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Even the well-nigh impossible combination of circumstances described above would not wreck the canal. The mitering lock gates could, of course, not be closed against the flowing stream, therefore, to stop the flow the emergency dam would be brought into play. The emergency dam is in the form of a bridge resting on a turntable on the side wall of the lock. It may be turned so as to span the lock and then be firmly bedded on each side. A series of steel girders with the upper ends fastened to the bridge would be lowered by cables into position, having the lower ends on a concrete sill provided for the purpose. Then steel plates would be forced down one by one, supported by the girders, and the opening would thus be gradually closed by a steel wall and the flow practically stopped. A floating caisson such as is used with dry docks would then be placed at the lake end of the lock on a seat provided for the purpose. The caisson carries a pumping plant for unwatering the lock, and repairs may thus be made. In the meantime traffic would use the other series of locks in both directions.
The locks are some of the most massive concrete work in the world. The dividing wall between the flights of three locks at Gatun, with the approach walls which are in extension of the dividing wall, forms a mass of concrete 60 feet thick, about 81 feet high, and over 1} miles long. The approach walls are of cellular construction. The dividing wall is built with the faces vertical and is solid for over half the height. Above the solid portion the center of the wall is filled with earth, except three superimposed tunnels. The lowest tunnel is used for drainage of the upper ones, the center tunnel for electric light and power cables, and the upper one as a passageway for employees to reach the various chambers containing machinery for operating the miter gates and the many valves. The exterior walls of the locks are of equal height with the central wall, and are from 45 feet to 50 feet thick at the floor-level; they diminish by steps on the back to a thickness of 8 feet at the top. The thickness of the floor is variable but is approximately 13 feet.
The emptying and filling of the locks is done through circular openings in the floor, each 3 feet 103 inches in diameter and having an area of 12 square feet. There are five of them in each line across the lock, and the lines are spaced 32 feet to 36 feet apart. In one 1000foot lock there are in all 105 openings, with a total area of 1260 square feet. Each row of five openings communicates with a cross-tunnel under the floor. Eleven of these cross-tunnels in each lock lead to the outside wall and there open into a culvert 18 feet in diameter, without the interposition of valves. The remaining ten tunnels, alternating with the others, lead to the center wall where a cylindrical valve allows each tunnel to communicate with the culvert in the center wall, which is also 18 feet in diameter. The center-wall culvert receives the tunnels from both locks. It extends the full length of the three locks at Gatun, and at the upper end it opens into Gatun Lake, while the lower end discharges into the sea-level canal. There are control valves at each end and also in the line of
Fig. 15. — Gatun Locks, showing towing locomotives handling coal barge. All vessels are towed in this manner. Locomotives are driven by pinion operating on rack visible at left of picture. Note lock gates folded into recess in wall.
the culvert at intermediate points corresponding to the locks. It is evident that, with a proper adjustment of the culvert valves, the water in any two lock chambers may be equalized by opening the cylindrical valves that allow the floor tunnels to communicate with the center culvert. This allows a very considerable saving of water in operation. With all culvert valves open, the center culvert may be used to discharge water from Gatun Lake into the sea. The side-wall culverts
. also extend the full length of the locks, and have control valves at the ends and at points corresponding to the ends of the locks, and at the subdivision points. They may be used in equalizing the water in any two locks that adjoin endwise, or in passing water into and out of the end locks. (See plan No. 6.)
All machinery connected with the locks is operated by electricity and has been specially designed for the canal work, based largely on previous experience with locks. There are 114 rising stem gate valves, 120 cylindrical valves, and 92 machines for operating the individual leaves of the large miter gates. The apparatus is equipped with remote control and position indicators, which show at the control switchboard the positions of the gates and valves during operation. Indicators also show the various water levels. All regular operating machinery will be controlled by means of interlocking switches, with one central switchboard for each of the three groups of locks at Gatun, Pedro Miguel and Miraflores. There is also local control and hand oper