Читать книгу The Panama Canal - Frederic J. Haskin - Страница 15

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The key to the whole Panama Canal is Gatun Dam, that great mass of earth that impounds the waters of the Chagres River, makes of the central portion of the canal a great navigable lake with its surface 85 feet above the level of the sea, and, in short, renders practicable the operation of a lock type of canal across the Isthmus.

Around no other structure in the history of engineering did the fires of controversy rage so furiously and so persistently as they raged for several years around Gatun Dam. It was attacked on this side and that; its foundations were pronounced bad and its superstructure not watertight. Doubt as to the stability of such a structure led some of the members of the Board of Consulting Engineers to recommend a sea-level canal. Further examination of the site and experimentation with the materials of which it was proposed to construct it, showed the engineers that it was safe as to site and satisfactory as to superstructure. The country had about accepted their conclusions, when, in the fall of 1908, there was a very heavy rain on the Isthmus, and some stone which had been deposited on the soil on the upstream toe of the dam, sank out of sight—just as the engineers expected it to do. A story thereupon was sent to the States announcing that the Gatun Dam had given way and that the Chagres River was rushing unrestrained through it to the sea. The public never stopped to recall that the dam was not yet there to give way, or to inquire exactly what had happened, and a wave of public distrust swept over the country.

To make absolutely certain that everything was all right, and to restore the confidence of the people in the big project, President Roosevelt selected the best board of engineers he could find and sent them to the Isthmus in company with President-elect Taft to see exactly what was the situation at Gatun.

They examined the site, they examined the material, they examined the evidence in Colonel Goethal's hands. When they got through they announced that they had only one serious criticism to make of the dam as proposed. "It is not necessary to tie a horse with a log chain to make sure he can not break away," observed one of them, "a smaller chain would serve just as well." And so they recommended that the crest of the dam be lowered from 135 feet to 115 feet. Still later this was cut to 105 feet. They found that the underground river whose existence was urged by all who opposed a lock canal, flowed nowhere save in the fertile valleys of imagination. The engineers had known this a long time, but out of deference to the doubters they had decided to drive a lot of interlocking sheet piling across the Chagres Valley. "What's the use trying to stop a river that does not exist?" queried the engineers, and so the sheet piling was omitted.

As a matter of fact, Gatun Dam proved the happiest surprise of the whole waterway. In every particular it more than fulfilled the most optimistic prophecies of the engineers. They said that what little seepage there would be would not hurt anything; the dam answered by showing no seepage at all. They said that the hydraulic core would be practically impervious; it proved absolutely so. Where it was once believed that Gatun Dam would be the hardest task on the Isthmus it proved to be the easiest. Culebra Cut exchanged places with it in that regard.

Gatun Dam contains nearly 22,000,000 cubic yards of material. Assuming that it takes two horses to pull a cubic yard of material it would require twice as many horses as there are in the United States to move the dam were it put on wheels. Loaded into ordinary two-horse dirt wagons it would make a procession of them some 80,000 miles long. The dam is a mile and a half long, a half mile thick at the base, 300 feet thick at the water line, and 100 feet thick at the crest. Its height is 105 feet.

Yet in spite of its vast dimensions it is the most inconspicuous object in the landscape. Grown over with dense tropical vegetation it looks little more conspicuous than a gradual rise in the surface of the earth. Passengers passing Gatun on the Panama Railroad scarcely recognize the dam as such when they see it, so gradual are its slopes. An excellent idea of the gentle incline of the dam may be had by referring to the accompanying figure, which shows the outlines of a cross section of the dam.

The materials of which it is constructed are also shown there. Starting on the upstream side there is a section made of solid material from Culebra Cut. Beyond this is the upstream toe of the dam, which is made of the best rock in the Culebra Cut. After this comes the hydraulic fill. This material is a mixture of sand and clay which, when it dries out thoroughly, is compact and absolutely impervious to water. It was secured from the river channel and pumped with great 20-inch centrifugal pumps into the central portion of the dam, where a veritable pond was formed; the heavier materials settled to the bottom, forming layer after layer of the core, while the lighter particles, together with the water, passed off through drain pipes. In this way the water was not only the hod carrier of the dam construction, but the stone mason as well. Where there was the tiniest open space, even between two grains of sand, the water found it and slipped in as many small particles as were necessary to stop it up.

A CROSS-SECTION OF THE GATUN DAM

Above the hydraulic fill on the upstream side is a layer of solid material, while that part of the face of the dam exposed to wave action is covered with heavy rock. The same is true of the crest. On the downstream half of the dam there is approximately 400 feet of hydraulic fill, then 400 feet of solid fill, then a 30-foot toe, and then ordinary excavated material.

