USE OF THERMONICS IN FOUNDATION STUDIES IN KARST TERRANE AT WOLF CREEK DAM, KENTUCKY, U.S.A.
William M. Turner, Ph.D.
In 1971, AGW scientists carried out a Thermonic survey of Wolf Creek Dam in Kentucky for the U.S. Army Corps of Engineers. The survey sought to determine whether there were distinct zones of subflow in the dam foundation. This case history is based on Turner (1977). The location of the dam is shown in Figure 1.
BACKGROUND
Wolf Creek Dam sits astride the Cumberland River near Jamestown, Kentucky. It was completed in 1955. In 1965 a mud boil appeared in the tailrace area of the dam. This led to speculation that subflow was occurring beneath the eastern edge of the embankment. Limited tracer studies using fluorescein dye indicated that ground water moved at 20 feet per minute in some areas.
Rapidly moving ground water in the foundation of an earthen dam presents a significant safety problem. In such cases, the high velocity ground water causes a low potential head in the foundation and water within the foundation moves downward and into it carrying with it the earthen material of the dam embankment. A condition known as piping occurs. If unchecked, piping can cause the failure of the dam itself. This was the cause of the Teton dam failure.
Extensive drilling, following the recognition of leakage, revealed a well-developed system of solutionally enlarged joints in the limestone foundation. This was not a complete surprise. Construction photographs of the key trench showed solution caverns in the limestone bedrock with workers standing in them.
GEOLOGIC SETTING
The Cumberland River, at the dam site, is a broad, meandering river that has entrenched itself into a relatively flat lying sequence of shale, sandstone, and limestone. At the dam site, a thin deposit of Quaternary alluvium and regolith debris overlies the Leipers Limestone of Upper Ordovician age. The Leipers Limestone is a bluish-gray to light-gray, arenaceous and argillaceous, medium- to coarse-grained, very fossiliferous limestone that weathers to a brown rubble and forms gentle slopes. It is 80 feet thick at the dam site.
The Leipers Limestone is overlain by the Cumberland Formation, a siliceous dolomite also of Upper Ordovician age.
Overlying the Cumberland Formation unconformably is the Chattanooga Shale of Devonian age.
The most extensive rock unit exposed in the vicinity of the dam and reservoir is the Fort Payne Formation of Lower Mississippian age. The Fort Payne Formation is comprised of siltstone, shale, and limestone. It is about 270 feet thick in the area of the dam.
The regional trend of major joints in the area is about N 54o W. and about N. 27o E.. Other joints appear to trend N. 36o E. and N. 42o E.. The axis of the dam embankment trends about N. 67o E..
During excavation of the key trench during construction of the dam, at least one large alluvium-filled solution cavity was encountered in the foundation near the left bank. Extensive drilling following recognition of leakage has revealed a well-developed system of joints in the limestone foundation of the dam.
PIEZOMETRIC DATA
Traditionally, piezometric information has been of greatest importance for analyzing the hydraulic characteristics of dam foundations. The piezometric information at Wolf Creek indicated:
1. A decrease in hydraulic head downstream from the dam.
2. The river downstream of the dam is a gaining stream.
3. The source of the waters flowing beneath the earthen dam embankment is the reservoir behind the dam.
4. A zone of high permeability exists in the dam foundation near the center of the dam.
5. No specific indications of narrow zones of high flow are evident.
THERMONICS
Conventional geophysical methods of ground-water exploration measure only the properties of the rock medium in which ground water occurs rather than measuring properties of the water itself. That is, the contrast in elasticity, density, electrical conductivity, and magnetic susceptibility in fractured rock is generally insufficient to enable resolution of features pertinent to ground-water occurrence and movement. Thermonics, however, measures a property of the ground water itself, namely its ability to redistribute rising geothermal heat. Thermonics is a powerful new tool for quantitative and qualitative analysis of ground-water-flow systems.
This paper deals only with the application of Thermonics to tracing leakage beneath earthen dam embankments in karst terrane.
Thermonics is based on three universally true physical principles:
1. There is a continuous flow of geothermal heat to the land surface from the hot interior of the earth.
2. Water has a high heat capacity.
3. Ground water is usually in motion. It moves more rapidly through zones of high permeability than through zones of low permeability under similar hydraulic head gradient.
The basis for Thermonics is the Stallman Equation.
In the saturated zone, geothermal heat is redistributed by moving ground-water such that the heat flux above the base of the saturated zone is not evenly distributed in space.
THERMONIC SURVEY
At the time of the Thermonic survey, a total of 150 piezometers had been installed in the dam embankment to define better the subflow system. We carried out the Thermonic survey by measuring temperature profiles of each piezometer. The piezometers terminated at the upper surface of the limestone bedrock, and no information was obtained in the zones of leakage themselves. Because of the embankment slope, terraces, and variable piezometer depths, temperature we corrected the data to make it comparable.
RESULTS
Because of the procedures required to correct the data for terrain differences and variable piezometer depth, isogeotherms could not be plotted. The locations of the zones of significant subflow beneath the dam were located using Thermonic data and the paths taken by subflow downstream of the main dam embankment were traced. Two of the significant subflow paths pass beneath sinkholes. Furthermore, the general trend of the major subflow paths corresponds well with the regional trend of joints in the limestone bedrock of the area. The accuracy of the locations of the major ground-water-flow paths depends greatly on the density of the data network.
The Corps of Engineers classified the original work by AGW scientists for six years pending the outcome of more work. Since the Thermonic study, an additional 150 piezometers were constructed. Data collected for almost 10 years from all of these piezometers and computer studies performed by the Nashville office of the Corps of Engineers, generally confirms the paths of subflow determined by the Thermonic survey (written communication, Euclid C. Moore, Chief, Engineering Division, January 21, 1976).
The Corps of Engineers embarked on a major grouting program to stabilize the dam foundation and prevent piping. The grouting program cost nearly as much as the original dam.
CONCLUSIONS
1. The flow directions in the zones of maximum subflow are consistent with the slope of the piezometric surface and are towards the river. Ground-water flow generally does not cross the equipotential lines at right angles. However, deviation from perpendicular intersection of flow streamlines and equipotential lines is expected because the Leipers Limestone is extremely heterogeneous and anisotropic for ground-water flow.
2. The zones of relatively high ground-water flow are probably solution channels and interconnected voids. Several zones of high flow pass directly below sink hole locations.
3. Examination of both piezometric and Thermonic information indicates that piezometric information alone is not sufficient to define the path of high ground-water flow.
4. The most significant conclusion reached is that Thermonics can define the entire foundation flow pattern of a dam foundation prior to construction provided the foundation is saturated. If Thermonics had been available at the time the Wolf Creek Dam was under study, perhaps a better and safer site could have been chosen. Or, with a complete definition of the flow system, the foundation could have been adequately treated before the dam was constructed, at a great saving in cost.
REFERENCES
Turner, W.M., 1977, Use of Thermonics in Foundation Studies in Karstic Terrane, in:
Hydrologic Problems in Karst Regions, eds. Ronald R. Dilamarter and Sandor C.
Csallany, Western Kentucky Press, pp. 459 - 462.
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