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Tue July 03, 2007 - National Edition
Wolf Creek Dam on the Cumberland River in south central Kentucky, is in the beginning stages of a $309 million repair project to control dangerous seepage under the structure’s mi.-long earth embankment. If left unchecked the seepage could threaten the embankment.
Seepage caused by the water from the reservoir puts a tremendous amount of pressure on the foundation, the U.S. Army Corps of Engineers said. This pressure carries away soil through openings in the foundation rock, eroding away the earthen embankment. The erosion can’t physically be seen but various scientific instruments embedded within the dam indicate the leakage. The exact rate or the full extent of the erosion is not known and even though there are no visible cracks in the dam, settlement of the dam has been noticed on a small scale.
Excessive seepage was first noticed at Wolf Creek Dam between 2002 and 2004 when the lake level was high, due to heavy rainfall. The Corps began to control the lake level in 2005 and decided that major rehabilitation of the dam was necessary. The corps later announced the mi.-long dam that impounds 101-mi.-long (162 km) Lake Cumberland was declared in high risk of failure.
In 2005 and 2006 the Corps performed an initial screening of more than 130 dam projects, which represent approximately 20 percent of the Corps’ 610 dams. The screened dams were believed to be the highest risk among those the Corps owns and operates.
As a result, the Corps identified six dam projects as high priority because they are critically near failure or have extremely high life and/or economic risk, which has made them a national priority for funding, studies, investigations and remedial work — Wolf Creek Dam is among the top six. The Corps asked an independent external panel of experts to review and assess these six dams to determine what measures need to be taken.
The panel is made up of six engineers who were called in by the Corps for independent advice on the aging dam, which was built to control flooding and bring electricity to rural Kentucky. And while the dam could fail in five years if something isn’t done, fixing it may take seven years.
If the Wolf Creek Dam were to suddenly give away, which would be the worst case scenario, there would be an estimated $3 billion-plus in property damages. Maps have been released by the U.S. Army Corps of Engineers showing five Kentucky counties and 15 Tennessee counties that would be threatened by flooding. Some well-known sites that would be affected include: LP Field, where the Tennessee Titans football team plays; the Country Music Hall of Fame; the Gaylord Opryland Resort and Convention Center; and the Roy Acuff Theater, where the Grand Ole Opry is located during peak tourist season.
Maps for the counties threatened by flooding can be found on www.lrn.usace.army.mil/WolfCreek.
As a first step, the lake level was lowered in January to 680 ft. (207 m) above sea level on Wolf Creek Dam to ease the pressure and reduce the chance of the embankment giving away.
Reducing the water depth behind the dam to 680 ft. reduces the load on the dam’s foundation 10 to 25 percent, the Corps said. The lake will remain at its reduced level until at least the end of 2007.
The decision to lower the water level came from Great Lakes and Ohio River Division Commander, following a recommendation from the Nashville Corps of Engineers District Commander. The decision was made considering the ramifications the drawdown would cause the communities upstream and downstream of the dam, as well as hydropower production, recreation and water supply, the Corps said. The lake supplies water for 200,000 people.
A six-generator-unit power plant, with a capacity of 270,000 kW, is located immediately downstream. Overall there will be a reduction in hydropower generation in the region. The reduced lake level affects the cold water that is available in Lake Cumberland and it may have an impact on downstream fossil fuel plants, since they require large amounts of cooling water to maintain operation.
Additionally, Wolf Creek Dam in Russell County impounds Lake Cumberland, creating the largest man-made lake east of the Mississippi River, creating a huge tourist industry. The lake attracts approximately 5 million recreational visitors each year, which is more than Yellowstone National Park’s approximately 3 million. According to Johnny Wilmore, chief of construction for the Corps’ Nashville District, tourists during the Memorial Day weekend commented they enjoyed their time on the water and it is still a great vacation spot, even with the lower water level. The Corps estimated a 50 percent reduction in visitation, which translates into a $23.6 million loss in direct sales, $8.5 million loss in personal income and a $38.4 million decline in trip spending within a 30 mi. radius.
