Evaluation of subsurface geological uncertainty in foundation design using electrical resistivity tomography and geotechnical methods

Uncertainty and inadequate subsurface geological data are significant factors contributing to the failure of engineering structures. Conventional site investigations typically rely on drilling multiple boreholes to obtain core samples for geotechnical analysis, including assessments of UCS and RQD. However, in regions like the Khorat Plateau of Northeast Thailand, foundation engineering faces challenges due to the highly variable thickness of hard sandstone beds interbedded with soft, weak mudstone layers that can critically affect bearing capacity and settlement behavior over short distances. This study integrates 2D ERT surveys with traditional borehole investigations to improve subsurface characterization for foundation design. ERT imaging successfully identified three primary subsurface layers: the earthen berm, intact sandstone bedrock, and weathered mudstone bedrock. The high-resolution tomograms correlated well with core log data, accurately delineating subsurface boundaries based on resistivity contrasts verified at borehole positions. While sandstone bedrock samples exhibited good to excellent RQD values, the correlation between RQD and resistivity was scattered due to factors such as elevation differences and moisture content. Nevertheless, ERT proved particularly valuable in detecting mudstone layers that might otherwise remain undetected. The study recommends ensuring a sandstone thickness of at least 5 m, as confirmed by ERT, for safe and reliable shallow foundation design. Furthermore, it highlights the potential of ERT to reduce the number of required boreholes and enhance the reliability of geotechnical assessments. This integrated methodology holds promise for application in similar geological contexts worldwide.