Problem Statement

Characterizing regolith layers is essential for understanding ground stability, soil–rock interactions, and construction suitability. Conventional borehole methods provide only point-specific information and often miss lateral variability. A non-invasive, high-resolution technique was required to map regolith thickness, detect lithological variations, and identify subsurface features such as boulders and air pockets near construction sites at IIT Tirupati.

Overview

A Ground Penetrating Radar (GPR) survey was conducted in selected construction zones of IIT Tirupati campus to assess regolith distribution and subsurface heterogeneity. A 250 MHz antenna was employed using the reflection method, balancing penetration depth with resolution to investigate the regolith cross-section up to several meters.

Methodology

• System Configuration: 250 MHz antenna, optimized for medium-depth penetration and subsurface imaging.

• Survey Design: Reflection profiling method was adopted to capture continuous subsurface cross-sections across target areas.

• Processing Workflow: Advanced GPR data processing steps including background removal, bandpass filtering, migration, and amplitude/depth-slice analysis were applied to refine imaging of regolith thickness and internal structures.

Key Findings

• Regolith Thickness: Distinct reflections allowed mapping of variable regolith thickness across the surveyed areas. Layer changes were evident based on compaction and material properties.

• Layer Compactness: Upper regolith layers showed relatively loose packing, while deeper layers displayed increased compactness and stronger reflection contrasts.

• Boulder Detection: Medium- to large-scale boulders were imaged after migration, providing evidence of subsurface heterogeneity and rock–soil interaction.

• Air Pockets & Loose Zones: Possible voids or air pockets were inferred near shallow layers where reflection discontinuities and low-amplitude zones were present.

• Soil–Rock Transition: Gradual changes from loose regolith to compact layers were well-resolved, supporting stratigraphic interpretation.

Engineering Impact

• Enabled non-destructive mapping of regolith thickness and variability.

• Provided critical insights into subsurface stability for construction site planning.

• Identified boulders and packing variations, important for excavation and foundation design.

• Detected potential air pockets that could pose risks for structural settlement.

Conclusion

The 250 MHz GPR survey effectively characterized regolith distribution and subsurface heterogeneity in IIT Tirupati construction zones. The ability to resolve compactness variations, boulders, and potential voids demonstrates the utility of GPR in geotechnical and construction-related investigations. This non-invasive approach provided valuable input for site evaluation, reducing reliance on invasive drilling and enhancing subsurface understanding.