SCI1205 PREPARED BY : ENGINEERING GEOLOGY Dr.S.Nandhakumar Page 1 of 27 UNIT - 5 ENGINEERING APPLICATIONS IN GEOLOGY GEOLOGICAL CONDITIONS NECESSRY FOR CONSTRUCTION OF DAMS DEFINITION A DAM may be defined as a solid barrier constructed at a suitable location across a river valley with a view of impounding water flowing through that river. (1) generation of hydropower energy; SELECTION OF SITES Topographically It would be a narrow gorge or a small valley with enough catchments area available behind so that when a dam is placed there it would easily store a calculated volume of water in the reservoir created upstream. This should be possible without involving significant uprooting of population, loss of cultivable land due to submergence or loss of existing construction. Technically The site should be as sound as possible: strong, impermeable and stable. Strong rocks at the site make the job of the designer much easy: he can evolve best deigns. Impermeable sites ensure better storage inventories.
SCI1205 PREPARED BY : ENGINEERING GEOLOGY Dr.S.Nandhakumar Page 2 of 27 Stability with reference to seismic shocks and slope failures around the dam, especially upstream, are a great relief to the public in general and the engineer in particular. The slips, slides, and slope failures around and under the dam and susceptibility to shocks during an earthquake could prove highly hazardous. Constructionally The site should not be far off from deposits of materials which would be required for its construction. All types of major dams require millions of cubic meters of natural materials — earth, sand, gravel and rock — for their construction. Economically The benefits arising out of a dam placed at a particular site should be realistic and justified in terms of land irrigated or power generated or floods averted or water stored. Dams are invariably costly structures and cannot be placed anywhere and everywhere without proper analysis of cost-benefit aspects. Environmentally The site where a dam is proposed to be placed and a reservoir created, should not involve ecological disorder, especially in the life cycles of animals and vegetation and man. The fish culture in the stream is the first sector to suffer a major shock due to construction of a dam. Its destruction may cause indirect effects on the population. These effects require as thorough analysis as for other objects. The dam and the associated reservoir should become an acceptable element of the ecological set up of the area. Geological Characters for Investigation Geology of the Area Preliminary geological surveys of the entire catchments area followed by detailed geological mapping of the reservoir area have to be conducted. These should reveal main topographic features, natural drainage patterns,
SCI1205 PREPARED BY : ENGINEERING GEOLOGY Dr.S.Nandhakumar Page 3 of 27 general characters and structures of rock formations such as their stratification, folding and faulting and igneous intrusions, and the trend and rate of weathering and erosion in the area. Geology of the site Lithology The single most important feature that must be known thoroughly at the site and all around and below the valley up to a reasonable depth is the Lithology, i.e. types of the rocks that make the area. Surface and subsurface studies using the conventional and latest techniques of geological and geophysical investigations are carried out. Such studies would reveal the type, the composition and textures of the rocks exposed along the valley floor, in the walls and up to the required depth at the base. Rocks are inherently anisotropic materials, showing variation in properties in different directions. Complex litho logy definitely poses challenging design problems. Structures This involves detailed mapping of planes of weakness like bedding planes, schistosity, foliation, cleavage, joints, shear zones, faults and fault zones, folding and the associated features. While mapping these features, special attention is given to recording their attitude, spacing and nature. Shear zones have to be searched, mapped and treated with great caution. In some cases, these may be developed to such an extent that the rock may necessitate extensive and intensive rock treatment (e.g. excavation, backfilling and grouting etc.).
SCI1205 PREPARED BY : ENGINEERING GEOLOGY Dr.S.Nandhakumar Page 4 of 27 Following is a brief account of the influence of more important structural features of rocks on dam foundations Dip and Strike The strength of sound, un fractured stratified rock is always greater when the stresses are acting normal to the bedding planes than if applied in other directions. This being so, horizontal beds should offer best support for the weight of the dam. But as is shown in a latter section, the resultant force is always inclined downstream. the most UNFAVOURABLE strike direction is the one in which the beds strike parallel to the axis of the dam and the dip is downstream It must be avoided as far as possible. Therefore, other conditions being same, beds with upstream dips are quite favorable sites for dam foundations. Faults These structures can be source of danger to the dam in a number of ways. Thus, The faulted rocks are generally shattered along the rupture surfaces; Different types of rocks may be present on either side of a fault plane. Hence, sites with fault planes require great caution in calculating the design strength in various sections of the dam. Dams founded on beds traversed by fault zones and on major fault planes are more liable to shocks during an earthquake compared to dams on non-faulted rocks.