Geological Investigation of Dams and Reservoirs

In order to detect geological flaws and to select a suitable site for the construction of dams and reservoirs, deep geological investigation of that particular area is carried out. The prime objective of this investigation may be summarized in the following lines.

1. Physiographic features of that particular area are studied during the geological investigation.
2. The nature and depth of filling in the river channel at the dam site are determined by means of geological investigation.
3. The favorable areas for the location of spillways, diversion tunnels, powerhouses, etc are delineated.
4. The lithological composition and structure of the rocks present in the area are also studied in this investigation.
5. Structural defects such as faults, fractures, joints, solution channels, etc in the rocks are also pointed out.
6. The potential leakage zones in the vicinity of the dam and reservoirs are also pointed out during the geological investigation.
7. The geological investigation is carried out in order to evaluate the rate of silting in the reservoirs.
8. The construction material in the vicinity of the dam is also located during the geological investigation.

Geological Survey:

Aerial photos and a geologic map of the area under investigation is obtained, which serves as a guide for a detailed geological survey. The geological investigation is normally started with the interpretation of aerial photos and maps, followed by fieldwork and laboratory testing, and ends with the preparation of the field report. This investigation may broadly be subdivided into three groups: (i) Study of physiography, (ii) Study of lithology, (iii) Study of geological structures. 

(1). Study of Physiography: The physiographic characters of the reservoir area are studied on the air photographs. This gives an idea about (i) the shape and size of the river valley, (ii) its stages of development, (iii) stability of valley slopes, (iv) and potential areas of scouring and erosion.

The bedrocks in the river valleys are often covered up with a layer of alluvium of variable thickness. It is necessary to know about the thickness of filling at the dam site. An idea about it may be obtained from the stage of development of the river valley and from the geophysical investigations.

In a river valley, the existence of a concealed fault is always suspected. This is particularly the case where a river cuts through a mountain range.

By studying the topography and rock outcrops of the valley sides, the potential areas for locating spillways, diversion tunnels, powerhouses, etc, may be delineated.

(2). Lithological Investigation: During fieldwork, a detailed geological map is prepared and the lithological characters of the rocks present in the area are examined.

The pervious and impervious strata are located and the potential leakage zones are delineated. Where rocks contain joints, fissures, solution channels, shear zones, and faults, there is much possibility of leakage and seepage of water.

The rocks present in the foundation and abutments of the dam must be sound enough to withstand the expected static and dynamic pressures. The rocks exposed in the reservoir rim must be resistant to the solution, erosion, and other damaging effects of water. By studying the lithology and topography of the catchment area of the river, the amount of silt likely to be contributed annually to the reservoir must be estimated.

Efforts should also be made to locate the construction materials near the dam site. This will cut down the transport cost considerably. The important construction materials are masonry stones for rip rap, gravels, and sands for aggregate, and clay for impervious filling.

(3). Investigation of Geologic Structure: The geological structures of the rocks present in the area, have an important bearing on the design and stability of the dam. A site where the strata dip upstream is preferred to that where they dip downstream.

The rocks existing in the foundation of a dam and reservoir walls should be watertight. If some potential leakage zones are present, they should be delineated. Further, the leakage of water through a porous bed may give rise to landslides. If rocks exposed in the foundation and abutments of the dam are highly jointed, the spacing and intensity of jointing are recorded. This will help in assessing the grouting problem. Faults if present, should be carefully delineated because they would require expensive treatment.

Related posts: 

1. Relationship of geology with civil engineering
2. Geology and Building-Stones
3. Most common rocks used as building stones
4. Geology of Dams
5. Problems associated with a dam/failure of dams
6. Geology of Reservoirs