What are the factors that must be considered during the geological investigation for locating a dam site?

Geological Factors for Dam Site Investigation

The geological investigation for a dam site is a multi-stage process, starting with reconnaissance surveys and culminating in detailed subsurface explorations. The factors considered can be broadly categorized into regional and site-specific considerations.

1. Regional Geological Setting

• Tectonic Activity: The seismic history of the region is crucial. Active fault lines near the dam site pose a significant risk. Seismic hazard assessment, including peak ground acceleration (PGA) and response spectra, is essential.
• Geological History: Understanding the geological evolution of the area helps predict potential weaknesses and instabilities. Past landslides, volcanic activity, or glacial events can indicate areas prone to future hazards.
• Regional Lithology: The types of rocks present in the region influence permeability, strength, and weathering characteristics. The presence of soluble rocks like limestone can lead to karst topography and seepage issues.
• Drainage Pattern: The regional drainage pattern provides insights into groundwater flow and potential erosion risks.

2. Site-Specific Geological Investigations

• Topography and Geology: Detailed topographic maps and geological maps are fundamental. The shape of the valley, the presence of narrow gorges, and the orientation of rock strata are important considerations.
• Rock Type and Properties:
  • Strength: Unconfined compressive strength (UCS), tensile strength, and shear strength of the foundation rocks are critical parameters.
  • Permeability: The permeability of the foundation and abutments determines the rate of seepage. High permeability can lead to excessive water loss and potential erosion.
  • Weathering: The degree of weathering affects the rock's strength and durability. Weathered rocks are more susceptible to erosion and failure.
  • Joints and Fractures: The presence, orientation, and spacing of joints and fractures significantly influence rock mass strength and permeability.
• Geological Structures:
  • Faults: Faults are zones of weakness and potential movement. Their proximity to the dam site must be carefully evaluated.
  • Folds: Folded strata can create zones of concentrated stress and potential instability.
  • Bedding Planes: The orientation of bedding planes relative to the dam axis can influence slope stability.
• Subsurface Exploration:
  • Drilling: Core drilling provides samples for laboratory testing and allows for the identification of subsurface geological conditions.
  • Geophysical Surveys: Seismic refraction, electrical resistivity tomography (ERT), and ground penetrating radar (GPR) are used to map subsurface structures and identify anomalies.
  • Excavation: Test trenches and exploratory excavations provide direct access to the foundation rocks.
• Seepage Analysis: Assessing potential seepage paths and rates is crucial. Flow nets are used to visualize groundwater flow patterns.
• Reservoir Geology: The geology of the reservoir area influences water quality, sedimentation rates, and potential for landslides into the reservoir.

3. Potential Geological Hazards

• Landslides: Slope stability analysis is essential to identify areas prone to landslides, especially in the reservoir area and along the abutments.
• Reservoir-Induced Seismicity (RIS): The impoundment of water in a reservoir can sometimes trigger earthquakes, particularly in areas with pre-existing faults.
• Karst Topography: The presence of sinkholes and underground cavities can lead to excessive seepage and potential dam failure.
• Erosion: Erosion of the abutments or foundation can undermine the dam's stability.
• Differential Settlement: Uneven settlement of the foundation can cause stress concentrations and structural damage.

Investigation Technique	Purpose
Core Drilling	Obtain rock samples for laboratory testing, identify subsurface geology.
Seismic Refraction	Determine subsurface layering and identify faults.
Electrical Resistivity Tomography (ERT)	Map subsurface structures and identify zones of weakness.
Flow Net Analysis	Visualize groundwater flow patterns and assess seepage rates.