Correlation between electrical resistivity tomography and Mackintosh probe in unsaturated soil

Subsurface investigation plays an important role in geotechnical engineering by providing information on soil conditions required for design and construction. Electrical Resistivity Tomography (ERT) and Mackintosh Probe (MP) are commonly used methods for evaluating subsurface characteristics. ERT is a non-destructive geophysical technique that measures the electrical properties of soils, whereas the Mackintosh Probe is an in-situ penetration test used to assess soil resistance and stiffness. This study investigates the relationship between ERT resistivity values and Mackintosh Probe blow counts in unsaturated tropical residual soils at Kempas, Johor. Laboratory tests including sieve analysis, Atterberg limits, and natural moisture content tests were conducted to characterize the soil properties. The sieve analysis results classified all samples as poorly graded sand (SP), while the Atterberg limit and moisture content tests indicated moderate to high plasticity and varying moisture conditions throughout the study area. Field investigations were carried out using a 2-D Electrical Resistivity Tomography survey and Mackintosh Probe testing were conducted at selected locations along two survey lines. The comparison of ERT and MP results revealed inconsistent trends between electrical resistivity and penetration resistance. While certain locations exhibited similar increasing patterns, most locations demonstrated contrasting responses. The findings indicate that electrical resistivity is strongly influenced by moisture content, degree of saturation, and pore water conductivity, whereas Mackintosh Probe resistance is governed primarily by soil density, stiffness, and particle interlocking. Consequently, a reliable proportional relationship between ERT and MP measurements could not be established. The study concludes that both methods should be considered complementary subsurface investigation techniques rather than interchangeable approaches for evaluating unsaturated tropical soils.