THE PANAMA CANAL 



173 



In case a ship should break the upper 

 lock gates and the Gatun Lake should 

 start to flow through the flight of locks 

 to the sea, a swing bridge is provided for 

 each upper lock, by means of which an 

 emergency dam can be built across the 

 locks. The operation of these swing- 

 bridge dams is as follows : The bridge is 

 first swung across the lock chamber and 

 heavy girders are then lowered, one end 

 of each of them finally resting against a 

 sill previously built in the bottom of the 

 lock entrance. Steel curtains are then 

 run down in tiers on tracks on the gir- 

 ders, gradually building a dam and stop- 

 ping the flow of water. The girders and 

 curtains are all lowered under power and 

 the entire operation can be carried on 

 through swiftly flowing water. 



It is purposed to allow no ship to go 

 through the locks of the Panama Canal 

 under its own steam. A ship will be re- 

 quired to land alongside the guide wall 

 at either end of the locks. 



Towing locomotives, four of them, will 

 run down the guide walls, pass lines to 

 the ship, two forward and two aft, and 

 will then tow the ship into the locks, hold 

 it during the time it is being raised or 

 lowered, and finally deliver it alongside 

 the guide wall at the other end. This is 

 a precaution that has never been taken 

 before in operating locks. One of the ex- 

 perimental towing locomotives is shown 

 in the picture on page 171. 



Ships are ordinarily allowed to pass 

 through locks under their own steam. 

 The wrong ringing of a bell or the mis- 

 understanding of a bell has ordinarily 

 been the cause of wrecking a lock gate. 



HIGH AND DRY BEEOW SEA-EEVEL 



One of the most difficult problems at 

 Gatun was the preparation of a founda- 

 tion at the lower end of the locks. It was 

 necessary to go 70 feet below sea-level 

 through soft mud to find rock suitable 

 for foundations. The material was so 

 soft that steam shovels could not be sup- 

 ported on it, so it was decided to do the 

 excavation by dredges. A sufficient width 

 of land between the space to be excavated 

 and the canal toward the Atlantic was 

 left to act as a dam when the excavation 

 was finally completed and unwatered. 



A dredge was allowed to cut a narrow 

 channel through this dam into the space 

 where the walls were to be built. This 

 dredge dug the entire space to a depth 

 40 feet below sea-level, which was its 

 limit. A dam was then placed across the 

 narrow entrance cut, with the result that 

 the dredge lowered itself as it continued 

 its work. When it had lowered itself to 

 30 feet below sea-level, it could excavate 

 to the required depth — 70 feet below, sea- 

 level. 



After completing the excavation, the 

 dredge pumped all the water out of the 

 space, leaving itself grounded 55 feet be- 

 low the level of the sea, in which position 

 it remained until the walls were com- 

 pleted. Water was then let in from the 

 sea; the dredge floated and cut its way 

 out (see page 179). 



THE GREAT GATUN DAM 



Gatun dam, across the Chagres Valley, 

 is about a mile and a half long, a third 

 of a mile thick at the base, with a top 

 elevation of 105 feet above sea-level. A 

 small hill existed in the center of the 

 valley at the clam site. While it was 200 

 feet below sea-level to rock in the portion 

 of the valley to the east of the central 

 hill and 260 feet to rock in that portion 

 to the west, rock was found at 40 feet 

 above sea-level in the central hill itself — 

 a fortunate condition, because a stable 

 channel for the temporary diversion of 

 the Chagres could be easily cut through 

 this hill, across which channel a masonry 

 spillway, founded on rock, could be built 

 later. 



A spillway through an earthen dam is 

 generally looked upon with disfavor. 

 While the Gatun Dam is earthen, it is 

 really two dams, one extending from the 

 locks to this central hill and one from 

 this central hill to the west side of the 

 valley. Rock existing at a high elevation 

 in this central hill made it an ideal loca- 

 tion for a spillway or waste weir. 



When the building of the Gatun Dam 

 was commenced, the Chagres River was 

 flowing through three channels at the 

 dam site — its own, the old French canal, 

 and a diversion channel dug by the 

 French to the west of Spillway Hill. The 

 first operation was to block the flow of 



