Integrated assessment of soil and rock penetration across the crystalline basement of the lower Thamirabarani river basin, southern India

This study aims to quantitatively evaluate soil and rock penetration characteristics and subsurface heterogeneity within the crystalline basement terrain of the Lower Thamirabarani River Basin, southern India, through integrated geophysical and geotechnical methods. Vertical Electrical Sounding (VES), Electrical Resistivity Imaging (ERI), and Magneto Telluric (MT) surveys delineated a sequence comprising surface calcrete, weathered quartzite or gneissic, fractured rock, and massive charnockite. Groundwater-bearing zones occur predominantly between 30 and 70 m depth, with apparent resistivity values ranging from 9–20 Ω·m in shallow aquifer zones, 70–300 Ω·m in weathered and fractured formations and exceeding 400 Ω·m in massive charnockite. Standard Penetration Test (SPT) results showed N-values ranging from 3 to 50, indicating weak silty–clayey deposits (N < 10), moderately strong sandy–gravel layers (N = 20–50), and refusal at dense gneissic bedrock. Grain-size analysis revealed sand as the dominant fraction (> 74%) with poor sorting and variable skewness, suggesting deposition under high-energy fluvial conditions. Spatial distribution of sediment fractions shows that coarse and medium sands dominate the central and southeastern sectors. At the same time, finer materials are more common in the southern and western parts of the basin. Correlation analysis shows that finer fractions form compressible, low-permeability layers, whereas coarser sediments exhibit higher load-bearing capacity and permeability. The results establish a quantitative relationship between lithological heterogeneity, penetration resistance, and groundwater occurrence in a crystalline rock river basin. The novelty of this study provides a site-specific integrated model of subsurface variability in a crystalline fluvial setting. It offers a robust framework for groundwater targeting, foundation design, and infrastructure planning in the Lower Thamirabarani River Basin and comparable hard-rock environments.