To improve the dam’s stability, the corps awarded a $14 million accelerated grouting contract last December to Advanced Construction Techniques, Ltd. of Toronto, Canada, to begin the foundation grouting process. Additional grouting will be placed after the accelerated portion is complete, bringing the total of the ACT Ltd. contract for foundation grouting to $51 million.
Another $8 million contract was awarded last September to VCI-Doyon JV, of Knoxville, Tenn., to relocate Halcomb’s Landing. The completion date for the grouting and the landing relocation is October 2007.
This fall, the Corps’ ongoing operation and maintenance authorities will judge whether grouting has helped the stability of the dam, and will decide what level the water should be kept at next year. It is possible during the grouting process that it will be decided that the seepage has been reduced enough that the water level is again increased to its normal 760 ft. (231 m), at which it impounds more than 6 million acre-ft. of water. However, the Corps said, there is no guarantee that pool levels will be increased until the work is totally complete.
Holes are dug approximately 300 ft. (91 m) into the ground to the base of the dam and then grout is pumped into the washed out areas until they are full. Preliminary grouting started in late December of 2006 where two lines of grout are placed, one in front of the earthern embankment and one along the dam, said Bill Peoples, chief of public affairs of the Corps.
Grout is a fluid form of concrete that can be pumped into the foundation voids under pressure. This grouting program will reduce seepage under the cutoff wall.
The current grouting is only a small portion of the project. Grouting is under way now and some critical sections will be done by the end of May.
“Our goal is to have the first grout line done by the October 2007 deadline,” Peoples said.
The grouting gallery in the bowels of the dam is the full length of the dam and so far approximately 1,000 ft. (304 m) has been completed.
“We’re just getting to the point of grouting on the outside of the dam on the platform and we expect to have that completed within the next month or so,” Wilmore said. “The single line for full length of the dam won’t be completed until January 2008.”
However, a large amount of grout has not been inserted to this point because this is beneath the concrete dam.
“The quantity is only a few hundred cu. ft., which a good sign the foundation is good and solid,” Wilmore said.
“Underneath the dam we took half of those drillings to elevation 375 ft. and we’re still not hitting any large holes, which is a good sign. We’ve completed a grout curtain 275 ft. under the dam, which is a second grout line that is just 200 ft. upstream; grout takes could be a little higher there,” Wilmore said.
“We anticipate that, with double grout curtain, we will use half a million cu. yds. of grout, which is still not a huge amount,” he added. There are some clay-filled cavities that could be fairly large. The grout will replace some clay to form a solid barrier. The grouting is to provide some stability in preparation of constructing the concrete seepage wall that should have a useful life of 50 to 100 years, Wilmore said.
Workers also are installing a concrete diaphragm wall through the entire length of the earthen embankment and upstream of the right-most concrete monoliths. The diaphragm wall will be constructed 75 ft. (22 m) deeper into the foundation than the previous wall. The plan was approved in August 2005 by officials at the Headquarters, U.S. Army Corps of Engineers.
The Corps plan is to install a grout curtain on both sides of the cutoff wall to keep from losing drilling fluids into any large voids and to keep water seepage from washing it away, Wilmore said.
The second phase is to install concrete cutoff walls in between those two lines of grout, which began in January 2007. The remedial construction is currently scheduled to be completed in 2014; however, options are being investigated to speed up the construction process to finish earlier, Peoples said.
The cutoff wall is actually the fix for the dam, Peoples said. The grout is to help stabilize the dam in preparation for the construction of the cutoff wall.
The concrete cutoff wall will be constructed through the embankment and the problem foundation rock as a long-term fix for leaks that will reinforce the purpose of the original wall. The wall will be at least 3 ft. thick and will be installed as deep as 275 ft. (83 m) so that it will be deeper than the deepest sections of the original wall and as much as 75 ft. deeper than the majority of the original wall. It will start immediately upstream of the right most concrete monoliths and run the length of the embankment into the right abutment, which will take it 1,650 ft. (502 m) beyond the existing wall. This portion of the project is expected to bring the total project to a cost of $309 million.
The cutoff wall will be approximately 1 mi. (1.6 km) long; seepage is from underneath through the rock foundation that is highly weathered and fairly hard with uneven rock that can cause some problems when the drills hit vertical crevices, Wilmore said. Crews are drilling approximately 300 ft. to reach the base of the dam.
