Investigation for reservoir site selection
• The most important factor influencing feasibility of proposed reservoir site is generally the location of the dam.
• After that, consideration must be given to the run-off characteristics of the catchment area, the watertightness of the proposed reservoir basin, the stability of the valley sides, the likely rate of sedimentation in the new reservoir, the quality of the water and, if it is to be a very large reservoir, the possibility of associated seismic activity.
• Topography: The area should be a wide natural valley preferably ending in a narrow gorge where a barrier (dam) could be placed.
• Initial estimates of storage capacity can be made from topographic maps or aerial photographs, more accurate information being obtained, where necessary, from subsequent surveying.
• Catchment areas and drainage densities can also be determined from maps and air photos.
• Reservoir volume can be estimated by planimetering areas upstream of the dam site for successive contours up to proposed top water level
Groundwater condition
• Amount of leakage of water from the reservoir is controlled by the depth of water table.
• If the water table is so near the ground surface, the water level in reservoir doesn't rise above it, no serious loss by leakage will occur.
• On the other hand if water table lies deep below the ground surface, the water table lies deep below the ground surface, the water level in the reservoir will stand above it and leakage will occur, the amount of which will depend on the permeability of the earth material it rests upon.
Permeability
• During geological investigation, it is necessary to locate the highly permeable layers present in the reservoir area.
• The rocks highly fissured, intensely jointed, faulted or have solution channels cause serious leakage from the reservoir
• Generally leakage will be more in rock dipping downstream than upstream
• In some cases ancient channels buried below the present valley floor, called buried valleys often provide passage for profuse leakage of reservoir water.
• Sedimentation in Reservoir: The amount of silt produced and supplied to the rivers depends mainly upon the lithological character and topography of the catchment area.
• The rivers flowing over the soft rocks and high gradient areas carry greater amount of silt.
• Reservoirs built on rivers which carry large amount of sediment, may silt up very soon and its water storage capacity may be reduced considerably.
• Silt traps may be constructed upstream in such rivers to prevent siltation on reservoirs.
• In those areas where streams carry heavy sediment loads, the rates of sedimentation must be estimated accurately in order that the useful life of any proposed reservoir may be determined.
Geology
• The rocks exposed in the reservoir rim must be resistant to solution, erosion and free of voids to stop leakage.
• If some potential leakage zones are present, they should be delineated.
• Highly jointed rocks should be investigated for joint intensity and spacing which will help in assessing the grouting.
• Faults if present should be delineated since they require treatment which may be expensive.
• Faults may be a passageway for water leakage as well as vulnerable or liable for earthquake effect.
• Movement on fault may initiate due to weight of water in the reservoir.
• If sand and gravel or other permeable bed are in the surface or vicinity of the proposed site, they may serious effect the efficiency of reservoir.
• Valley sections in competent, hard resistant rocks like granite, quartzite, gneiss provide excellent sites.
• Presence of shearing zone, water table, active faults and characteristics of rocks in the sub-surface are done applying geophysical techniques.
• Presence of weakened rock mass or soil in the rim of the reservoir may be the cause of slumps or landslides into the reservoir.
• Increase in water level will further weaken them and hence cause slumping increasing the stability and siltation problem.
• The load of the structures consists of its own weight (dead load) and weight of the loads applied to the structure intermittently is the live load.
• These loads are transmitted to the foundation vertically.
• Besides dead and live load, lateral forces are also acting on the structure mostly by wind and earthquake.
• The primary considerations for foundation support are bearing capacity, settlement, and ground movement beneath the foundations.
• When bedrock is near the surface, if the rocks are massive intact it provides excellent supporting foundation material.
• Discontinuous, weathered and fragile rocks should be removed or treated properly before laying foundation.
• Granite, gneiss, quartzite provide best foundation.
• Fine grained sedimentary rock have higher shear strength than coarse grained rocks.
• Presence of discontinuity reduces the shear strength of rocks.
• Finer material like clay and silt have lesser compressive strength than coarser material such as gravel and coarser sand.
• The ultimate bearing power of the supporting soil should be calculated so that to design the foundation care should be taken that the unit load at the base of a spread footing should not be larger than the safe bearing power of the supporting soil.
• The soil are tested for load test. A load test usually consist of loading a certain area with the design load and afterwards with 150% thereof and observing settlement which should not be larger than some prescribed values.
• Settlement can present a problem in clayey soils, so that the amount that is likely to take place when they are loaded needs to be determined.
• The commonly accepted basis of design is that the total settlement of a footing should be restricted to about 25 mm.
• During excavation for foundation, care should be taken about the stability of the excavation walls.
• High groundwater table may cause problem and the foundation structures should be made water proof.
• Measures should be taken against possible damage of the structure by hydrostatic uplift which may lift the floor slabs and crack walls.
• To obtain groundwater data, observation pipe should be used in the bore holes.
• For large multi-storied complexes number of boreholes depending upon the relationship between the expected variability of the foundation material are recommended.
• With the complete soil profile, SPT of the bore holes data should also be obtained.
• The geologic nature of subsurface material may be important in the interpretation of the lab test. For eg. For much fractured rock, UCS test may be of less value.
• In alluvial or glacial deposit, lab results ma apply only to a very small area adjacent to borehole where the sample was taken.
• For foundation investigation for power plants, the influence of vibrations and sensitivity to settlements are given special attention.
• Even a slight excess of settlement can throw the generators out of alignment.
• The vibrations produced by the generators are transmitted to the rocks by keying the walls of structure to the rock with steel rods.
• In such case, the rock has to be suitable for the placement of such rods and should resist disintegration from continuous vibration in the rod.
• In case of foundations in hill or on a steep hillside, the uphill rock should be examined for fissures, crack, closely spaced joints and presence of cliffs for possible landslides.
