Geological Factors Influencing Site Selection
Some of the important geological factors influencing site selection dam are compactness and bearing capacity of foundation rock, geological structures, attitude of beds, thickness of overburden, and width of the river valley. These are described as follows.
Compactness
Any of the igneous, sedimentary or metamorphic rock may be present at the dam site. The compactness of a rock depends upon its mineral composition, hardness, homogeneity, and massiveness. Shale is the most common sedimentary rock followed by sandstone.
Shale may be compaction-shale or cementation-shale. The cementation-shale being stronger is preferable. Moreover, compaction-shale is soft and slakes when subjected to alternate wetting and drying, and has the tendency to flow away under load, causing the structure to settle.
The compactness of rocks like quartzite, granite, and basalt is quite more than sandstone, shale and
limestone, and therefore, the area characterized with massive quartzite is more suitable.
Although limestones, dolomites, and marbles are soluble rocks, they also are strong enough to support the weight of a dam if massive in nature. However, these rocks if have solution channels and caverns may need their grouting to check leakage.
Bearing Capacity
The bearing capacity of a rock is the load that can be withheld by any rock without failure. It depends upon the cohesive forces among grains of the rock and its hardness. Quartzite has the largest bearing capacity amongst different rocks.
Geological Structures
It is one of the most important parameters for the site selection of dams. The rocks in the area of interest should be free from fault and joints. The study of drainage from toposheet, satellite images, and fi eld investigations are carried out to identify the location and magnitude of fault, fold, and joints.
The presence of such weak zones at major scales would certainly prohibit the site for the construction of a dam and a reservoir. The stability of dams in different folded and faulted conditions has been enumerated below.
Folded Region
The limbs of the fold may be treated as dipping beds or strata of the site selected for the construction of a dam. The upstream limb forms the most suitable place, followed by the crest of the folded region. The downstream limb of a fold should not at all be selected for constructing a dam (Fig. 9.9).
Faulted Region
The occurrence of a fault is a tectonic phenomenon. However, the area with faults is never suitable for the construction of a dam; the bearing capacity of foundation rock is reduced drastically. Also, the area is prone to earthquakes if faults are reactivated.
Furthermore, faults accelerate the seepage of water and in turn, the dam becomes unsafe. However, if the situation demands to construct of the dam, the area may be prioritized as follows:
Case 1
If a fault plane is on the upstream side, and beds have also an upward dip then the faulted plane should be sealed so that the dam is safe (Fig. 9.10).
Case 2
If a fault plane is on the upstream side, but beds have a downward dip, the construction of the dam must be avoided (Fig. 9.10).
Case 3
If a fault occurs on the downstream side then there is no threat to the construction of the dam (Fig. 9.10).
Attitude of Rock Formations/Beds
The dip direction of existing formation/beds is important for the stability of dams. The dip direction of beds when following the direction of the upstream side is supposed to be the most suitable site.
This is because the resultant force acts against the toe of the dam. In addition, the beds may have different amounts of dip which can affect the stability of the dam. Differential conditions of dip direction throughout The extent and consequences of overload, fold, and defects are discussed below.
Condition 1
When the beds have a dip amount of 10°–45° towards the upstream side, the resultant force of the water would be negated and the dam would be safe. When the beds have a dip amount greater than 45° towards the upstream side, the resultant force of the water would be negated but it would be a little dangerous as compared to a previous condition [Fig. 9.11(a)].
Condition 2
When the beds have a dip amount of 10°–45° towards the downstream side, the resultant force of the water would be accelerated and the dam would be unsafe. When the beds have a dip amount greater than 45° towards the downstream side, the resultant force of the water would be more accelerated and it would be devastating [Fig. 9.11(b)].
Condition 3
When the beds have the vertical disposition, which is rarely found, the foundation remains water-tight and the dam will not be subjected to uplift pressure created by the dam [Fig. 9.11(c)].
Thickness of Overburden
The thickness of loose sediments or debris, below which the hard or consolidated rock formation exists is known as overburden. The thickness of the overburden can be estimated by an electrical resistivity survey and borehole drilling.
Since the thickness of the overburden affects the cost as well as the stability of dam structures, it is essential to excavate and remove the overburden. The greater thickness of overburden is not a suitable criterion for the construction of a dam.
Stream/River Condition
A narrow river channel is the most suitable place for the construction of the proposed dam because it can be blocked with a relatively small dam. Apart from it, the slope of the stream bed should be gentle and free from the meandering of the river. The cost of the dam is proportional to the width of the stream.
Also Raed: