Some of the most common and frequently occurring problems associated with dams are the factors, which cause a dam to fail. Most of the dam failures, which have occurred in the past are not due to faulty design or poor construction, but mainly due to neglect of the geological flaws. The main geological problems that are usually associated with the dam sites are as follows;
- Dams built on Shale
- Dams constructed on Soluble Rocks
- Dams built on strata dipping upstream
- Dams constructed on strata dipping downstream
- Dams built across the strike of rocks
- Dams constructed on jointed and permeable rocks
- Dams constructed on Faults
- Abutment problems
(1). Dams constructed on Shale:
There are two types of shales, (i) Cementation Shales, (ii) Compaction Shales. The first type of shale is stronger and does not disintegrate when subjected to continual wetting and drying. On the other hand, the compaction shales are soft and absorbent. The slake when subjected to alternate wetting and drying. Their bearing strength is low and they become plastic when wet. 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 difficulty. If dams have to be built on compaction shales, heavier structures like gravity dams should be avoided. After excavating the weathered rock either concrete should be placed immediately without delay or its surface should be coated with asphalt to avoid swelling and caving.
(2). Dams built on Soluble Rocks:
The soluble rocks include limestones, dolomites, and marbles. These rocks are generally sufficiently strong to support the weight of the dam, but they may contain underground solution channels and caverns. If such solution channels are present at the dam site, the leakage through them may be on such a large scale that the reservoir may not hold water for long. The treatment of such openings is very expensive therefore, they should be carefully looked for in the soluble rocks before the construction of a dam.
(3). Dams on Strata Dipping Upstreams:
The dams located on rocks dipping upstream represent ideal foundation conditions. They are the most capable of supporting the weight of dams and the pressure of reservoirs because the resultant of these two forces act nearly at right angles to the bedding planes of rocks. Further, the upstream dip of rocks does not allow the water in the reservoir to percolate beneath the dam. As a result, the leakage of water and the development of uplift pressure will be minimum.
(4). Dams on strata dipping downstream:
The dams constructed on rocks dipping downstream may not be safe due to the following reasons.
- The percolation of water may lubricate the junctions of rock beds, which may facilitate the 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 rocks and increased the seepage of water.
- The water, which enters into the opening of rocks below the dam, causes the development of uplift pressure, which tends to decrease the stability of the structure.
- In this figure R is the resultant of the weight of the dam and pressure of the reservoir water. In this case, this resultant acts nearly parallel to the bedding planes and endangers the stability of the dam.
(5). Dams built across the strike of 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 of varying properties. In such a case, there are chances of unequal settlement of dams. Further, as the bedding planes of strata lie across the axis of the dam, there is a possibility of serious leakage of water not only through the porous beds but through bedding planes also.
(6). Dams built 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 build 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.
(7). Dams built on faults:
Faults are the most troublesome features if they are encountered across the length of the dam. It is better to avoid fault zones for the construction of a dam. The fault zones cause the following troubles.
- It is difficult to seal the fault zones and prevent leakage of the water from the reservoir at a reasonable cost.
- The rocks may weather up to a great depth along a fault zone. This requires digging and scraping of the weathered rock to a great depth and refining the trench with concrete.
- The crushed and fissured rocks that exist along a fault zone in the foundation, have to be grouted intensively to increase their bearing strength.
- Along a fault, some displacement of strata is always expected, particularly during an earthquake. such a movement will not only reopen the fault fissure but also rupture the dam simultaneously.
- A site where the fault is known to have been active in recent years should always be discarded.
(8). Abutment problems of the dams:
Careful attention should be given to the orientation of joints, bedding planes, foliations, and weak zones that are present in the abutment 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 resist the pressure without being crushed.
- Relationship of geology with civil engineering
- Geology and Building-Stones
- Most common rocks used as building stones
- Geology of Dams
- Geology of Reservoirs