Dams
• Dams are barriers constructed across a river valley to impound water.
• Dams are built mainly for controlling floods, irrigation, electricity generation and for urban water supply.
• Dams may be constructed for a specific use or it may be ‘multipurpose’ which serves more than one use.
• Dams are generally classified according to its use, hydraulic design, construction design and material used in it.
Geological investigation for dam site
• Topography: Topography in large measures dictates a first choice of dams. Generally a dam site is ideal where a valley is constricted with steep rock slopes. A narrow stream flowing between high, rock walls would naturally suggest a concrete overflow dam whereas a low, rolling plain would suggest an earth-fill dam.
• Geology and foundation condition: Foundation condition depend upon the geological character and thickness of the strata which are to carry the weight of the dam, their inclination, permeability and relation to underlying strata, existing faults and fissures.
• Foundations are better on igneous rocks and hard metamorphic rocks like granite, gneiss, quartzite etc. than on sedimentary rocks like shale, phyllite, slate and schist etc.
• The removal of disintegrated rocks and sealing of seams and fractures by grouting will frequently be necessary.
• Rocks like limestone are usually cavernous with numerous solution channels. These channels not only provide path for water percolation but also may collapse and destabilize the setting.
• Weathered rock pose serious problems of instability and require special treatment. The weathering of rocks give rise to formation of clays and gritty soils where the weathering is complete.
• Gravel Foundation, if well compacted, are suitable for earth-fill, rock-fill and low concrete gravity dam.
• Silt or fine sand foundation can be used for the support of low concrete gravity dam but not rock-fill dam. The main problem is settlement, excessive percolation loss, and need of protection of the foundation at the downstream toe from erosion.
• Poorly consolidated sediments like silt, sand and gravel have low bearing strength and become weaker still when moistened.
• Clay foundation can be used for the support of earth-fill dam but require special treatment. There may be considerable settlement of the dam if the clay is unconsolidated and the moisture content is high.
• Occasionally situations may occur where reasonably uniform foundations of any of the foregoing descriptions cannot be found and where a non-uniform foundation of rock and soft material must be used if the dam is to be built. Alteration of hard and soft rocks do not provide safe condition for the foundation of dam.
Geological structures
• Generally dam axis should be chosen perpendicular to the strike of the geological structures.
• Steep beds upstream is favourable for foundation, if bed dips moderately upstream, the day may shear off along the bedding plane.
• Dam founded on the limb of an anticline dipping upstream is favourable but on the limb of a syncline with a downstream plunge is unsuitable.
• Fault zone is an unstable zone. If permeable bed is present and dam rests on beds dipping moderately downstream, water can percolate through the beds and fault. It is difficult to seal a fault zone and prevent leakage.
• Highly jointed rocks even though hard may be worse than a soil foundation. It reduces the strength of rock and creates problem related to leakage.
• Valley slopes along the dam axis should be free from existing any type of landslides or possible unsuitability of slopes.
• Earthquake shocks cause slope failures, landslides, loss of contact of the dam with its abutment, sliding of dam itself and failure of dam.
• Seismic zonation map must be consulted during the site investigation. In seismically active area, it is necessary to assess the degree of earthquake tremors and design must include provisions for the added loading and increased stresses.
• Recognition and delineation of active faults and analysis of historical records of past occurrences are among many seismological investigations to be carried out.
• Construction of a dam requires large quantity of construction materials like soil, rock, concrete and aggregates. The most economical type of dam will often be the one for which materials are to be found in sufficient quantity with a reasonable distance from the site. So availability of such materials nearby the proposed site should be assessed.
• Permeability test of the foundation material should be carried out during site selection. Water tightness of the dam is an important factor so permeability of foundation material should be within tolerance limit.
• Buried channels or valley abandoned by the rivers in the past and now filled with alluvial deposit contain numerous large boulder which allow passage to the impounded water. Careful exploration including drilling and seismic sounding can be used to such hidden topography.
• Construction of a dam modifies the ecosystem and hydrological regimes of both upstream and downstream.
• Erosion and landslide in reservoir basin, evaporation lost, reservoir-induced seismicity, destruction of flora and fauna, resettlement of displaced people which should be investigated and should be kept at minimum.
Treatment of Dam Foundation
• Poor geological condition can be improved by improving load bearing properties and controlling seepage.
• If the foundation is on the highly jointed rocks, grouting is usually done to seal the joints and fractures thus increasing the strength as well as sealing against seepage.
• In soil foundation, soil treatments methods are applied to increase its strength making it suitable for construction of dam.
• The dam area and reservoir area are made impermeable to water to a certain level by use of natural material or synthetic materials (geo-textiles).
• Grout curtains are used to seal off water to some level around the dam area.
