Geological and Geophysical Investigation in Civil Engineering

Geological and Geophysical Investigation in Civil Engineering
Site Investigation
  • Definition: Site investigation involves determining the layers of natural soil deposits that will support a proposed structure along with their physical properties.
Purpose of Soil Investigation Program
  1. Foundation Selection: Identify suitable foundation type and depth for the structure.
  2. Load-Bearing Capacity: Evaluate the soil's capacity to support the anticipated loads.
  3. Settlement Estimation: Estimate potential settlement that can occur under the load.
  4. Foundation Problems Identification: Detect issues like expansive soil, collapsible soil, or implications of nearby landfills.
  5. Groundwater Table Establishment: Identify the groundwater level which impacts structural integrity.
  6. Lateral Earth Pressure Prediction: Analyze earth pressure for structures like retaining walls and bulkheads.
  7. Construction Method Formulation: Plan feasible construction methods given subsoil conditions.
Exploration Program
  • Purpose: To obtain stratification and engineering properties of underlying soils efficiently.
  • Focus on properties such as strength, deformation, and hydraulic characteristics to maximize information while minimizing costs.
Subsurface Exploration Stages
  1. Information Assembly: Gather details on structure dimensions, spacing, and local building codes.
  2. Area Reconnaissance: Conduct site visits to observe existing adjacent structures and regional geology. Look for signs of instability like cracks or sagging structures.
  3. Preliminary Site Investigation: Perform initial borings or create test pits to assess soil types and groundwater presence.
  4. Detailed Site Investigation: Implement deeper exploratory methods based on preliminary findings to confirm subsurface conditions.
  5. Depth and Spacing of Borings: Determine necessary depths based on expected stress increase under the foundation and spacing requirements based on soil variability.
    • General spacing guidelines include:
      • Multistory buildings: 10-30 meters
      • Industrial plants: 20-60 meters
      • Highways: 250-500 meters
Methods of Investigation
  • Test Pits: Useful for visual inspection, limited depth (18-20 feet).
  • Boring Methods:
    • Auger Borings: Common and simple method; suitable in various soils.
    • Wash Boring: Uses water to loosen soil, allowing removal and analysis.
    • Percussion Drilling: Employs heavy chisels to grind soil, suitable for creating slurry.
    • Probing Methods: Provide resistance measurements based on the soil's response to penetrative force.
    • Geophysical Methods: Use seismic and electrical methods to ascertain subsurface characteristics.
Soil and Rock Sampling
  • Types of Soil Samples: Disturbed vs. Undisturbed; undisturbed samples are crucial for testing compressibility and strength properties.
  • Rock Coring: Essential for assessing rock quality, using methods like diamond coring for better-quality samples.
Groundwater Conditions
  • Essential to understand groundwater for geotechnical analysis; include measuring levels, pressures, and the permeability of materials.
Field Strength Tests
  • Include methods like:
    1. Vane shear test (VST)
    2. Standard Penetration Test (SPT)
    3. Cone Penetration Test (CPT)
    4. Plate Load Test (PLT)
  • SPT is significant for measuring resistance of soil, indicating its capacity to support structures.
Geotechnical Design Reports
  • Comprehensive documentation after exploration detailing investigation scope, findings, and recommendations.
  • Typically includes:
    • Project description, site conditions
    • Geological setting
    • Field exploration details
    • Subsoil conditions
    • Recommendations for foundation design and allowances for loads.
    • Conclusions and limitations of the investigation.