The Panama Canal
Features of Construction
The Isthmus of Panama connects the two continents
through an elbow or segment of an arc running almost east and west. The canal runs
more nearly north and south than east and west, and the Pacific end of it is east of the
Atlantic end. The starting point in Limon Bay lies at latitude 9º 23' north by
longitude 79º 56' north by 79º 32' west.
The distance by air from shore to shore of this narrow part of the Isthmus is about 30 miles. The canal is 43.84 nautical miles in length from deep water to deep water. It passes through a varied and picturesque country, at places rugged, and where Gaillard Cut [Culebra Cut] goes through the Continental Divide the lowest point was formerly some 700 feet above sea level. The route selected has, in general, followed the valley of the Mindi and Chagres Rivers on the Atlantic slope of the divide, and the valley of the Rio Grande on the Pacific slope. Sea-level channels were dredged inward from either end of the canal as far as practicable -- that is, from deep water in the Pacific northward to Miraflores, and from deep water in the Atlantic southward to Gatun -- and two artificial lakes were formed by damming the waters of the rivers at higher levels, one, the Miraflores Lake, extending between Miraflores and Pedro Miguel, with surface 54 2/3 feet above sea level, and the other, Gatun Lake, extending from Pedro Miguel to Gatun, with surface 85 feet above sea level. Gaillard Cut, which is approximately 8 miles long, forms the southern arm of the Gatun Lake. The locks at Miraflores, Pedro Miguel, and Gatun are used as elevators for raising and lowering vessels between the levels mentioned.
From the initial station in Limon Bay, on the Atlantic side, the canal runs almost due south 7 miles in a sea-level section reaching to the valley of the Chagres at Gatun. Here is the great Gatun Dam, nearly a mile and a half long, closing a gap through the western end of the Quebrancha Range. The dam is an artificial ridge formed by pumping an impervious core of dredged clay and sand between parallel ridges of "toes" of rock and earth. Its construction, across swampy bottoms, was considered the most difficult feature of the canal. The top was smoothed over with earth, and the part of the slope on the lake side, lying between levels 10 feet above and 10 feet below the normal water surface, has been riprapped with hard rock to protect against wave erosion.
As completed, Gatun Dam is about half a mile wide at the base and 100 feet wide at the top, which is 103.5 feet above sea level. It contains 10,728,965 cubic yards of wet fill and 12,229,104 cubic yards of dry fill, a total of 22,958,069 cubic yards, which is more than one-sixth of the total excavation from Gaillard Cut to date.
Near the center of the dam is a concrete spillway, for discharging the surplus waters of the lake into the lower channel of the Chagres. The discharge channel is 285 feet wide and 1,200 feet long; and the spillway dam across its upper end is 808 feet long, being in the form of an arc of a circle. The top of this dam is 69 feet feet above sea level, and is surmounted by regulating gates 20 feet feet high, the tops of which are accordingly at elevation 89 feet, or 2 feet above the proposed maximum elevation of the lake. The 14 regulating gates are installed between vertical concrete piers and are raised and lowered by means of chains running over sheaves at the top of the piers and down through the piers to the operating machinery in the body of the dam. The operating machinery is accessible by means of a tunnel through the center of the spillway dam, and may be operated by remote control from a switchboard in the hydroelectric station, which is situated on the east side of the spillway discharge channel. When all the gates are open the discharge of water is greater than any known rate of run-off from the Chagres watershed. Dropping down the 60-foot slope of the ogee and striking against the baffle piers at the bottom, the water makes a turbulent fall which is one of the most beautiful sights on the Isthmus.
The hydroelectric station uses water from Gatun Lake for driving three turbo-generators of 2,000-kilowatt capacity each, which supply electricity for the operation of the lock and spillway machinery, the terminal shops and adjacent facilities, and for the lighting of the locks and the canal villages and fortifications. Transmission over the Zone is effected through four substations and a connecting high voltage transmission line which follows the main line of the Panama Railroad.
Gatun Lake, impounded by Gatun Dam, has an area of 164 square miles when its surface is at the normal elevation of 85 feet above sea level, and is the largest artificially formed lake in the world. The area of the watershed tributary to the lake is 1,320 square miles. During the rainy season, from April to the latter part of December, the run-off from this basin exceeds considerably the consumption of water, and the surplus is discharged through the spillway of Gatun Dam. Toward the end of rainy season the surface of the lake is raised to about 87 feet above sea level, in order to afford a surplus or reserve supply to keep the channel full to operating depth during the dry season, in part of which the consumption and evaporation are in excess of the supply. It is calculated that when this level has been attained at the beginning of the dry season the reserve is sufficient to assure a surface elevation of at least 79 feet at the end of the dry season in spite of the consumption at the hydroelectric station, and allowing 41 passages of vessels through the locks each day with the use of the full length of the chambers, or 58 lockages a day when the shorter sections of the chambers are used and cross filling employed which would usually be the case. This is a greater number of lockages than can be made in one day.
