Underestimated Arsenal posted
"Foundation conditions at Guntersville Dam presented interesting engineering and construction problems, the solutions of which are thought to be unique," Verne Gongwer, the Tennessee Valley Authority's project engineer on the dam in Alabama, wrote in the Proceedings of the American Society of Civil Engineers in March 1940. The problems were under the river. The Tennessee there runs over limestone, and over time water had dissolved parts of it. Test borings showed a fairly level rock floor. They did not show what was inside it: cracks and bedding planes widened by water into open seams and channels, some only partly filled with loose, porous material. This Army aerial photograph was taken on December 22, 1938, from 1,500 feet. You are looking upstream. The river is pouring over the new spillway. The lock walls stand on the far bank. On the near bank, behind a curved row of round steel cells, the powerhouse is going up. The dam was built in three stages: the navigation lock first, then the spillway, then the powerhouse and its intakes. To build on the riverbed, each stage needed its own cofferdam: a ring of round cells made of interlocking steel sheet piles, driven down to the rock, filled with gravel, and then pumped dry inside so the work could go on. The first went round the lock. When it was driven, the piles seated firmly on the rock, and their tops looked "very uniform and satisfactory." But one open seam ran the full distance beneath it. Several times the river came in underneath, in leaks the engineers called blows, and filled the cofferdam. Crews ran barge loads of sand bags, gravel and baled hay out to the river side to plug the openings. Nobody could find every entrance to the seams. About six weeks of work were lost. So for the second cofferdam, the one for the spillway, the engineers changed the order of the work. Before pumping it out, they drilled down through the center of the cells every 10 feet and pumped grout, a thin cement mix, into every seam in the rock below. Men watched the river and the cofferdam for bubbles, froth or the color of cement, the signs that grout was escaping, and slowed or stopped until it began to stiffen. It held. The filling of Wheeler Reservoir, the next lake downstream, had raised the water outside as high as the river rises that had always flooded the first cofferdam. Yet the leakage was so small that one 6-inch pump, running part time, could handle it. The work inside took about four months. The drilling and grouting cost less than all the patching on the first one. The speed mattered. The engineers needed to close the third cofferdam, for the powerhouse, and turn the whole river through the spillway before the winter floods. Between December 14 and 18, 1937, they did it, and the third cofferdam was grouted the same way. That is the curved row of cells in this photo, a year later, with the Tennessee still running over the spillway beside it. On January 16, 1939, less than four weeks after this picture, the lake behind the dam began to fill. Today Guntersville Dam stands 94 feet [28.7m] high and 3,979 feet [1213m] across the river, with a visitor center and hiking trails on both banks. Most of the laborers and tradesmen who built it came from within 60 miles of the site. The lake they made is now the largest in Alabama.
The grout worked almost too well. In some cells of the second cofferdam it rose up into the gravel fill and set hard, and Verne Gongwer wrote that it "produced concrete-like masses, many of which were of sufficient hardness to require drilling and blasting." That gravel was meant to be dredged out and reused to fill the third cofferdam. He called it "one minor drawback," and the third cofferdam was grouted the same way. About the photograph. It is an Army aerial print, lettered on its face with the date, December 22, 1938, the time, 10 A.M., and the height, 1,500 feet. The mount says it was received from Maxwell Field, Alabama, on January 12, 1939. Today it is in the National Archives at College Park, Maryland, among the records of the Army Air Forces, numbered 18-AA-1-18. I cropped off the archive's mount with its handwritten notes, and the bottom strip of the print, where the photographer's lettering is written in white across the trees. Nothing else was changed. What the photo cannot show is the grouting itself. It happened down in the rock, out of sight. What you see is the result: the river going over the spillway, and the powerhouse being built beside it. In those last weeks of 1938, Gongwer notes, a suction dredge was pumping gravel out of the cofferdam cells. One honest note on dates. The Encyclopedia of Alabama gives January 24, 1939 as the day the dam was completed. I have used the date in Gongwer's own paper instead, January 16, 1939, when the filling of the lake actually began. The words on the photo rest on the same paper. The first steel wall is the lock cofferdam, which filled with river water several times and cost about six weeks. The next wall is the spillway cofferdam, grouted before it was pumped out, which needed only part-time use of one 6-inch pump. The main source is Verne Gongwer's paper "Foundation Conditions and Treatment at Guntersville Dam", in the Proceedings of the American Society of Civil Engineers, March 1940. The magazine Construction Methods, in its December 1938 issue, named him as the Tennessee Valley Authority's project engineer and described the three stages: lock, spillway, then powerhouse. The dam was not built by a contractor. The introduction to the set of TVA papers in that issue says it plainly: "The projects were built with the Authority's forces." That means the Tennessee Valley Authority's own crews, most of them laborers and tradesmen from within 60 miles of the site. They were the men on the barges with the sand bags and baled hay, and the drill crews who sealed the rock. Guntersville Dam is at 3464 Snow Point Road, Guntersville, Alabama, and the Tennessee Valley Authority keeps a visitor center there. |
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