Coupled mechanical-electrical behavior and microstructural mechanisms of Cu²⁺ contaminated red clay

Abstract

The increasing prevalence of heavy metal soil contamination, particularly involving Cu²⁺, poses significant challenges in environmental geotechnics, underscoring the need for more robust methods to evaluate the engineering behavior of affected soils. An integrated approach combining direct shear, unconfined compression, and real-time electrical resistivity testing was employed to characterize the coupled mechanical-electrical responses of Cu²⁺-contaminated red clay. Significant exponential correlations (R² >0.90) were established between key strength parameters (e.g., shear strength, cohesion, internal friction angle, unconfined compressive strength) and corresponding resistivity metrics, supporting the development of non-destructive predictive models for strength degradation. Complementary multiscale analyses using X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) further elucidated the mineralogical and microstructural transformations, including kaolinite/goethite dissolution, surface roughening, and pore coarsening. These coupled transformations are primarily driven by Cu²⁺-induced alteration of the electrical double layer (EDL), progressive microstructural degradation, and acid-induced mineral dissolution. These findings establish both a theoretical foundation and practical framework for employing electrical resistivity as a diagnostic indicator of strength degradation in contaminated soils, with implications for in-situ monitoring, remediation strategies, and sustainable geotechnical design.