The Chagres Valley is a wide one until it reaches Gatun. Here it narrows down to a mile and a half. It is across this valley that the Gatun Dam is thrown in opposition to the seaward journey of the Chagres waters. At the halfway point across the valley there was a little hill almost entirely of solid rock. It happened to be planted exactly at the place the engineers needed it. Here they could erect their spillway for the control of the water in the lake above.

GATUN LAKE PLAN OF THE GATUN DAM AND LOCKS

The regulation of the water level in Gatun Lake is no small task, for the Chagres is one of the world's moodiest streams. At times it is a peaceful, leisurely stream of some 2 feet in depth, while at other times it becomes a wild, roaring, torrential river of magnificent proportions. Sometimes it reaches such high stages that it sends a million gallons of water to the sea between the ticks of a clock.

In controlling the Chagres, the engineers again took what on any private work would have been regarded as absurd precaution. In the first place, Gatun Lake will be so big that the Chagres can break every record it heretofore has set, both for momentary high water and for sustained high water, and still, with no water being let out of the lake, it can continue to flow that way for a day and a half without disturbing things at all. It could flow for two days before any serious damage could be done. Thus the canal force might be off duty for some 45 hours, with the outlet closed, before any really serious damage could be done by the rampage of the river.

But of course no one supposes that it would be humanly possible that two such contingencies as the highest water ever known, and everybody asleep at their posts for two days, could happen together. When the water in the lake reached its normal level of 87 feet the spillway gates would be opened, and, if necessary, it would begin to discharge 145,000 feet of water a second. This is 17,000 feet more than the record for sustained flow heretofore set by the Chagres. But if it were found that even this was inadequate the culverts in the locks could be brought into play, and with them the full discharge would be brought up to 194,000 feet a second, or 57,000 more than the Chagres has ever brought down. But suppose even this would not suffice to take care of the floods of the Chagres? The spillway is so arranged that as the level of the water in the lake rises the discharging capacity increases. With the spillway open, even if the Chagres were to double its record for continued high water, it would take many days to bring the lake level up to the danger point—92 feet. When it reached that height the spillway would have a capacity of 222,000 feet, which, with the aid of the big lock culverts, would bring the total discharge up to 262,000 feet a second—only 12,000 cubic feet less than double the highest known flow of the Chagres.

But this is only characteristic of what one sees everywhere. Whether it be in making a spillway that would accommodate two rivers like the Chagres instead of one, or in building dams with 63 pounds of weight for every pound of pressure against it, or yet in building lock gates which will bear several times the maximum weight that can ever be brought against them, the work at Panama was done with the intent to provide against every possible contingency.

The spillway through which the surplus waters of Gatun Lake will be let down to the sea level, is a large semicircular concrete dam structure with the outside curve upstream and the inside curve downstream. Projecting above the dam are 13 piers and 2 abutments, which divide it into 14 openings, each of them 45 feet wide. These openings are closed by huge steel gates, 45 feet wide, 20 feet high, and weighing 42 tons each. They are mounted on roller bearings, suspended from above, and are operated by electricity. They work in huge frames just as a window slides up and down in its frame. Each gate is independent of the others, and the amount of water permitted to go over the spillway dam thus can be regulated at will.

When a huge volume of water like a million gallons a second is to be let down a distance of about 60 feet, it may be imagined that unless some means are found to hold it back and let it descend easily, by the time it would reach the bottom it would be transformed into a thousand furies of energy. Therefore, the spillway dam has been made semicircular, with the outside lines pointing up into the lake and the inside lines downstream, so that as the water runs through the openings it will converge all the currents and cause them to collide on the apron below. This largely overcomes the madness of the water. But still further to neutralize its force and to make it harmless as it flows on its downward course, there are two rows of baffle piers on the apron of the spillway. They are about 10 feet high and are built of reinforced concrete, with huge cast-iron blocks upon their upstream faces. When the water gets through them it has been tamed and robbed of all its dangerous force. The spillway is so constructed that when the water flowing over it becomes more than 6 feet deep it adheres to the downstream face of the dam as it glides down, instead of rushing out and falling perpendicularly.

The locks are situated against the high hills at the east side of the valley, after which comes the east wing of the dam, then the spillway, then the west wing of the dam, which terminates on the side of the low mountain that skirts the western side of the valley. With the hills bordering the valley and the dam across it, the engineers have been able to inclose a gigantic reservoir which has a superficial surface of 164 square miles. It is irregular in shape and might remind one of a pressed chrysanthemum, the flower representing the lake and the stem Culebra Cut. The surface of the water in this lake is normally 85 feet higher than the surface of the water seaward from Gatun and Miraflores. The lake is entirely fresh water supplied by the Chagres River. The accompanying figure shows the profile of the canal.