Crews are using a resident sonic method to drill through the hard compacted clay embankment, which is a different method than normally would be used on this type of project because of a lot of pressure that could fracture the embankment. The sonic method is more common in landfills or areas where there could be contamination, Wilmore said.
In the rock, four-in. (10 cm) diameter holes are being drilled with a Cubex rock drill and a Prosonic Abort Long Ear lf90d to core holes every 100 ft. (30 m) in order to examine the core.
To install the grout curtain below crews are using two Atlas Topco Dimacs to drill 3.5 to 4-in. (8.9 to 10.1 cm) holes for the grouting.
“They start with 20 ft. spacing and close that to 10 ft. and they may close it to 5 ft. spacing, depending on how much grout it takes,” Wilmore said. “It could even be closed to 2.5 ft. For the concrete wall, we anticipate the contractors will consider at least two alternative methods of installing the wall; one is drilling overlapping circular holes; and the second is using a hydro mill, which is a larger device that has wide cutting heads that could excavate a panel three to four ft. wide. Crews would overlap panels 9 ft. deep, rather than overlapping holes. We haven’t gotten the conception that that will be good because of the hardness of the rock. We don’t think we will know until the bidders/proposers come in with proposals and demonstrate that this can be done, probably late this year or early next year. We don’t expect to award that contract until January of 2008.”
Between 50,000 to 75,000 cu. yds. (38,000 to 57,000 cu m) of concrete is expected to be used on the wall. “This is a huge excavation job, not one that requires huge quantities of material because the wall is not very thick,” Wilmore said. “Constructing a concrete wall this deep is fairly unique; it is basically a trench and we’re overlapping holes to place concrete in.”
The corps is confident the concrete wall will be deep enough to fix the problems at Wolf Creek.
“What we have learned from working below the concrete dam is the dam is still in very good shape; that was not unexpected. We really thought the dam was solid and that has been verified because the voids are not huge,” Wilmore said. “So far it looks as though we will not have to lower the lake further, but no decision has been made yet to raise the level either.”
The Corps also is building a work platform for future wall work that is 60 ft. (18 m) wide and approximately 1 mi. long that is almost complete, Wilmore added.
Wolf Creek Dam has seen leakage since it was first constructed 55 years ago. The 5,736 ft.-long (1,748 m) Wolf Creek Dam is a combination rolled-earth fill and concrete gravity structure. It has a maximum height of 258 ft. (78 m) above founding level. U.S. Highway 127 crosses the top of the dam.
Because the dam was built in terrain where there are channels and caverns in the limestone underneath it, leakage has been inevitable.
Builders tried to cut off those potential channels for leaks during construction in the 1940s and 1950s and again in the late 1960s when there was almost a catastrophic breach near where the dirt and concrete sections join, the Major Rehabilitation Report said.
The corps injected 290,000 cu. ft. (8,200 cu m) of grout into the foundation and the embankment in the bedrock (soil portion of the dam) to stave off failure, and then in the 1970s, it built a concrete wall inside the earthen section of the dam. But because of technological and budget limits, the wall didn’t extend the whole length of the earthen section, and it didn’t go deep enough into the porous rock under the dam.
Although some seepage is normal, in recent years the seepage levels have gradually increased. Pictures in the report show where large wet spots have formed on the ground — a sign that water from the lake is seeping past the dam. A chart shows that 37 wet spots were found in 2004.
Engineers in the district’s Nashville office are looking at the idea of building a “roller-compacted concrete dam” just down the Cumberland River from the existing dam as an alternative to the wall. A roller-compacted dam would be constructed with layers of concrete, each compacted with heavy equipment. The concrete would be a “really dry mix” unlike the more familiar soupy concrete that wouldn’t support compaction.
The structure would connect the east side of the Cumberland River bank to the existing concrete portion of the dam, covering land now occupied by a national fish hatchery, a recreation area and other dam facilities. And it could take 10 to 15 years to build, Wilmore added.
The study on the roller compacted dam should be completed mid to late June, Wilmore said. “That would be a drastic alternative that would mean lowering the lake; everyone is hoping to avoid that. We’re fairly certain the concrete cutoff will be adequate, but we can’t say that with complete certainty.” CEG