Geological Investigations for Dams, Tunnels, Highways, Railways, and Bridges | Geology Optional for UPSC PDF Download

Geological Investigation for Site Selection of Dams & Reservoirs

Dams are barriers constructed across a river valley to impound water. They serve various purposes such as controlling floods, irrigation, electricity generation, and urban water supply. Dams can be classified based on their use, hydraulic design, construction design, and materials used.

Dams

• Dams are mainly built for controlling floods, irrigation, electricity generation, and urban water supply.
• They can be constructed for specific purposes or be multipurpose, serving more than one use.

Reservoir

• A reservoir is an enlarged natural or artificial lake created by a dam to store water.
• Reservoirs can be formed by controlling a stream that drains an existing body of water.

Geological Investigation for Dam Sites & Reservoirs

• Topography plays a crucial role in the initial selection of dam sites.
• Geological investigations are essential for assessing the suitability of sites for dam construction.

Author: Gaurav Srivastava, Civil Engineering Department, FOET, LU

Dam Site Selection and Foundation Conditions

Dam Site Selection:

• A dam site is typically ideal in a valley that is constricted with steep rock slopes.
• For instance, a narrow stream flowing between high, rock walls suggests a concrete overflow dam.
• Conversely, a low, rolling plain would recommend an earth-fill dam.

Geology and Foundation Condition:

• Foundation condition depends on geological factors such as the character and thickness of the strata.
• Foundations are more stable on igneous and hard metamorphic rocks like granite, gneiss, and quartzite.
• However, sedimentary rocks like shale, phyllite, slate, and schist may pose challenges.
• The removal of disintegrated rocks and sealing of seams and fractures through grouting is often necessary.
• Rocks like limestone tend to be cavernous, with solution channels that can lead to instability.

Foundation Types:

• Gravel foundations, if well compacted, are suitable for earth-fill, rock-fill, and low concrete gravity dams.
• Silt or fine sand foundations can be used for low concrete gravity dams but not for rock-fill dams.
• Poorly consolidated sediments like silt, sand, and gravel have low bearing strength, especially when moistened.
• Clay foundations can support earth-fill dams but require special treatment to mitigate settlement risks.

Author: Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Dam Foundation Considerations

• Unconsolidated and high moisture content may lead to non-uniform foundations.
• Dam axis should ideally be perpendicular to geological structures' strike.
• Steep beds upstream are favorable for foundation stability.
• Dams on the limb of an anticline dipping upstream are favorable.
• Fault zones are unstable areas prone to leakage issues.
• Highly jointed rocks, despite hardness, can lead to strength reduction and leakage problems.
• Valley slopes along the dam axis should be free from landslides.
• Earthquake shocks can cause various failures, necessitating seismic considerations in design.

Test Questions:

• What are the challenges of building on unconsolidated, high-moisture content foundations?
• Why is it important for the dam axis to be perpendicular to geological structures' strike?
• Discuss the significance of steep beds upstream for dam foundations.
• Explain why dam placement on the limb of an anticline dipping upstream is preferable.
• How do fault zones pose risks to dam foundations, and what challenges do they present?
• Why can highly jointed rocks, despite their hardness, be problematic for dam foundations?
• What considerations should be made regarding valley slopes along the dam axis?
• How do earthquake shocks impact dam structures, and what design considerations are necessary?

Source: Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Summary

• Recognition and delineation of active faults and analysis of historical records are crucial in seismological investigations.
• Availability of construction materials like soil, rock, concrete, and aggregates nearby a dam site is essential for economic construction.
• Permeability tests of foundation materials should be conducted during site selection to ensure water tightness of the dam.
• Buried channels or valleys filled with alluvial deposits may contain large boulders that need exploration techniques like drilling and seismic sounding.
• Construction of a dam impacts ecosystems, hydrological regimes, erosion, landslides, evaporation, seismic activity, flora, fauna, and displaced populations.
• Treatment of dam foundations involves improving geological conditions, controlling seepage, grouting joints in rocks, and applying soil treatment methods.

Test Questions:

• Why is it important to assess the availability of construction materials near a dam site?
• What exploration techniques can be used to uncover hidden topography in buried channels?
• How does the construction of a dam impact the ecosystem both upstream and downstream?
• What methods can be employed to improve poor geological conditions in dam foundations?

Author: Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Dam Area, Reservoir Area, and Tunnel Construction

• The dam area and reservoir area are made impermeable to water using natural or synthetic materials like geo-textiles.

Tunnel:

• Grout curtains are utilized to seal off water to a certain level around the dam ends.
• Tunnel is a nearly horizontal underground passage open at both ends.
• There are different types of tunnels based on their use, such as traffic tunnels, hydropower headrace tunnels, and public utility tunnels.
• Tunnels can be driven through rock or earth mass using methods like blasting in mining.
• In soft ground, tunnels may be excavated using boring machines with walls supported by liner plates.

Geological Investigations for Tunnels:

• Geological investigations are crucial for selecting tunnel sites.

• Classifications include:

  • Selection of Tunnel Route (Alignment): The final choice of alignment depends on geological factors, with the least negative geological factors being preferred.

  • Selection of Excavation Method: The method of excavation is an important decision in tunnel construction.

Test Questions:

• Why are grout curtains used in tunnel construction?
• What are the different types of tunnels based on their use?
• Why are geological investigations essential in the selection of tunnel sites?
• Explain the importance of selecting the right alignment for a tunnel.
• Discuss the significance of the excavation method in tunnel construction.

Key Objectives of Geological Investigations for Tunneling

• Tunneling is a complex process with high costs involved.
• Proper planning is crucial to minimize costs and ensure successful excavation.
• Excavation methods are closely linked to the type of rocks present.
• Geological investigations aim to determine the nature of rocks along the tunnel alignment.

Selection of Design for the Tunnel:

• Geological composition along the tunnel alignment influences tunnel design.
• Rock strength dictates whether circular, D-shaped, horse-shoe shaped, or rectangular tunnels are suitable.
• In self-supporting strong rocks, D-shaped or horse-shoe tunnels may be appropriate.
• Circular tunnels are preferred in soft ground or weak rocks with unequal lateral pressure.

Assessment of Cost and Stability:

• Geological investigations impact excavation method, dimensions, and supporting system, influencing project cost estimates.
• Tunnels in hard and massive rocks are inherently stable.
• Tunnels in difficult grounds may collapse without proper geological understanding.

Assessment of Environmental Hazards:

• Tunneling processes can disturb the environment through vibrations.
• Environmental impacts should be considered for all tunneling projects.

Author: Gaurav Srivastava, Civil Engg. Deptt., FOET, LU