Integrated geotechnical and electrical resistivity approaches for investigating rainfall-induced slope failure in a mountainous highway embankment

A rainfall-induced slope failure occurred in October 2022 along Highway No. 1088 in northern Thailand. To investigate the failure mechanism, an integrated approach combining geotechnical, geological, and geophysical methods was employed. Borehole drilling and laboratory testing were conducted to evaluate soil stratigraphy, strength parameters, and water content, while 2D electrical resistivity imaging (ERI) was used to identify subsurface profiles and failure surfaces over a large area. The failure surface was inferred at depths between 3.00–6.45 m based on borehole data, characterized by low N₆₀-values and water contents exceeding the plastic limits. The 2D ERI results delineated a low-resistivity zone (10–20 Ωm) at depths between 6.00–9.00 m, indicating a preferentially saturated and mechanically weak layer consistent with the failure surface. Integration of the 2D ERI profiles revealed a northwest-oriented slope movement direction, aligning with field observations of fracture patterns and regional fault orientation. Geological investigation of outcrops supported the identification of semi-consolidated interbedded clay and gravel layers with high weathering susceptibility. The study demonstrates that the integration of resistivity imaging with conventional geotechnical investigations provides a robust framework for assessing failure surfaces and slope movement in complex geomaterials. These findings are essential for slope stability analysis, early-warning system design, and the development of targeted countermeasures in rainfall-prone mountainous regions.