Integrated Geotechnical-Mineralogical Characterization of Crystalline Basement Soils: Implications for Pavement Design and Performance in Tropical Terrains

In tropical regions, soils derived from crystalline basement rocks are widely used in road construction; however, their engineering performance is often inadequately characterized due to limited spatial sampling and insufficient integration of geological controls.

This study evaluates the geotechnical and mineralogical characteristics of crystalline-derived soils to assess their suitability for pavement applications.

Fifty samples collected across multiple lithological units were analyzed for index properties, compaction parameters, California Bearing Ratio (CBR), and mineral composition using X-ray diffraction (XRD).

The soils are predominantly fine-grained, with fine content ranged up to 59.6% and generally poor gradation.

Atterberg limits indicate low to moderately high plasticity (liquid limit: 14.3–53%; plasticity index: 1.0 - 24.0%), while soil activity ranges from 0.3 to 2.85, schist-derived soils exhibiting the highest variability (mean 1.85) across the lithologies.

Compaction results show mean maximum dry density and optimum moisture content of 1.80 Mg/m³ and 16.9%, respectively.

Unsoaked CBR values (40–54%) indicate moderate strength, but a marked reduction under soaked conditions (18–25%) reflects significant moisture susceptibility.

XRD analysis reveals that the dominant clay minerals, expressed as mean values are kaolinite (31.5%), pyrophyllite (18.7%), chlorite (18.4%, and illite (15.7%).

This finding offers a mechanistic explanation for the observed swelling behavior and the pronounced reduction in strength under saturated conditions which highlights the need for careful material selection and appropriate stabilization measures in pavement design.

The study demonstrate that variations in parent lithology and clay mineral assemblages exert a strong control on the engineering performance of the soils, thus emphasizing the importance of integrating mineralogical data with conventional geotechnical testing to improve the reliability of subgrade evaluation in crystalline basement terrains.