Subsurface Soil Evaluation Using 2D Electrical Resistivity Tomography for Foundation Studies at Najaf City, Iraq

Abstract

At two locations in Al-Najaf City, Al-Ghadeer Village and Zain Al-Abideen Street, a two-dimensional Electrical Resistivity Tomography survey was carried out using the ABEM Terra  LS2 and three electrode configurations: Wenner, Schlumberger, and Dipole-Dipole. Assessing subsurface geotechnical conditions and identifying weak or abnormal zones were the goals of the study. Each array made a unique contribution to the interpretation: The Schlumberger attained deeper penetration and better vertical resolution close to the groundwater-saturated layers. The Dipole-Dipole provided the best lateral resolution for mapping possible cavity-related anomalies. Whereas the Wenner offered high signal stability and dependable detection of shallow lithological variations. According to drilling data (BH-13, BH-15, and BH-16), low resistivity anomalies (0.27–9.5 Ωm) in Al-Ghadeer Village were associated with sandy clay and localized dissolution characteristics. In Zain Al-Abideen Street, very low resistivity zones (<1.0 Ωm) beneath paved surfaces matched field-observed subsidence, indicating weak saturated soils, while deeper high-resistivity layers corresponded to dry gypsum-rich sands (BH-1). Integrating Electrical Resistivity Tomography with borehole information enhanced the interpretation confidence. The upper layers (0.5–2.5 m) are generally suitable for shallow foundations, but deeper weak zones require caution. Localized cavities likely associated with gypsum dissolution may trigger future ground collapse. The Schlumberger array gave the best vertical detail, especially at deeper depths, while the Dipole-Dipole array was more sensitive to horizontal changes. The Wenner array gave balanced results but with slightly higher RMS errors. Additional investigations, such as 3D ERT imaging or geotechnical load testing, are recommended, and soil improvement or deep foundations may be necessary in hazardous zones.