This circumstance serves to enforce the apparent paradox that the great area of Lake Gatun is in several respects an element of safety, not, as the layman might suppose, of danger. The hydrostatic pressure upon the dam depends, of course, solely upon the depth of water, not upon the area of the lake, while the greater the contents of the reservoir the more nearly stagnant are its waters.
As there is to be no lock at La Boca, the dams shown there on the plan and profile will not have to be constructed, so that it is not necessary to deal with the questions to which they formerly gave rise.
In the vicinity of the locks at Pedro Miguel and Milaflores, however, dams have to be constructed to hold up the water. At both places the dams will be short, and will be founded upon hard rock,[16] and in each case the head of water to be held up will only be about 40 feet, instead of 80, as at Gatun. The construction of the dams at Pedro Miguel and Milaflores is not, therefore, regarded with anxiety.
[16] "Canal Zone Pilot," pp. 316-317.
The great Gatun dam remains the one important experiment in the whole scheme of the high-level Canal, and much attention is being devoted to the planning of this work. The alluvial foundation is a disadvantage shared by the Bohio site formerly chosen, and all other sites in the lower Chagres valley; so that, having decided upon the Panama route, and a high-level canal, there appears to be no alternative to the construction of a dam upon this kind of bottom. The details of the proposed structure, as elaborated in April, 1908, were as follows:—
The length of the great earthen dam at Gatun is 7,700 feet, its breadth no less than 2,060 feet. The weight of the dam per linear foot is more than 60 times the horizontal pressure of the water in the lake, so that the pressure could not move the whole mass; and the weight of the dam is spread over such a great width that it is not thought that the ground will sink beneath it. The form of the plan and section is shown on the map, and an idea of the topography may be obtained from the photographs, which I took in April, 1908. The south-eastern end of the dam abuts on the hill of hard, fine-grained, argillaceous sandstone in which the lock-site is being excavated.
The dam, according to these plans, is not to be merely superposed upon the surface, as originally proposed in 1905. Embedded in its earthy mass there is to be a puddled core, and a trench will be excavated to a level of 40 feet below the sea (-40 feet) for the lower part of this core. Nor is this all that is to be done to check seepage beneath the earthen dam. From the bottom of the trench excavated for the puddled core, sheet piling, made of 4-inch timbers, is to be driven down for another 40 feet, so that sheet piling and puddled core together will form an impervious barrier to -80 feet; that is to say, 80 feet below the surface-level of the sea, or about 85 feet below the lowest natural surface of the ground. The puddled core is carried up through the earthen dam to the level of +90, that is to say, 5 feet above the level of the lake, which is to be 85 feet above sea. The crest of the dam will be +135 feet, i.e., 50 feet above the level of the lake; this excess of height being to provide top weight for increased stability of the whole structure, and also for the purpose of compacting the underlying material. The underwater slopes of the earthy materials have been reduced from the 1:3 of 1905 to 1:5. On the other hand, it has been decided that the width of 2,625 feet given in 1905 was in excess of utility, and that a reduction of between 500 and 600 feet can be made without loss of strength or efficiency.
EXCAVATION IN THE CUT.