Some Engineering Problems Of The Panama Canal In Their Relation To Geology And Topography

ROCK MATERIAL USED IN CANAL CONSTRUCTION. 41 run-off is small; the rocks during that time carry all the water they can possibly contain. Secondly, just how the water is going to separate and break apart the minute particles of a fine-grained rock which are cemented together, without deforming movements to grind and disintegrate the rock, is difficult to understand. The Gatun rock might possibly become soft through deforming movement and mixing with water, but such movements are impossible where there is no lateral force, but only a downward or compressional load from the weight of the dam. This weight will tend to keep the rock in the same solid condition in which it originally was and is now. Solution had a chance to complete its effects centuries before the dam was built. If the weight of Gatun, Lake is going to force water into the Gatun formation and cause it to soften into a semimud that will allow the dam to sink, then all the islands in Gatun Lake and all the hills around it, which are of the same formation, should also have their bases softened in the same way, and sink into the depths. Reports that the Gatun dam and locks are in any clanger from sinking have not the slightest foundation on geologic data. The third question, as to the best type of dam to adopt, was answered by the application of a few simple engineering and geologic principles. It was recognized that the cheapest dam would be one of earth and rock. Therefore, instead of dumping the traiiiloads of material brought from Culebra Cut and nearer excavations on some spoil bank, they were dumped on the Gatun. Dam. In other words, this dam site was made a great dumping ground whore many trains could run out and have their loads plowed off by machine unloaders. It was evident that if the base was made broad enough the dam would bo amply strong to resist any water pressure that might develop, and would be nearly as secure as a ridge of hills. Also, an adequately wide base would provide a satisfactory foundation for the dam. In. the matter of water-tightness, two main facts wore evident : If the dam was high enough, it would sink into any local areas of soft and spongy rock or soil that might be under it, and thus prevent leakage around its base; and if very fine material could be calked in. so as to fill the interstices between the coarser fragments, the dam would be practi- cally water-tight. It was to be nearly half a mile wide at its base and about 105 feet high, so it had ample base area for its elevation to make the foundation secure and to resist the head of water that Gatun. Lake was expected to develop. In order to get the interstices between tho coarser fragments filled with fine clay so as to make the dam water-tight, the outside rims of it were built of coarse material (fig. 5). Then, suction dredges, operating in the river channels and flats on either side of the dam, pumped mud into the basin thus formed. This drained into the openings between the larger frag-