The dams are barriers which are constructed across rivers to store water. They are built mainly to control floods, for irrigating lands, for generating electricity and for supplying water to industries and cities. A Dam that serves more than one purpose is called a multipurpose dam.
Geological Problems Associated with Dam Site
Most of the dam failure that has occurred in the past is not due to mainly design or construction but mainly due to neglect of the geological flaws. The main geological problems that are usually met with at dam site are as follows.
Dams on Shales
Shales are of two types
The cementation shales are stronger and do not disintegrate when subjected to wetting and drying. The compaction shales on the other hand are soft and they slake when subjected to alternate wetting and drying. Their bearing strength is low and they become plastic when wetted.
The compaction shales have a tendency to flow away from the loaded area and therefore the structure settles. Swelling and caving may result during the excavation work which may cause trouble. If dams have to build on compaction shales, heavier structures like gravity should be avoided.
Dams on Soluble Rocks
The soluble rocks include limestones, dolomites and marbles. These rocks are generally sufficient strong to support the dam, but they may contain underground solution and channels and cavers. If such solution channels are present at dam site, the leakage through them may be on such a large scale that the reservoir may not hold water for long.
Dams on Strata Dipping Upstream
The dam located on rocks dipping upstream represent ideal foundation conditions, they are the most capable of supporting the weight of dams and the pressure of reservoir because the resultant of these two forces acts nearly at right angles to the bedding planes of rocks. Further the upstream dip of the rocks does not allow the water in the reservoir to percolate below the dam. As result the leakage of water and the developments of uplift pressure will be minimum.
Dams on Strata Dipping Downstream
The dams constructed on rock dipping downstream may not be safe due to the following reasons.
The percolation of water may lubricate the junction of rock beds which may facilitate sliding of dams.
The water percolating through the strata dissolves the cementing material of rocks and enlarges the openings by mechanical erosion. This undermines the strength of the rocks and increases the seepage of water.
The water which enters into opening of rocks below the dam causes the development of uplift pressure which tends to decrease the stability of the structure.
Dams Built Across Strike or Rocks
The best foundation condition is when only one uniform rock is present along the length of a dam. If a dam is aligned across the strike of strata, its foundation will be on different rock types varying properties. In such case, there are chances of unequal settlement of the dam. Further as the bedding planes of strata lie across the axis of the dam, there is a possibility of serious leakage of water not through the porous bed but through bedding planes also.
Dams on Jointed and Permeable Rocks
Where highly fissured, jointed and permeable rocks exist below the dam, they will not only cause leakage of water, but also built uplift pressure at the base of the dam. The uplift pressure acts opposite to the weight of the structure and it may cause sliding. Such rocks may be consolidated by grouting.
Dams on Faults
Faults are most troublesome if they are encountered across the length of the dam. It is better to avoid fault zones for the construction of dams. The faults zones cause the following troubles.
It is difficult to seal the fault zones and prevent leakage of water from the reservoir at reasonable cost.
The crushed and fissured rocks that exist along a fault zone in the foundation have to be grouted intensively to increase their bearing.
Along a fault some displacement of strata is always expected, particularly during and earthquake. Such movement will not only reopen the fault fissure but also rupture the dam.
A site where the fault is known to have been active in recent year should always be discarded.
Careful attention should be given to the orientation of joints, bedding planes, foliation and weak zones that are present in the abutments rocks. If such weak zones lie parallel to the thrust of water in the reservoir, the stability of the structure may be endangered, the rocks that exist in the abutments of an arch dam, should be strong enough to resists the pressure without being crushed.