The Panama Canal. Frederic J. Haskin
a single year.
Wherever one turns he finds things which furnish collateral evidence of the magnitude of the work. The Sanitary Department used each year 150,000 gallons of mosquito oil, distributed thousands of pounds of quinine, cut and burned millions of square yards of brush, and spent half a million dollars for hospital maintenance.
No other great engineering project has allowed such a remarkable "margin of safety"—the engineering term for doing things better than they need to be done. The engineers who dug the canal took nothing for granted. No rule of physics was so plain or so obvious as to escape actual physical proof before its acceptance, when such proof was possible. No one who knows how the engineers approached the subject, how they resolved every doubt on the side of safety, and how they kept so far away from the danger line as actually to make their precaution seem excessive can doubt that the Panama Canal will go down in history as the most thorough as well as the most extensive piece of engineering in the world.
CHAPTER III
GATUN DAM
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.
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.
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,