• The most important factor influencing feasibility of proposed reservoir site is generally the location of the dam.
• After that, consideration must be given to the run-off characteristics of the catchment area, the watertightness of the proposed reservoir basin, the stability of the valley sides, the likely rate of sedimentation in the new reservoir, the quality of the water and, if it is to be a very large reservoir, the possibility of associated seismic activity.
• Topography: The area should be a wide natural valley preferably ending in a narrow gorge where a barrier (dam) could be placed.
• Initial estimates of storage capacity can be made from topographic maps or aerial photographs, more accurate information being obtained, where necessary, from subsequent surveying.
• Catchment areas and drainage densities can also be determined from maps and air photos.
• Reservoir volume can be estimated by planimetering areas upstream of the dam site for successive contours up to proposed top water level
Groundwater condition
• Amount of leakage of water from the reservoir is controlled by the depth of water table.
• If the water table is so near the ground surface, the water level in reservoir doesn't rise above it, no serious loss by leakage will occur.
• On the other hand if water table lies deep below the ground surface, the water table lies deep below the ground surface, the water level in the reservoir will stand above it and leakage will occur, the amount of which will depend on the permeability of the earth material it rests upon.
Permeability
• During geological investigation, it is necessary to locate the highly permeable layers present in the reservoir area.
• The rocks highly fissured, intensely jointed, faulted or have solution channels cause serious leakage from the reservoir
• Generally leakage will be more in rock dipping downstream than upstream
• In some cases ancient channels buried below the present valley floor, called buried valleys often provide passage for profuse leakage of reservoir water.
• Sedimentation in Reservoir: The amount of silt produced and supplied to the rivers depends mainly upon the lithological character and topography of the catchment area.
• The rivers flowing over the soft rocks and high gradient areas carry greater amount of silt.
• Reservoirs built on rivers which carry large amount of sediment, may silt up very soon and its water storage capacity may be reduced considerably.
• Silt traps may be constructed upstream in such rivers to prevent siltation on reservoirs.
• In those areas where streams carry heavy sediment loads, the rates of sedimentation must be estimated accurately in order that the useful life of any proposed reservoir may be determined.
Geology
• The rocks exposed in the reservoir rim must be resistant to solution, erosion and free of voids to stop leakage.
• If some potential leakage zones are present, they should be delineated.
• Highly jointed rocks should be investigated for joint intensity and spacing which will help in assessing the grouting.
• Faults if present should be delineated since they require treatment which may be expensive.
• Faults may be a passageway for water leakage as well as vulnerable or liable for earthquake effect.
• Movement on fault may initiate due to weight of water in the reservoir.
• If sand and gravel or other permeable bed are in the surface or vicinity of the proposed site, they may serious effect the efficiency of reservoir.
• Valley sections in competent, hard resistant rocks like granite, quartzite, gneiss provide excellent sites.
• Presence of shearing zone, water table, active faults and characteristics of rocks in the sub-surface are done applying geophysical techniques.
• Presence of weakened rock mass or soil in the rim of the reservoir may be the cause of slumps or landslides into the reservoir.
• Increase in water level will further weaken them and hence cause slumping increasing the stability and siltation problem.
• The load of the structures consists of its own weight (dead load) and weight of the loads applied to the structure intermittently is the live load.
• These loads are transmitted to the foundation vertically.
• Besides dead and live load, lateral forces are also acting on the structure mostly by wind and earthquake.
• The primary considerations for foundation support are bearing capacity, settlement, and ground movement beneath the foundations.
• When bedrock is near the surface, if the rocks are massive intact it provides excellent supporting foundation material.
• Discontinuous, weathered and fragile rocks should be removed or treated properly before laying foundation.
• Granite, gneiss, quartzite provide best foundation.
• Fine grained sedimentary rock have higher shear strength than coarse grained rocks.
• Presence of discontinuity reduces the shear strength of rocks.
• Finer material like clay and silt have lesser compressive strength than coarser material such as gravel and coarser sand.
• The ultimate bearing power of the supporting soil should be calculated so that to design the foundation care should be taken that the unit load at the base of a spread footing should not be larger than the safe bearing power of the supporting soil.
• The soil are tested for load test. A load test usually consist of loading a certain area with the design load and afterwards with 150% thereof and observing settlement which should not be larger than some prescribed values.
• Settlement can present a problem in clayey soils, so that the amount that is likely to take place when they are loaded needs to be determined.
• The commonly accepted basis of design is that the total settlement of a footing should be restricted to about 25 mm.
• During excavation for foundation, care should be taken about the stability of the excavation walls.
• High groundwater table may cause problem and the foundation structures should be made water proof.
• Measures should be taken against possible damage of the structure by hydrostatic uplift which may lift the floor slabs and crack walls.
• To obtain groundwater data, observation pipe should be used in the bore holes.
• For large multi-storied complexes number of boreholes depending upon the relationship between the expected variability of the foundation material are recommended.
• With the complete soil profile, SPT of the bore holes data should also be obtained.
• The geologic nature of subsurface material may be important in the interpretation of the lab test. For eg. For much fractured rock, UCS test may be of less value.
• In alluvial or glacial deposit, lab results ma apply only to a very small area adjacent to borehole where the sample was taken.
• For foundation investigation for power plants, the influence of vibrations and sensitivity to settlements are given special attention.
• Even a slight excess of settlement can throw the generators out of alignment.
• The vibrations produced by the generators are transmitted to the rocks by keying the walls of structure to the rock with steel rods.
• In such case, the rock has to be suitable for the placement of such rods and should resist disintegration from continuous vibration in the rod.
• In case of foundations in hill or on a steep hillside, the uphill rock should be examined for fissures, crack, closely spaced joints and presence of cliffs for possible landslides.