• Dams are barriers constructed across a river valley to impound water.
• Dams are built mainly for controlling floods, irrigation, electricity generation and for urban water supply.
• Dams may be constructed for a specific use or it may be ‘multipurpose’ which serves more than one use.
• Dams are generally classified according to its use, hydraulic design, construction design and material used in it.
Geological investigation for dam site
• Topography: Topography in large measures dictates a first choice of dams. Generally a dam site is ideal where a valley is constricted with steep rock slopes. A narrow stream flowing between high, rock walls would naturally suggest a concrete overflow dam whereas a low, rolling plain would suggest an earth-fill dam.
• Geology and foundation condition: Foundation condition depend upon the geological character and thickness of the strata which are to carry the weight of the dam, their inclination, permeability and relation to underlying strata, existing faults and fissures.
• Foundations are better on igneous rocks and hard metamorphic rocks like granite, gneiss, quartzite etc. than on sedimentary rocks like shale, phyllite, slate and schist etc.
• The removal of disintegrated rocks and sealing of seams and fractures by grouting will frequently be necessary.
• Rocks like limestone are usually cavernous with numerous solution channels. These channels not only provide path for water percolation but also may collapse and destabilize the setting.
• Weathered rock pose serious problems of instability and require special treatment. The weathering of rocks give rise to formation of clays and gritty soils where the weathering is complete.
• Gravel Foundation, if well compacted, are suitable for earth-fill, rock-fill and low concrete gravity dam.
• Silt or fine sand foundation can be used for the support of low concrete gravity dam but not rock-fill dam. The main problem is settlement, excessive percolation loss, and need of protection of the foundation at the downstream toe from erosion.
• Poorly consolidated sediments like silt, sand and gravel have low bearing strength and become weaker still when moistened.
• Clay foundation can be used for the support of earth-fill dam but require special treatment. There may be considerable settlement of the dam if the clay is unconsolidated and the moisture content is high.
• Occasionally situations may occur where reasonably uniform foundations of any of the foregoing descriptions cannot be found and where a non-uniform foundation of rock and soft material must be used if the dam is to be built. Alteration of hard and soft rocks do not provide safe condition for the foundation of dam.
Geological structures
• Generally dam axis should be chosen perpendicular to the strike of the geological structures.
• Steep beds upstream is favourable for foundation, if bed dips moderately upstream, the day may shear off along the bedding plane.
• Dam founded on the limb of an anticline dipping upstream is favourable but on the limb of a syncline with a downstream plunge is unsuitable.
• Fault zone is an unstable zone. If permeable bed is present and dam rests on beds dipping moderately downstream, water can percolate through the beds and fault. It is difficult to seal a fault zone and prevent leakage.
• Highly jointed rocks even though hard may be worse than a soil foundation. It reduces the strength of rock and creates problem related to leakage.
• Valley slopes along the dam axis should be free from existing any type of landslides or possible unsuitability of slopes.
• Earthquake shocks cause slope failures, landslides, loss of contact of the dam with its abutment, sliding of dam itself and failure of dam.
• Seismic zonation map must be consulted during the site investigation. In seismically active area, it is necessary to assess the degree of earthquake tremors and design must include provisions for the added loading and increased stresses.
• Recognition and delineation of active faults and analysis of historical records of past occurrences are among many seismological investigations to be carried out.
• Construction of a dam requires large quantity of construction materials like soil, rock, concrete and aggregates. The most economical type of dam will often be the one for which materials are to be found in sufficient quantity with a reasonable distance from the site. So availability of such materials nearby the proposed site should be assessed.
• Permeability test of the foundation material should be carried out during site selection. Water tightness of the dam is an important factor so permeability of foundation material should be within tolerance limit.
• Buried channels or valley abandoned by the rivers in the past and now filled with alluvial deposit contain numerous large boulder which allow passage to the impounded water. Careful exploration including drilling and seismic sounding can be used to such hidden topography.
• Construction of a dam modifies the ecosystem and hydrological regimes of both upstream and downstream.
• Erosion and landslide in reservoir basin, evaporation lost, reservoir-induced seismicity, destruction of flora and fauna, resettlement of displaced people which should be investigated and should be kept at minimum.
Treatment of Dam Foundation
• Poor geological condition can be improved by improving load bearing properties and controlling seepage.
• If the foundation is on the highly jointed rocks, grouting is usually done to seal the joints and fractures thus increasing the strength as well as sealing against seepage.
• In soil foundation, soil treatments methods are applied to increase its strength making it suitable for construction of dam.
• The dam area and reservoir area are made impermeable to water to a certain level by use of natural material or synthetic materials (geo-textiles).
• Grout curtains are used to seal off water to some level around the dam area.