Learn About Geological Exploration: SPT, Core Drilling and Reverse Circulation

Subsurface exploration is a fundamental process in geotechnical engineering, mining, and geology, aimed at understanding the physical properties and composition of the earth beneath the surface. It provides critical data for designing foundations, assessing slope stability, exploring mineral resources, and mitigating geological hazards. Among the various techniques employed, the Standard Penetration Test (SPT), Core Drilling, and Reverse Circulation (RC) Drilling are widely used methods, each with distinct applications and advantages.

1. Standard Penetration Test (SPT)

The Standard Penetration Test is an in-situ dynamic penetration test primarily used in geotechnical site investigations to evaluate the geotechnical properties of soils and weak rocks.

• Method: A thick-walled split-barrel sampler is driven into the ground at the bottom of a borehole by a drop hammer with a standard weight (63.5 kg) and height (760 mm). The number of blows required to drive the sampler through three consecutive 150 mm intervals is recorded. The sum of blows for the last two intervals (300 mm of penetration) is reported as the SPT N-value.
• Applications:
  • Estimating soil strength parameters (e.g., friction angle, relative density).
  • Evaluating liquefaction potential of sandy soils during earthquakes.
  • Determining bearing capacity for shallow foundations.
• Advantages: Simple, cost-effective, and provides immediate data for preliminary soil classification.
• Limitations: Less reliable in cohesive soils or gravelly layers; results can be influenced by drilling practices and equipment.

2. Core Drilling

Core drilling is a technique used to retrieve intact cylindrical samples (core samples) of rock and sometimes soil for detailed visual inspection, laboratory testing, and geological analysis.

• Method: A rotary drill with a diamond- or alloy-impregnated core bit cuts an annular ring into the subsurface, leaving a central core that enters a core barrel. The core barrel is retrieved periodically to extract the sample.
• Applications:
  • Geotechnical characterization of rock mass (e.g., RQD - Rock Quality Designation).
  • Mineral exploration and ore grade estimation.
  • Structural geology studies (e.g., fault zones, bedding planes).
• Advantages: Provides high-quality, undisturbed samples for comprehensive analysis (strength, permeability, mineralogy).
• Limitations: Expensive, time-consuming, and less effective in unconsolidated or fractured formations.

3. Reverse Circulation (RC) Drilling

Reverse Circulation Drilling is a fast and efficient method primarily used in mineral exploration and large-scale geotechnical projects to obtain representative chip samples from depth.

• Method: A dual-wall drill pipe is used: the drilling fluid (air or water) is pumped down the annular space between the inner and outer tubes, flushing cuttings up through the inner tube to the surface. This prevents sample contamination and allows continuous sample collection.
• Applications:
  • Rapid exploration for minerals (e.g., gold, copper, iron ore).
  • Geochemical analysis and resource estimation.
  • Investigating deep soil profiles or alluvial deposits.
• Advantages: High penetration rates, minimal cross-contamination, cost-effective for deep holes.
• Limitations: Samples are disturbed (chips rather than cores), limiting detailed geotechnical testing; not suitable for cohesive soils or requiring intact samples.

Conclusion

Each of these methods—SPT, Core Drilling, and RC Drilling—serves a unique purpose in subsurface investigation. The choice depends on project objectives, material types, depth requirements, and budget constraints. While SPT is ideal for shallow soil assessments, core drilling excels in detailed rock characterization, and RC drilling offers speed and efficiency for mineral exploration. Integrating these techniques often provides the most comprehensive understanding of subsurface conditions.

If you have any needs in this regard, please contact JCDRILL to learn more and get a quote for the equipment.