National Geographic : 1968 Dec
The slope was slippery and steep, the sun was out again, and as we made our way up, I felt as if the heat and the humidity might soon dissolve me. We stopped 280 feet above the river. A sign said Pa Mong Dam Site, Hole 108. Energetic Thai technicians and a Swedish drill powered by a noisy Volkswagen motor were bringing up three-inch-thick cylinders of rock. "Samples of siltstone," said Mabbott. "You've got to seal them fast, in hot wax, or they'll disintegrate. We send them to Denver for analysis. You can't design a dam until you know the geology." And the topography. As Mabbott explained 752 the problems, I began to see why, with all the money in the world, the Pa Mong Dam couldn't possibly be finished in a hurry. The quality of the rock determines how heavy and high a dam it can support. Enor mous areas must be surveyed to calculate how much water would be stored at various heights. These factors would dictate the strength of the dam, its thickness and shape. Mabbott drew a graph. "Pa Mong at 325 feet above the riverbed would hold 3.4 tril lion cubic feet of water. Now we think we can go to 360 feet, to give us 3.7 trillion-two and a half times as much as Hoover Dam in the States. Maybe we could go to 390 .... " When would he know? "We'll have good guesses by 1969, and the Statistics to stagger the imagination: The Pa Mong Dam will stand at least 325 feet high, stretch close to a mile, and back up more than twice as much water as Lake Mead holds behind Hoover Dam - enough to irrigate 5 million acres in Thailand and Laos. Annual power output will be 20 billion kilowatt-hours- 1 /2 times that of Grand Coulee Dam, largest U. S. hydroelectric producer. Americans carry on vital roles in the Mekong Project. Lyle W. Mabbott, right, chief engineer for the Pa Mong Dam, confers with Jay Olson, who supervises geological studies at the site. Both are in Thailand on loan from the U. S. Bureau of Reclamation.