Geotechnical Investigation of Hydel Power Sites
It is a well-known fact that almost all the dam sites are also the sites of hydal power plants. The generation of electricity is one of the most important objective of a dam constructions along with irrigation and flood controls. So during preparations of a geological report a geological expert must keep in mind all the criteria suitable for a hydal power sites along with dam sites. This should not be much difficult as the geotechnical considerations for the two are almost same and the success of dams itself ensure the success of a hydal power plant.
Though during five-year plan much hydal power projects have been completed successfully, the advent of recent geomatic tools has added new dimension to the site suitability location analysis for a hydal project. It has been realized very long back by the earth science expert that the remote sensing and GIS has the potential of accurate analysis for a site locations. Remote sensing technique can be applied to the geological applications like lithologic mapping, lineament analyses, tectonic studies, structural mapping, geomorphic studies, ground water and mineral explorations and geothermal studies etc. Single site location analysis can often be approached with some form of ranking or scoring process, which is based upon a set of geological parameter from a landscape of potential sites. Score of area are screened or filtered out of these scores are to low to be acceptable. This process can be carried out by a number of GIS product.
The geotechnical expert has to observe and record various geological parameters and then translate it into practical construction and maintenance of such projects. The involvement of geologists in such projects is of paramount importance mainly during the planning stages. A geologist will be of great help in the analysis of the geological parameters of the site, which can be summarized as follows:
- Topographical and Drainage characteristics
- Geological structures and rock types
- Hydrogeological parameter of the site
- Geophysical investigation of the site
- Environmental considerations
Topographical characteristics
Topographical features are of the foremost importance in the construction of any engineering project. Mainly the topographical maps provide the information about the topography of the construction site. However, in the modern time, the use of GPS, Ariel photography and Remote sensing techniques, have added a new dimension to the topographical data capture methodology. The broad spectrum of the area is provided by satellite imageries and aerial photographs. It illustrates geomorphologic features, linear features, and rock characteristics and drainage pattern. GIS is a useful tool for automating the map production process. The captured data can be analysed through GIS, which later on can be represented in the form of Digital Elevation Model ( DEM). A variety of terrain features can be constructed from DEM to support terrain dependent analyses. They are usually associated with secondary terrain structures defined in terms of surface shape and drainage structure. The terrain parameters deducted from DEM can be utilised to find out the exact location of a particular project.
Geological structures and rock types
Structural investigation of the project site is the most important consideration in the success of the structure. The stability and durability of any engineering project depends on the careful analyses of the structural parameters of the terrain, as selection of favorable site depends upon a particular structural element possessed by the rock. Most of the projects require a strong foundation on the stable rocks. Along with rock types, the site should be properly investigated for the plane of weaknesses, viz. Bedding planes, Schistosity, joints, fractures, folding, fault, fault planes, shear zones etc. Remote sensing and aerial photographs are the latest techniques in the field of geological mapping. The data gathered from these methods, supplemented by geological map will provide a complete geological information about the area. There are instances, in which, Dam failure has resulted due to negligence of the geological aspects of the terrain. St. Francis Gravity Dam (USA), 62.5 mts high and 213 mts long, was constructed in 1926 across San Francisquito Canyon. The Dam rested on a basement, which consisted of Schist, conglomerate, Sandstone and Shale formation. The contact between these heterogeneous rocks is traversed by a fault. Displacement along this fault resulted in shearing and fracturing of low strength conglomeratic rock formation and on 12 March 1928, Dam failed. This illustration clearly emphasizes the importance of geological considerations in any engineering project.
During 1970s the arrival of multispectral scanner facilitated the geological and geomorphological mapping by the method of extrapolations of local observations to regional scale for correlation and comparisons of features, delineation of linear geological parameters and monitoring of dynamic geological and geomorphic features. Therefore geological and geomorphological mapping of areas with poor accessibility such as rugged and mountain terrain was now possible.
Though geological map can be prepared on regional scale (1: 250000) and (1: 50000) scale in the case of detail mapping by the remote sensing technique; large scale lithological and structural map can be prepared by the use of higher resolutions aerial photography.