Page:Popular Science Monthly Volume 74.djvu/444

440 It should be noted, however, that there has been little deepening of the canal excavation on the backbone of the isthmus at an near Culebra below the levels reached by the French companies. The work thus far has required the widening of the cut. Whether, hereafter, under the requirement for securing the depth to the same rate of progress can be maintained, as now shown by the record, is a question which can best be answered by the engineers now in charge of the work. It is probable that the future records will show gradually decreasing rate of progress if measured by yards of excavation only.

When the building of the dams contemplated by the minority, from high ground at Corozal to Sosa Hill, was undertaken, it was soon found that these structures were ill advised. The muck and mud of the "Manglares" could not support the load of a moderate fill. There was much settling and lateral displacement and the work was abandoned. This was in no way fatal to the lock project, because it merely involved the shifting of the prepared locks at Sosa Hill back to Miraflores, where excavation for the dam was in progress.

The work on the main dam, the dam at Gatun, has barely commenced. This dam will be of earth; it will have a crest length of about 7,000 feet, and a breadth of base, measured up and down the stream, of nearly one half mile. The greater part of the material of which the dam is to be composed will be put in place by the hydraulic method. The material will be transported by water in pipes, a a practically impervious and safe body of earth will be thus deposited across the valley of the Chagres at Gatun. The recent settling ad slipping of a rock fill that was being placed along the up-stream toe of the proposed structure is of no great significance. There will be other slips until displacement of layers of yielding material, under and particularly near the edges of the fills, is sufficient to secure stability. The doubts concerning the feasibility of a structure at this point have nor arisen from fear of such slips, but from the uncertainty relating to a movement of water at considerable depth under the dam in porous deposits, the extent and permeability of which, notwithstanding information obtained by borings and shafts, must necessarily remain more or less conjectural. It is feared by some that such underflow may threaten the safety of the dam and by others that a serious loss of water may thus result.

All such fears are far fetched. There is bu little basis for them. There can be no injury to the dam from a moderate underflow if suitable precautions are take to let the water come to the surface at the down-stream toe of the dam through sand, gravel and broken rock