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.