LOCATION OF ZONES OF RAPID GROUND-WATER FLOW IN THE FOUNDATION OF TARBELA DAM, WEST PAKISTAN
William M. Turner, Ph.D.
Tarbela Dam is sits astride the Indus River, 60 miles 97 km) north of Rawalpindi. The site of the dam is shown in Figure 1.
The earthen embankment of the dam is sited on a thick sequence of fluvial boulder, cobble, pebble and sand deposits of the Siwalik System. These deposits occupy the paleo-channel of the Indus River which had deeply incised its channel into the underlying metasedimentary rock of the area as the Himalayas underwent uplift. The fluvial deposits vary in thickness from about 200 feet (61 m), at a location about one mile (1.6 km) upstream of the dam, to perhaps more than 700 feet (213 m) at the downstream side of the dam.
The fluvial deposits are saturated and are characterized by very high transmissivity and very rapid ground-water flow. Several zones of extremely high transmissivity were defined based on ground-water level information, tracer studies, pumping-in tests, grain-size analysis and grout-take studies. Project engineers were greatly concerned that high velocity ground water in the foundation beneath the dam could cause piping and dam failure. Their solution to the problem was to construct a thick clay blanket above the fluvial deposits between the main dam embankment and the diversion channel upstream of the dam.
The purposes of the Thermonic study reported here were to:
• Locate the zones of very rapid ground-water flow using; and,
• Determine if the zones of rapid ground-water flow extended upstream of the diversion channel where no clay blanket was planned.
PROCEDURE
An evaluation of daily air and river water temperatures at the Tarbela dam site indicated that the air temperature throughout the year is always warmer than the temperature of the Indus River water. The mean annual air and water temperatures at the Tarbela dam site from December 1971 to December 1972 were 23.55 and 13.50 degrees Centigrade, respectively.
This large temperature differential causes the average temperature within the fluvial deposits to be colder than the average temperature of the soil at the land surface. Thus, the saturated fluvial sediments act as a heat sink with respect to both the interior of the earth and the land surface. According to Thermonic theory, the strength of the heat sink is proportional to the ground-water flow rate. The ground-water flow rate is controlled by the aquifer transmissivity and the hydraulic gradient. It is possible to identify the zones of highest ground-water flow rate within the fluvial foundation of the dam by means of Thermonic measurements taken in the unsaturated zone.
RESULTS
We located three zones of very rapid ground-water flow using Thermonic data collected from existing piezometers at depths of between 15 and 20 feet (4.6 and 6.1 m) below the land surface. All measurements were made in the unsaturated zone above the water table.
The ground-water-flow net drawn on the surface of the water table depicted three zones of rapid ground-water flow that are nearly coincident with the axes of zones of rapid flow determined by Thermonic methods.
However, Thermonic analysis projected the zones of rapid ground-water flow upstream of the diversion channel whereas traditional flow-net methods could not.
CONCLUSIONS
From the Thermonic study of the foundation of Tarbela Dam, it was concluded that the zones of rapid ground-water flow can be determined by Thermonic methods. These zones of high flow corresponded to zones of very high aquifer transmissivity.
We concluded that Thermonic methods are preferred for mapping aquifer transmissivity because of the greatly reduced cost involved.
SUBSEQUENT EXPERIENCE
Project engineers chose to disregard the results of the Thermonic analysis and chose not to place a clay blanket upstream of the diversion channel.
After completion of the dam, one of the gates in the dam became stuck during the filling of the reservoir and it was necessary to drain the reservoir. After the reservoir was emptied, project engineers inspected the clay blanket upstream of the dam. They noticed an abundance of sink holes where the blanket material had piped into the foundation alluvium. The distribution of the sink holes was in excellent agreement with the location of the high transmissivity, rapid-flow zones identified by AGW scientists.
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