Into the locks at Gatun will go 2,000,000 cubic yards of concrete if the original estimate is adhered to. A statistician estimates that it would build a wall 8 feet wide and 12 feet high and 133 miles long—which would just about wall off the state of Delaware from the rest of the Union.
The side walls of each of the locks are practically monoliths, constructed of concrete poured into great steel frames or moulds where it hardens into a solid mass. They are based in the main on bed rock, though it was found on making tests that the bed rock was not of sufficient extent to support the guide walls as well, so one of these is therefore made cellular to lighten its weight, which rests on piles of 60 feet long capped and surrounded with concrete. This wall was built by slow stages and allowed to stand in order that its settlement might be uniform. An examination of the picture below will make clear the method of constructing the lock walls, for in it are shown the completed monoliths and a steel form half completed with men preparing it for the concrete therein. Col. Sibert describes the details of the work thus:
“The locks proper are founded on rock and the heavy masonry is completed. This rock foundation was not of sufficient extent, however, at available elevations, for supporting the guide walls. Under that guide wall extending into the lake the underlying rock at the south end is about 150 feet below sea level, and the overlying material is soft. This wall is cellular in construction. It is composed of four longitudinal walls about 2 feet thick with cross walls about 17 feet apart, all built of reinforced concrete.
Photo by Underwood & Underwood
BUILDING A MONOLITH
“The natural ground underlying the wall was about 8 feet above sea level. On this ground a wide fill with a very flat slope was constructed to elevation plus 35, and through this piles about 60 feet long, 4-foot centers, were driven and a heavy reinforced concrete slab built around the heads of the piles, on which was erected the cellular structure. There was a continual slow settlement of this wall as its construction progressed. It was brought to a height of 61 feet above sea level through its entire length in order that the settlement might extend over the whole base before any part was brought to full height.
A CULVERT IN THE LOCK WALL
“The north guide and flare walls are yet to be built. It will be necessary to go to a depth of about 70 feet below sea level through very soft material in order to uncover the rock on which to build the flare walls. Under the guide wall itself the rock is at a still lower elevation, and a pile foundation will probably be constructed, the piling going to rock. The material in this space was too soft to hold up steam shovels, and it was decided to do the general excavation by suction dredges. These dredges cut their way into the space where the walls in question are to be built, making a channel just wide and deep enough for their passage. They then widened out the cut and deepened it to 41 feet below sea level. An earthen dam was then built across the narrow entrance cut, shutting off the connection with the sea, and as the dredges worked they were lowered. They are now floating at an elevation of 32 feet below sea level and can remove the material to the depth required. After the excavation is completed it is proposed to have the dredges excavate a sump 65 feet below sea level and lower the water to 50 feet below sea level in order to test the stability of the sides of the cut. If there is no sliding the pit will be filled with water; the dredges floated out; the dam across the entrance channel replaced and the excavation unwatered for the construction of the walls first referred to.