flowing into the Atlantic Ocean at Fort Lorenzo. Now the river may rage, coming down from its upper watershed with as great a flood as it has ever been known to carry, but the lake into which it flows is so extensive that angry torrents of the Chagres are swallowed up and scarcely disturb the placid surface. when the lake reaches eighty-seven feet, the spillway has a capacity of 145,000 cubic feet per second, which is 17,000 cubic feet more than the Chagres has ever carried. In addition to this the big culverts in the locks can dispose of 50,000 cubic feet per second. At ninety-two feet the capacity of the spillway and locks becomes 262,000 cubic feet per second, which would take care of two Chagres rivers instead of one. during the rainy season observers of the Chagres are placed above Gamboa, who telephone the stages of the river to the operators at Gatun. There is no danger that the operators at Gatun will ever be caught napping for a period of two days while the Chagres is on a rampage, and inasmuch as the capacity of the spillway, even at the normal lake level, is greater than the water-carrying capacity of the Chagres at its flood stage, the Chagres has been robbed of all its terrors. It has been transformed from a menace to the canal into a burden-bearer of commerce. Not only will it bear the burden of maintaining the level of the lake in the thirty-five miles stretch from Gatun to Pedro Miguel, but it will also be called upon to furnish the water which will drive the turbines of the hydro-electric plant at Gatun spillway, where the power for operating the locks, operating the terminal facilities, and lighting the Canal Zone settlements will be generated. Furnishing the water for the middle thirty-seven mile stretch of the canal, for the locks, and for the hydro-electric plant, the Chagres River has been made the major factor of the great trans-isthmian highway. It has become the friend of the lock canal, where it would have been the foe of a sea-level canal.
The first suggestion of Gatun as a proper site for the locks on the Atlantic isthmus was made in 1879 by Godin de Lepinay, who was chief engineer of bridges and roads in France. He tried to induce de Lesseps to give real consideration to engineering figures on his proposition to build a lock-type canal, but de Lesseps dodged the issue. De Lepinay then wrote his own report, setting forth objections to a sea-level canal, and presenting the advantages of a lock canal at Gatun. That report has been proved a very far-seeing one by experience at Panama. The reasoning he employed against a sea-level canal and that which he urged in favor of a lock canal, is amply vindicated by facts ascertained in the actual construction of the highway. when the Walker Commission took up the matter, they followed de Lesseps's plan for a dam at Bohio. The first chief engineer of the present canal, John F. Wallace, took little interest in investigations at Gatun, because he advocated a sea-level canal. It was the second chief engineer, John F. Stevens, who recommended the site at Gatun, and upon his careful investigations the decision to build the dam and locks there was reached. One of the principal objections urged to a lock canal was that a stable dam could not be built at Gatun. How little this objection was justified is shown by the success that attended the construction of Gatun Dam.
The fight against a lock canal was kept up even after that type had been authorized by congress, and those who opposed the lock canal centered their attack upon Gatun Dam. They declared that underground rivers flowed beneath the valley of the Chagres, and that there was no hope of building there a dam which would impound the waters of Gatun Lake. Colonel Goethals, as soon as he took charge, determined to ascertain just what the situation was beneath the surface of the valley. He called to his aid Caleb M. Seville, one of the foremost earth-dam experts of the world, who had been in charge of the work on the Wachusetts Dam in Massachusetts. They honeycombed the site of Gatun Dam with borings, and sank test pits here and there, so that they could go down and see with their own eyes the various strata of the proposed foundation of the dam. Never was the site of any structure explored more thoroughly than was the site of Gatun Dam. They found that the alleged rivers beneath the valley of the Chagres had no existence except in the imagination of those who opposed the building of a dam at Gatun. After they had satisfied themselves as to the site, they next took up the work of determining the properties of the materials with which the dam was to be constructed. They modeled experimental dams drawn to scale, and brought varying water pressures against them. They filled huge steel cylinders with the material they proposed to put into the dam, and exerted a water pressure upon it equal to a head of water twice that of Gatun Lake; and the trace of seepage through three feet of this material was nothing but perfectly clear water. It was obvious that if three feet of the material was impervious under such a pressure, there could be no question about the 800 feet of it to be used in the dam.
When the stone in one of the "toes" of the dam sank in 1908, a sensational dispatch was sent to a New Orleans newspaper stating that the dame had given way. This report was spread broadcast, and so stirred the nation -- for the people had not stopped to think that there was as yet no dam at Gatun to give way -- that President Roosevelt asked President-elect Taft to go to Panama to investigate the situation. The net result of this investigation was that the engineers who accompanied Mr. Taft -- Messrs. F.P. Stearns, Arthur . Davis, Henry A. Allen, James D. Schuyler, John R. Freeman, and Allen Hazen -- decided that the dam was being built with a grater margin of safety than even the utmost precaution required. they reported that the crest of the dam should be cut down thirty feet, and that there was no necessity for driving interlocking sheet piling across the valley as a precaution against imaginary underground rivers.
The completed Gatun Dam has proven a happy surprise. It has shown itself to be water-tight beyond all engineering demands; it has become such a part and parcel of the topography of the Gatun region that it seems to be nothing more than a hill of very gentle slope, thrown by the hand of Nature across the valley at Gatun. Nature has adopted it as a part of her own work, having covered it with a dense cover of jungle growth which relieves it of every indication of artificiality.
At Pedro Miguel the valley to be closed up was half a mile wide. Here a very small dam, thrown from the locks to the hills on the west, acts as a cork for the Gatun Lake bottle, of which Culebra Cut is the neck. At Miraflores there are two small dams. the construction of the west dam is similar to that at Gatun, and runs almost parallel with the locks. This was made necessary in order to control a small river during the period of lock construction at Miraflores. More than a dozen of these small rivers were haltered and led about by the canal diggers during the work of completing the canal. Some of them were forced to change their beds several times before they were finally permitted to resume an age-long repose.
At the outset of the work at Panama it was intended to create a large lake at the Pacific end of the canal, by throwing dams across depressions between Ancon Hill, Sosa Hill, and the hills to the west of the canal. It was found, however, that instead of costing $4,000,000, dams at these points would cost $11,000,000. Furthermore, locks and dams at Sosa Hill would have been exposed to an enemy's fire from Panama Bay, while Miraflores is out of the range of hostile guns. these considerations led to the abandonment of the projected lake, and the digging of a sea-level canal from the Pacific to Miraflores followed.
from The History of the
Panama Canal by Ira E. Bennett
Historical Publishing Company, 1915
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February 15, 1999