The creation of the lake made it possible to have a channel 45 feet deep with its bottom at 40 feet above sea level. by following the valley of the Chagres as far as Gamboa, 24 miles of channel were thus completed with relatively little excavation. At the same time the lake, by backing water far up the valleys of the Chagres and its tributaries, deadens the currents of the rivers before they reach the canal channel and decreases silting to a minimum.
At Gamboa the Chagres Valley turns sharply to the east and the line of the canal leaves it for the heavy cut through the Continental Divide. Gaillard Cut, forming the passageway between the opposite slopes of the divide, is 7.97 miles long, 300 feet wide at the bottom, and from 45 to 65 feet in depth. The great depth of the Cut is responsible for the magnitude of the slides, which are breaks in the bans, due to the pressure of the material. The elementary phenomena of slides are encountered in almost any kind of cutting or trenching through earth; the great depth of the Gaillard cut has caused similar breaks even in ordinarily firm rock. The slides are responsible for 35,158,225 cubic yards of additional excavation to February 1, 1915, To that date the total excavation from the Cut has been 117,077,044 cubic yards. The Cut is an arm of Gatun Lake and its bottom is accordingly 40 feet above sea level.
At the south end of the cut, on the Pacific slope of the divide,the waters are held back by Pedro Miguel Dam and Lock. The dam is of earth, protected by rock riprap at the water levels, and is 1,400 feet long, extending from a high hill on the west to the lock, which is set at the base of a high hill on the east.
Below Pedro Miguel Lock and Dam is a small lake, Miraflores Lake, through which the channel passes to Miraflores Locks, which effect the transit between Miraflores Lake and the Pacific entrance channel. the surface is normally 55 feet above sea level. Its area is 1.88 square miles, and it may always be kept at full depth by supplying water from Gatun Lake, as to fill it completely from Gatun Lake would lower the surface of the latter less than 6 inches. The length of the canal channel through it is 1.4 miles. The lake is impounded by an earth dam 2,700 feet long, connecting with Miraflores Locks from the west, and by a concrete spillway dam to the east of the locks, 500 feet long, on which are mounted eight regulating gates similar to those on the Gatun Spillway.
The transits between the several levels of the canal -- between the Atlantic and Gatun Lake, between Gatun Lake (Gaillard Cut) and Miraflores, and between Miraflores Lake and the Pacific -- are effected by means of massive locks of concrete and gates of steel.
Three sets of locks were built -- one set in three successive levels at Gatun, a set with one lift at Pedro Miguel, and set of two lifts at Miraflores. The differences in levels overcome at the three places are, respectively, 85, 30 1/3, and 54 2/3 feet, the latter varying according to the tide in the Pacific, figure given being for mean tide.
Each lock consists of two parallel chambers, which effect a double-tracking of the channel and allow vessels going in opposite directions to use the same flight of locks simultaneously. All the chambers have the same length, 1,000 feet, and width, 110 feet; the depth of water in the locks varies from approximately 81 feet when a boat is being locked down and 45 feet when a boat is being locked up, and there is always a minimum depth over the gate sill of 45 feet.
At the upper and lower ends of each set of locks the center wall was extended approximately 1,250 feet to form a long pier, against which entering vessels can bring up before entering the chambers of the locks proper; and the side walls were flared out at an angle of 60º to form a funnel-shaped entrance. Both the center-approach walls and the flare walls are fitted with strips of timbers resting on helical springs to form buffers for the vessels, and the outer end of the center wall is fitted with a resilient fender of heavy timbers.
The channels of the lock are blocked by massive steel gates which cut off the flow of water and divide the locks into chambers. The flow of water into and from the chambers is effected through culverts running longitudinally through the bottoms of the side and center walls and feeding into the chambers through lateral culverts running under the floors of the chambers and emptying upward. The culverts are controlled by valves.
In all 92 leaves, forming 46 gates of two leaves each, are used in the three sets of locks. Each leaf is 65 feet long, so that then two are swung together to form a closed gate they meet in the center of the 110-foot width at an obtuse angle. The leaves are so set that this junction always points upstream against the downward pressure of the water; this pressure accordingly forces them firmly together and affords an element of safety, since a gate can not be opened until the water on both sides has been equalized. The leaves range from 47 to 82 feet in height, according to location, and they weigh from 390 to 730 tons each. The 82-foot gates occur only at the lower end of Miraflores Locks, where they are necessary on account of tidal conditions.
The leaves are hinged to anchorages in the walls and are swung back and forth like ordinary gates. They are moved by machines driven by electric motors. In fact, every piece of mechanism in the locks is actuated by electricity, and this has made possible a central control by which an operator at a central switchboard can cause every movement of the lock equipment except the running of the towing locomotives, which are under the control of individual operators riding on them, and the handling of the emergency dams.
[Editor's Note, 10/11/99: Madden Dam was built in the 1920s to supplement the water supplied by Gatun Lake for all Canal operations. There was a severe drought a couple of years ago which did threaten Canal operations. There are plans right now for the immediate building of another dam. Approximately 8,500 families will be displaced by the building of this dam.]
From The Official Handbook
of the Panama Canal, 1915
October 11, 1999
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