GIS-Based Subsurface Heterogeneity Index Mapping Using Multi-Depth SPT Data for Geotechnical Infrastructure Planning

Conventional interpretation of Standard Penetration Test (SPT) data using average N-values fails to capture depth-wise variability and abrupt stratigraphic transitions. A Subsurface Heterogeneity Index (SHI) was developed as a composite metric that integrates depth-wise resistance gradients, anomaly detection, the Variation Index, and Shannon entropy to quantify multidimensional subsurface variability. The framework was applied to SPT data from 178 boreholes across the Nagpur region, India. The Variation Index ranged from 0.00 to 0.92, and entropy reached up to 1.10, reflecting a wide spectrum of subsurface conditions. SHI values ranged from 0.00 to 1.00, enabling spatial classification into low (~ 63%), moderate (~ 8%), and high (~ 29%) heterogeneity zones. A significant proportion of boreholes exhibited abrupt changes in SPT resistance, highlighting weak interlayers and irregular stratification that conventional averaging failed to capture. GIS-based interpolation of SHI produced a continuous heterogeneity map, enabling identification of critical zones requiring detailed investigation. Validation confirmed a weak correlation (R² ≈ 0.03) between SHI and mean SPT resistance, demonstrating that SHI captures variability independent of average soil strength. The proposed framework offers a computationally efficient tool for geotechnical zoning, foundation planning, and preliminary site investigation in geologically complex, data-limited environments.