Dams are the solid barrier constructed across a river valley for impounding water. Geological investigation for dam site is performed in the surface or subsurface and carried out either by direct or indirect method.
The important geological requirement which should be considered in the selection of dam site are as follows:
Narrow river valley
Bedrocks with shallow depths
Horizontal strata
Tilted beds
Narrow river valley:
At the proposed site, if the river valley is narrow, only a small dam is required, which means that the cost of dam construction will be least. On the other hand, if the valley is wide, a bigger dam is necessary which means the construction cost will be very high. But such sites should not be blindly selected without further investigations because sometimes it may have severe defect which may lead to serious leakage from the dam.
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Dams are the solid barrier constructed across a river valley for impounding water. Geological investigation for dam site is performed in the surface or subsurface and carried out either by direct or indirect method.
The important geological requirement which should be considered in the selection of dam site are as follows:
Narrow river valley
Bedrocks with shallow depths
Horizontal strata
Tilted beds
Narrow river valley:
At the proposed site, if the river valley is narrow, only a small dam is required, which means that the cost of dam construction will be least. On the other hand, if the valley is wide, a bigger dam is necessary which means the construction cost will be very high. But such sites should not be blindly selected without further investigations because sometimes it may have severe defect which may lead to serious leakage from the dam.
Bedrocks at shallow depths:
To ensure the safety and stability of the dam, it has to be necessarily on very strong and very stable rock. If such bedrock is at shallow depth, cost of dam will be ultimately lower. On the other hand, if such bedrock occur at great depth, the cost of foundation will be very high because it enables extensive work of excavation of loose overburden and concrete refilling. if the thickness of the loose overburden is negligible, the country rocks may be found to occur exposed frequently along the river valley.
Horizontal strata:
This geological situation is good at the dam site because the load of dam acts perpendicular to the bedding plane, which means that the beds are in an advantageous position to bear the load with full competence. Further, the seepage of reservoir water that may take place beneath the dam is effectively prevented by the weight of dams which acts vertically downward. Thus, the possible uplift pressure which is dangerous to the stability of the dam is effectively reduced. This also means that the leakage of the reservoir water is checked. However, if the strata are composed of alternating hard and soft rocks like quartzites and shales, it will be undesirable.
Tilted beds:
There are many condition of tilting of beds affecting the dam load differently as follows:
Beds with gentle upstream slope($10^0-30^0$)
Such situation is ideal and much better and this is a typical example where the occurrence of a geological structure brings the advantage to the associated rock of a given site. This is because the resultant force acts more or less perpendicular to the bedding plane which are dipping in the upstream side. Hence, rocks are the best positioned to take the load effectively.
Vertical beds:
In the case of vertical beads, it will not pose problem of uplift. However, it will not have any advantage in terms of competence of rocks.
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