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The slides as a feature of the canal operations have received, perhaps, more than their share of attention, and have been made use of by alarmist press agents. Whereas the amount of American excavation for the whole canal is 232,353,000 cubic yards, the total additional amount of material to be removed, due to the slides, is estimated to be approximately 29,431,000 cubic yards, or about 127 per cent of the total excavation, or a little over 22 per cent of the dry excavation, or about 30 per cent of the excavation in the Culebra Cut. This is, relatively, the same condition that would be encountered on a difficult sewer or water-pipe trench. Recent activity of the slides will increase the amounts.
The slides are not to be minimized, however, as the amount of material involved is equivalent to the excavation for 60 of the largest dry docks and they have served to considerably prolong the time of completing the Culebra Cut. Careful attention has been given them by the commission, and the geologist employed by them has made a thorough examination and report on the subject.
Slides have developed at those points where the side slopes of the canal excavation were left too steep; that is, where not enough material was taken out to provide a stable bank, having in view the soft or weakened character of the material. By removing additional material, stability of the banks results and the slides are thus resolved into a problem of the ultimate amount of excavation. Very large areas are involved. The West Culebra slide, for instance, covers 68 acres and involves the excavation of over 11,000,000 cubic yards. The Cucaracha slide covered over 50 acres and involved over 5,300,000 cubic yards. The East Culebra slide covers an area of 55 acres and will mean a total excavation of about 8,000,000 cubic yards. The slides are uncertain. It was thought that the Cucaracha slide would come to rest because of some strengthening dikes of hard rock, but the pressure of the material above was sufficient to break or shear the rocks, allowing the material to move, which it did very rapidly, filling the almost completed canal.
The slides are of four different characters, each due to different conditions. The first is produced where a bed of clayey materials, with or without superimposed formations, rests on a bed of harder material which pitches toward the canal. If the surface is sufficiently lubricated by the ground waters, the superincumbent mass slowly moves into the canal. The second class results where faults or great geologic cracks in the rocks occur, and where the fault is at such an angle that the material in the canal prism held the rocks from sliding and, upon excavation of this material, there is nothing to prevent a sliding into the canal. The third and most subtle and difficult form of slide, which is locally known as a break or structural break, begins by a vertical settling of the banks. Cracks form in the surface of the ground and the material between cracks settles a little more on the side toward the canal, causing the cracks to open and a series of steps to form; at the same time the bottom of the canal rises. The rocks in which this phenomenon occurs are some of the weakest in the canal and are composed of loosely cemented rounded particles. Very little stress will break the cementitious material, and the rounded particles then flow freely on one another; there is nothing to prevent their sliding, as would be the case if they interlocked like pieces of broken stone. The masses do not slide directly into the canal, but the high part of the bank drops vertically, slides some, and forces the bottom of the excavation to rise. It reaches a state of equilibrium by a different method than the normal sliding
The fourth form of slide is that due to erosion and weathering induced both by physical and by chemical forces. Just as every cliff has as its foot a talus of broken bits of weathered rock, so the banks of the canal will weather, and limited amounts of material will wash into the canal. Vegetation will retard this. The rest will have to be dug or later dredged from the canal.
The serious problems of the slides will have been solved when the canal is entirely finished. No one can predict with certainty, however, that they will have been altogether eliminated by that time.* New slides may develop or old ones extend. But the slides are usually slow to develop, and the material can be rapidly excavated should any occur. The commission will no doubt leave for the maintenance force a fleet of dredges and other excavating apparatus, with which the problems could be met should they arise. It may
be added that it is most fortunate that we did not adopt the sea-level canal, for with 85 feet greater depth the slides in the cut might easily have been fourfold the present extent, and they might, indeed, have been a problem of the first magnitude.
* A slide in October, 1914, temporarily closed the canal to traffic, after vessels had been regularly passing through for several weeks.
Miraflores Lake is a very much smaller body of water than Gatun Lake, and lies between the locks at Pedro Miguel and those at Miraflores. (See plan No. 1.) It is a little over 12 miles long and about 1} miles wide at its widest part. It was at first intended to have the lower locks built close to Balboa instead of at Miraflores. If this had been done, the lake would have had an area of 7 square miles instead of, as built, only one square mile, and would have afforded a very good navigable channel of 5 miles, instead of only 11 miles, as at the present time. This was the scheme proposed by the Board of Consulting Engineers in their report of 1906. The change from the board's plan to the one finally adopted involved an increased cost of about $10,000,000, and was apparently disadvantageous, so far as the physical characteristics of the canal are concerned. The reasons for the change were that close to Balboa the locks would be subject to hostile gun-fire from the Bay of Panama, and that the preliminary work on the dam at Balboa connecting with Sosa Hill showed that a suitable foundation did not exist.
The water level of Miraflores Lake will be carried at an elevation 55 feet above mean tide. Miraflores Lake occupies a portion of the valley of the Rio Grande River, and at its lower end the Miraflores locks have been constructed in the line of this valley. Dams extend from the lock walls to neighboring hills, which are close by, so as to enclose the lake. The dam on the west side of the locks makes an apparently unnecessary sweep to the south, but the object is to capture the flow of the Cocoli River for use in the lake and to prevent the water from giving trouble in the valley below the locks. The flow from the Rio Grande and Pedro Miguel Rivers, and from one or two other smaller streams, also enters the lake. The water from Gatun Lake which is used in the single lock at Pedro Miguel will flow into Miraflores Lake. The water consumption from Miraflores Lake is that due to evaporation and lockages through the two sets of locks at Miraflores, and the amount used will be in excess of the supply from the rivers during the dry season. The difference will be made up from water allowed to flow into Miraflores Lake from Gatun Lake. During the rainy season there may be an excess of water, and this will be discharged through a spillway having gates exactly like those for the Gatun Dam spillway. The capacity of the gates was not, however, designed from the estimated flow thus obtained, but was based on the larger flow which would enter Miraflores Lake in case all the gates in one of the Pedro Miguel locks should be wrecked, and the full head of water from Gatun Lake should flow uninterruptedly through one of the Pedro Miguel locks. The discharge from the spillway is into the old channel of the Rio Grande River, over which the spillway is built. After following the old channel for about one mile, the water will be carried through the Rio Grande diversion for about 14 miles, when it